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2. AN905
Brushed DC Motor Fundamentals
Author: Reston Condit
Stator
Microchip Technology Inc. The stator generates a stationary magnetic field that
surrounds the rotor. This field is generated by either
permanent magnets or electromagnetic windings. The
INTRODUCTION different types of BDC motors are distinguished by the
construction of the stator or the way the electromag-
Brushed DC motors are widely used in applications netic windings are connected to the power source.
ranging from toys to push-button adjustable car seats. (See Types of Stepping Motors for the different BDC
Brushed DC (BDC) motors are inexpensive, easy to motor types).
drive, and are readily available in all sizes and shapes.
This application note will discuss how a BDC motor
works, how to drive a BDC motor, and how a drive
Rotor
circuit can be interfaced to a PIC® microcontroller. The rotor, also called the armature, is made up of one
or more windings. When these windings are energized
PRINCIPLES OF OPERATION they produce a magnetic field. The magnetic poles of
this rotor field will be attracted to the opposite poles
The construction of a simple BDC motor is shown in generated by the stator, causing the rotor to turn. As the
Figure 1. All BDC motors are made of the same basic motor turns, the windings are constantly being
components: a stator, rotor, brushes and a commutator. energized in a different sequence so that the magnetic
The following paragraphs will explain each component poles generated by the rotor do not overrun the poles
in greater detail. generated in the stator. This switching of the field in the
rotor windings is called commutation.
FIGURE 1: SIMPLE TWO-POLE BRUSHED DC MOTOR
N
NORTH
SOUTH
Brushes
Commutator
Field
Magnet
Axle Armature or Coil
 2004 Microchip Technology Inc. DS00905A-page 1

3. AN905
Brushes and Commutator Shunt-Wound
Unlike other electric motor types (i.e., brushless DC, Shunt-wound Brushed DC (SHWDC) motors have the
AC induction), BDC motors do not require a controller field coil in parallel (shunt) with the armature. The
to switch current in the motor windings. Instead, the current in the field coil and the armature are indepen-
commutation of the windings of a BDC motor is done dent of one another. As a result, these motors have
mechanically. A segmented copper sleeve, called a excellent speed control. SHWDC motors are typically
commutator, resides on the axle of a BDC motor. As the used applications that require five or more horsepower.
motor turns, carbon brushes slide over the commutator, Loss of magnetism is not an issue in SHWDC motors
coming in contact with different segments of the so they are generally more robust than PMDC motors.
commutator. The segments are attached to different
rotor windings, therefore, a dynamic magnetic field is FIGURE 3: SHUNT-WOUND DC
generated inside the motor when a voltage is applied MOTORS
across the brushes of the motor. It is important to note
that the brushes and commutator are the parts of a
BDC motor that are most prone to wear because they Brush
are sliding past each other. DC Shunt
Voltage Field
Supply
TYPES OF STEPPING MOTORS
Armature
As mentioned earlier, the way the stationary magnetic
field is produced in the stator differentiates the various
types of BDC motors. This section will discuss the
different types of BDC motors and the advantages/ Series-Wound
disadvantages of each. Series-wound Brushed DC (SWDC) motors have the
field coil in series with the armature. These motors are
Permanent Magnet ideally suited for high-torque applications because the
current in both the stator and armature increases under
Permanent Magnet Brushed DC (PMDC) motors are
load. A drawback to SWDC motors is that they do not
the most common BDC motors found in the world.
have precise speed control like PMDC and SHWDC
These motors use permanent magnets to produce the
motors have.
stator field. PMDC motors are generally used in appli-
cations involving fractional horsepower because it is
more cost effective to use permanent magnets than FIGURE 4: SERIES-WOUND DC
wound stators. The drawback of PMDC motors is that MOTORS
the magnets lose their magnetic properties over time.
Some PMDC motors have windings built into them to Series
prevent this from happening. The performance curve Field
(voltage vs. speed), is very linear for PMDC motors. DC
Current draw also varies linearly with torque. These Voltage Armature
motors respond to changes in voltage very quickly Supply
because the stator field is always constant. Brush
FIGURE 2: PERMANENT MAGNET DC
MOTORS
Armature
Brush
DC
Voltage
Supply
Permanent
Magnet Poles
DS00905A-page 2  2004 Microchip Technology Inc.

4. AN905
Compound-Wound Note that in each circuit there is a diode across the
motor. This diode is there to prevent Back Electromag-
Compound Wound (CWDC) motors are a combination netic Flux (BEMF) voltage from harming the MOSFET.
of shunt-wound and series-wound motors. As shown in BEMF is generated when the motor is spinning. When
Figure 5, CWDC motors employ both a series and a the MOSFET is turned off, the winding in the motor is
shunt field. The performance of a CWDC motor is a still charged at this point and will produce reverse
combination of SWDC and SHWDC motors. CWDC current flow. D1 must be rated appropriately so that it
motors have higher torque than a SHWDC motor while will dissipate this current.
offering better speed control than SWDC motor.
FIGURE 6: LOW-SIDE BDC MOTOR
FIGURE 5: COMPOUND-WOUND DC DRIVE CIRCUIT
MOTORS
VCC
Series
Field Brush
DC
D1
Voltage Shunt
BDC
Supply Field Motor
Armature
To Controller
R1
BASIC DRIVE CIRCUITS
Drive circuits are used in applications where a control-
R2
ler of some kind is being used and speed control is
required. The purpose of a drive circuit is to give the
controller a way to vary the current in the windings of
the BDC motor. The drive circuits discussed in this
section allow the controller to pulse width modulate the
voltage supplied to a BDC motor. In terms of power FIGURE 7: HIGH-SIDE BDC MOTOR
consumption, this method of speed control is a far more DRIVE CIRCUIT
efficient way to vary the speed of a BDC motor
compared to traditional analog control methods. VCC
Traditional analog control required the addition of an
inefficient variable resistance in series with the motor.
To Controller
VCC
R2
BDC motors are driven in a variety of ways. In some
cases the motor only needs to spin in one direction.
Figure 6 and Figure 7 show circuits for driving a BDC R1
motor in one direction. The first is a low-side drive and
the second is a high-side drive. The advantage to using
the low-side drive is that a FET driver is not typically
needed. A FET driver is used to:
D1
1. bring the TTL signal driving a MOSFET to the BDC
Motor
potential level of the supply voltage,
2. provide enough current to drive the MOSFET(1),
3. and provide level shifting in half-bridge
applications.
Note 1: The second point typically does not apply Resistors R1 and R2 in Figure 6 and Figure 7 are
to most PICmicro® microcontroller important to the operation of each circuit. R1 protects
applications because PIC microcontroller the microcontroller from current spikes while R2
I/O pins can source 20 mA. ensures that Q1 is turned off when the input pin is
tristated.
 2004 Microchip Technology Inc. DS00905A-page 3

5. AN905
Bidirectional control of a BDC motor requires a circuit Note the diodes across each of the MOSFETs (D1-D4).
called an H-bridge. The H-bridge, named for it's These diodes protect the MOSFETs from current spikes
schematic appearance, is able to move current in either generated by BEMF when the MOSFETs are switched
direction through the motor winding. To understand off. These diodes are only needed if the internal
this, the H-bridge must be broken into its two sides, or MOSFET diodes are not sufficient for dissipating the
half-bridges. Referring to Figure 8, Q1 and Q2 make up BEMF current.
one half-bridge while Q3 and Q4 make up the other The capacitors (C1-C4) are optional. The value of
half-bridge. Each of these half-bridges is able to switch these capacitors is generally in the 10 pF range. The
one side of the BDC motor to the potential of the supply purpose of these capacitors is to reduce the RF
voltage or ground. When Q1 is turned on and Q2 is off, radiation that is produced by the arching of the
for instance, the left side of the motor will be at the commutators.
potential of the supply voltage. Turning on Q4 and
leaving Q3 off will ground the opposite side of the
motor. The arrow labeled IFWD shows the resulting
current flow for this configuration.
FIGURE 8: BIDIRECTION BDC MOTOR DRIVE (H-BRIDGE) CIRCUIT
VSUPPLY
CTRL1 Q1 D1 C1 C3 D3 Q3 CTRL3
Motor
R1
IFWD
R3
BDC
IRVS
IBRK
CTRL2 Q2 D2 C2 C4 D4 Q4 CTRL4
R4
R2
The different drive modes for and H-bridge circuit are There is one very important consideration that must be
shown in Table 1. In Forward mode and Reverse mode taken into account when designing an H-bridge circuit.
one side of the bridge is held at ground potential and All MOSFETs must be biased to off when the inputs to
the other side at VSUPPLY. In Figure 8 the IFWD and IRVS the circuit are unpredictable (like when the microcon-
arrows illustrate the current paths during the Forward troller is starting up). This will ensure that the
and Reverse modes of operation. In Coast mode, the MOSFETs on each half-bridge of the H-bridge will
ends of the motor winding are left floating and the never be turned on at the same time. Turning
motor coasts to a stop. Brake mode is used to rapidly MOSFETs on that are located on the same half-bridge
stop the BDC motor. In Brake mode, the ends of the will cause a short across the power supply, ultimately
motor are grounded. The motor behaves as a genera- damaging the MOSFETs and rendering the circuit
tor when it is rotating. Shorting the leads of the motor inoperable. Pull-down resistors at each of the MOSFET
acts as a load of infinite magnitude bringing the motor driver inputs will accomplish this functionality (for the
to a rapid halt. The IBRK arrow illustrates this. configuration shown in Figure 8).
TABLE 1: H-BRIDGE MODES OF
OPERATION
Q1 Q2 Q3 Q4
(CTRL1) (CTRL2) (CTRL3) (CTRL4)
Forward on off off on
Reverse off on on off
Coast off off off off
Brake off on off on
DS00905A-page 4  2004 Microchip Technology Inc.

6. AN905
SPEED CONTROL The ECCP module (short for Enhanced Capture
Compare and PWM) provides the same functionality as
The speed of a BDC motor is proportional to the voltage the CCP module with the added capability of driving a
applied to the motor. When using digital control, a full or half-bridge circuit. The ECCP module also has
pulse-width modulated (PWM) signal is used to gener- auto-shutdown capability and programmable dead
ate an average voltage. The motor winding acts as a band delay.
low pass filter so a PWM waveform of sufficient
frequency will generate a stable current in the motor Note: Microchip Application Note AN893 gives a
winding. The relation between average voltage, the detailed explanation of configuring the
supply voltage, and duty cycle is given by: ECCP module for driving a BDC motor.
The application note also includes
firmware and drive circuit examples.
EQUATION 1:
VAVERAGE = D × VSUPPLY FEEDBACK MECHANISMS
Though the speed of a BDC motor is generally propor-
Speed and duty cycle are proportional to one another. tional to duty cycle, no motor is ideal. Heat, commutator
For example, if a BDC motor is rated to turn at 15000 wear and load all affect the speed of a motor. In
RPM at 12V, the motor will (ideally) turn at 7500 RPM systems where precise speed control is required, it is a
when a 50% duty cycle waveform is applied across the good idea to include some sort of feedback mechanism
motor. in the system.
The frequency of the PWM waveform is an important Speed feedback is implemented in one of two ways.
consideration. Too low a frequency will result in a noisy The first involves the use of a speed sensor of some
motor at low speeds and sluggish response to changes kind. The second uses the BEMF voltage generated by
in duty cycle. Too high a frequency lessens the the motor.
efficiency of the system due to switching losses in the
switching devices. A good rule of thumb is to modulate
Sensored Feedback
the input waveform at a frequency in the range of 4 kHz
to 20 kHz. This range is high enough that audible motor There are a variety of sensors used for speed feed-
noise is attenuated and the switching losses present in back. The most common are optical encoders and hall
the MOSFETs (or BJTs) are negligible. Generally, it is a effect sensors. Optical encoders are made up of
good idea to experiment with the PWM frequency for a several components. A slotted wheel is mounted to the
given motor to find a satisfactory frequency. shaft at the non-driving end of the motor. An infrared
So how can a PIC microcontroller be used to generate LED provides a light source on one side of the wheel
the PWM waveform required to control the speed of a and a photo transistor detects light on the other side of
BDC motor? One way would be to toggle an output pin the wheel (see Figure 9). Light passing through the
by writing assembly or C code dedicated to driving that slots in the wheel will turn the photo transistor on. As
pin(1). Another way is to select a PIC microcontroller the shaft turns, the photo transistor turns on and off with
with a hardware PWM module. The modules available the passing of the slots in the wheel. The frequency at
from Microchip for this purpose are the CCP an ECCP which the transistor toggles is an indication of motor
modules. Many of the PIC microcontrollers have CCP speed. In the case of positioning applications, an
and ECCP modules. Refer to the product selector optical encoder will also provide feedback as to the
guide to find the devices having these features. position of the motor.
Note 1: Microchip Application Note AN847 FIGURE 9: OPTICAL ENCODER
provides an assembly code routine for
pulse-width modulating an I/O pin in
firmware.
slotted
The CCP module (short for Capture Compare and wheel
PWM) is capable of outputting a 10-bit resolution PWM
waveform on a single I/O pin. 10-bit resolution means
that 210, or 1024, possible duty cycle values ranging Photo Transistor
from 0% to 100% are achievable by the module. The IR LED
advantage to using this module is that it automatically
generates a PWM signal on an I/O pin which frees up
Front View Side View
processor time for doing other things. The CCP module
only requires that the developer configure the parame-
ters of the module. Configuring the module includes
setting the frequency and duty cycle registers.
 2004 Microchip Technology Inc. DS00905A-page 5

7. AN905
Hall effect sensors are also used to provide speed Back Electro Magnetic Flux (BEMF)
feedback. Like optical encoders, hall effect sensors
require a rotary element attached to the motor and a Another form of velocity feedback for a BDC motor is
stationary component. The rotary element is a wheel BEMF voltage measurement. BEMF voltage and speed
with one or more magnets positioned on its outer rim. A are proportional to one another. Figure 11 shows the
stationary sensor detects the magnet when in passes locations where BEMF voltage is measured on a
and generates a TTL pulse. Figure 10 shows the basic bidirectional drive circuit. A voltage divider is used to
components of a hall effect sensor. drop the BEMF voltage into the 0-5V range so that it
can be read by an analog-to-digital converter. The
BEMF voltage is measured between PWM pulses
FIGURE 10: HALL EFFECT SENSOR
when one side of the motor is floating and the other is
grounded. At this instance in time the motor is acting
magnet wheel
like a generator and produces a BEMF voltage
magnet proportional to speed.
hall effect
sensor
Front View Side View
FIGURE 11: BACK EMF VOLTAGE MEASUREMENT
VSUPPLY
CTRL1 Q1 C1 C3 Q3 CTRL3
R1
R3
Motor
BEMF BDC BEMF
CTRL2 Q2 C2 C4 Q4 CTRL4
R4
R2
DS00905A-page 6  2004 Microchip Technology Inc.

8. AN905
All BDC motors behave slightly differently because of
differences in efficiency and materials. Experimenta-
tion is the best way to determine the BEMF voltage for
a given motor speed. A piece of reflect tape on the
shaft of the motor will allow a digital tachometer to
measure the RPM of the motor. Measuring the BEMF
voltage while reading the digital tachometer will give a
correlation between motor speed and BEMF voltage.
Note: Microchip Application Note AN893
provides firmware and circuit examples for
reading the BEMF voltage using a
PIC16F684.
CONCLUSION
Brushed DC motors are very simple to use and control,
which makes them a short design-in item. PIC
microcontrollers, especially those with CCP or ECCP
modules are ideally suited for driving BDC motors.
REFERENCES
AN893 Low-Cost Bidirectional Brushed DC Motor
Control Using the PIC16F684.
AN847 RC Model Aircraft Motor Control.
www.howstuffworks.com
www.engin.umich.edu/labs/csdl/me350/motors/dc/
index.html
 2004 Microchip Technology Inc. DS00905A-page 7

9. AN905
NOTES:
DS00905A-page 8  2004 Microchip Technology Inc.

10. Note the following details of the code protection feature on Microchip devices:
• Microchip products meet the specification contained in their particular Microchip Data Sheet.
• Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
• There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
• Microchip is willing to work with the customer who is concerned about the integrity of their code.
• Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device Trademarks
applications and the like is intended through suggestion only
The Microchip name and logo, the Microchip logo, Accuron,
and may be superseded by updates. It is your responsibility to dsPIC, KEELOQ, MPLAB, PIC, PICmicro, PICSTART,
ensure that your application meets with your specifications.
PRO MATE, PowerSmart and rfPIC are registered
No representation or warranty is given and no liability is
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© 2004, Microchip Technology Incorporated, Printed in the
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 2004 Microchip Technology Inc. DS00905A-page 9

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DS00905A-page 10  2004 Microchip Technology Inc.

12. AN887
AC Induction Motor Fundamentals
created naturally in the stator because of the nature of
Author: Rakesh Parekh
the supply. DC motors depend either on mechanical or
Microchip Technology Inc.
electronic commutation to create rotating magnetic
fields. A single-phase AC induction motor depends on
extra electrical components to produce this rotating
INTRODUCTION magnetic field.
AC induction motors are the most common motors Two sets of electromagnets are formed inside any motor.
used in industrial motion control systems, as well as in In an AC induction motor, one set of electromagnets is
main powered home appliances. Simple and rugged formed in the stator because of the AC supply connected
design, low-cost, low maintenance and direct connec- to the stator windings. The alternating nature of the sup-
tion to an AC power source are the main advantages of ply voltage induces an Electromagnetic Force (EMF) in
AC induction motors. the rotor (just like the voltage is induced in the trans-
Various types of AC induction motors are available in former secondary) as per Lenz’s law, thus generating
the market. Different motors are suitable for different another set of electromagnets; hence the name – induc-
applications. Although AC induction motors are easier tion motor. Interaction between the magnetic field of
to design than DC motors, the speed and the torque these electromagnets generates twisting force, or
control in various types of AC induction motors require torque. As a result, the motor rotates in the direction of
a greater understanding of the design and the the resultant torque.
characteristics of these motors.
This application note discusses the basics of an AC Stator
induction motor; the different types, their characteris- The stator is made up of several thin laminations of
tics, the selection criteria for different applications and aluminum or cast iron. They are punched and clamped
basic control techniques. together to form a hollow cylinder (stator core) with
slots as shown in Figure 1. Coils of insulated wires are
BASIC CONSTRUCTION AND inserted into these slots. Each grouping of coils,
together with the core it surrounds, forms an electro-
OPERATING PRINCIPLE magnet (a pair of poles) on the application of AC
Like most motors, an AC induction motor has a fixed supply. The number of poles of an AC induction motor
outer portion, called the stator and a rotor that spins depends on the internal connection of the stator wind-
inside with a carefully engineered air gap between the ings. The stator windings are connected directly to the
two. power source. Internally they are connected in such a
way, that on applying AC supply, a rotating magnetic
Virtually all electrical motors use magnetic field rotation
field is created.
to spin their rotors. A three-phase AC induction motor
is the only type where the rotating magnetic field is
FIGURE 1: A TYPICAL STATOR
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13. AN887
Rotor Speed of an Induction Motor
The rotor is made up of several thin steel laminations The magnetic field created in the stator rotates at a
with evenly spaced bars, which are made up of synchronous speed (NS).
aluminum or copper, along the periphery. In the most
popular type of rotor (squirrel cage rotor), these bars EQUATION 1:
are connected at ends mechanically and electrically by f-
the use of rings. Almost 90% of induction motors have N s = 120 × --
P
squirrel cage rotors. This is because the squirrel cage
where:
rotor has a simple and rugged construction. The rotor
NS = the synchronous speed of the stator
consists of a cylindrical laminated core with axially
magnetic field in RPM
placed parallel slots for carrying the conductors. Each
P = the number of poles on the stator
slot carries a copper, aluminum, or alloy bar. These
f = the supply frequency in Hertz
rotor bars are permanently short-circuited at both ends
by means of the end rings, as shown in Figure 2. This
total assembly resembles the look of a squirrel cage, The magnetic field produced in the rotor because of the
which gives the rotor its name. The rotor slots are not induced voltage is alternating in nature.
exactly parallel to the shaft. Instead, they are given a To reduce the relative speed, with respect to the stator,
skew for two main reasons. the rotor starts running in the same direction as that of
The first reason is to make the motor run quietly by the stator flux and tries to catch up with the rotating flux.
reducing magnetic hum and to decrease slot However, in practice, the rotor never succeeds in
harmonics. “catching up” to the stator field. The rotor runs slower
than the speed of the stator field. This speed is called
The second reason is to help reduce the locking ten- the Base Speed (Nb).
dency of the rotor. The rotor teeth tend to remain locked
under the stator teeth due to direct magnetic attraction The difference between NS and Nb is called the slip. The
between the two. This happens when the number of slip varies with the load. An increase in load will cause
stator teeth are equal to the number of rotor teeth. the rotor to slow down or increase slip. A decrease in
load will cause the rotor to speed up or decrease slip.
The rotor is mounted on the shaft using bearings on The slip is expressed as a percentage and can be
each end; one end of the shaft is normally kept longer determined with the following formula:
than the other for driving the load. Some motors may
have an accessory shaft on the non-driving end for
EQUATION 2:
mounting speed or position sensing devices. Between
the stator and the rotor, there exists an air gap, through Ns – Nb
which due to induction, the energy is transferred from % slip = ------------------- x100
-
Ns
the stator to the rotor. The generated torque forces the
where:
rotor and then the load to rotate. Regardless of the type
NS = the synchronous speed in RPM
of rotor used, the principle employed for rotation
Nb = the base speed in RPM
remains the same.
FIGURE 2: A TYPICAL SQUIRREL CAGE ROTOR
End Ring Conductors End Ring
Shaft
Bearing Bearing
Skewed Slots
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14. AN887
TYPES OF AC INDUCTION MOTORS phase induction motor is required to have a starting
mechanism that can provide the starting kick for the
Generally, induction motors are categorized based on motor to rotate.
the number of stator windings. They are:
The starting mechanism of the single-phase induction
• Single-phase induction motor motor is mainly an additional stator winding (start/
• Three-phase induction motor auxiliary winding) as shown in Figure 3. The start wind-
ing can have a series capacitor and/or a centrifugal
Single-Phase Induction Motor switch. When the supply voltage is applied, current in
the main winding lags the supply voltage due to the
There are probably more single-phase AC induction main winding impedance. At the same time, current in
motors in use today than the total of all the other types the start winding leads/lags the supply voltage depend-
put together. It is logical that the least expensive, low- ing on the starting mechanism impedance. Interaction
est maintenance type motor should be used most between magnetic fields generated by the main wind-
often. The single-phase AC induction motor best fits ing and the starting mechanism generates a resultant
this description. magnetic field rotating in one direction. The motor
As the name suggests, this type of motor has only one starts rotating in the direction of the resultant magnetic
stator winding (main winding) and operates with a field.
single-phase power supply. In all single-phase Once the motor reaches about 75% of its rated speed,
induction motors, the rotor is the squirrel cage type. a centrifugal switch disconnects the start winding. From
The single-phase induction motor is not self-starting. this point on, the single-phase motor can maintain
When the motor is connected to a single-phase power sufficient torque to operate on its own.
supply, the main winding carries an alternating current. Except for special capacitor start/capacitor run types,
This current produces a pulsating magnetic field. Due all single-phase motors are generally used for
to induction, the rotor is energized. As the main applications up to 3/4 hp only.
magnetic field is pulsating, the torque necessary for the
Depending on the various start techniques, single-
motor rotation is not generated. This will cause the
phase AC induction motors are further classified as
rotor to vibrate, but not to rotate. Hence, the single-
described in the following sections.
FIGURE 3: SINGLE-PHASE AC INDUCTION MOTOR WITH AND WITHOUT A
START MECHANISM
Capacitor Centrifugal Switch
Rotor Rotor
Input Main Input
Power Power Main
Winding
Winding
Start Winding
Single-Phase AC Induction Motor Single-Phase AC Induction Motor
without Start Mechanism with Start Mechanism
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15. AN887
Split-Phase AC Induction Motor FIGURE 5: TYPICAL CAPACITOR
The split-phase motor is also known as an induction START INDUCTION MOTOR
start/induction run motor. It has two windings: a start Capacitor Centrifugal Switch
and a main winding. The start winding is made with
smaller gauge wire and fewer turns, relative to the main
Rotor
winding to create more resistance, thus putting the start
winding’s field at a different angle than that of the main
winding which causes the motor to start rotating. The
main winding, which is of a heavier wire, keeps the
Input
motor running the rest of the time. Power Main
Winding
FIGURE 4: TYPICAL SPLIT-PHASE AC
INDUCTION MOTOR Start Winding
Centrifugal Switch They are used in a wide range of belt-drive applications
like small conveyors, large blowers and pumps, as well
Rotor as many direct-drive or geared applications.
Permanent Split Capacitor (Capacitor
Run) AC Induction Motor
Input
Power Main A permanent split capacitor (PSC) motor has a run type
Winding capacitor permanently connected in series with the
start winding. This makes the start winding an auxiliary
Start Winding winding once the motor reaches the running speed.
Since the run capacitor must be designed for continu-
The starting torque is low, typically 100% to 175% of the ous use, it cannot provide the starting boost of a start-
rated torque. The motor draws high starting current, ing capacitor. The typical starting torque of the PSC
approximately 700% to 1,000% of the rated current. The motor is low, from 30% to 150% of the rated torque.
maximum generated torque ranges from 250% to 350% PSC motors have low starting current, usually less than
of the rated torque (see Figure 9 for torque-speed 200% of the rated current, making them excellent for
curve). applications with high on/off cycle rates. Refer to
Good applications for split-phase motors include small Figure 9 for torque-speed curve.
grinders, small fans and blowers and other low starting The PSC motors have several advantages. The motor
torque applications with power needs from 1/20 to design can easily be altered for use with speed control-
1/3 hp. Avoid using this type of motor in any applications lers. They can also be designed for optimum efficiency
requiring high on/off cycle rates or high torque. and High-Power Factor (PF) at the rated load. They’re
considered to be the most reliable of the single-phase
Capacitor Start AC Induction Motor motors, mainly because no centrifugal starting switch is
required.
This is a modified split-phase motor with a capacitor in
series with the start winding to provide a start “boost.”
Like the split-phase motor, the capacitor start motor FIGURE 6: TYPICAL PSC MOTOR
also has a centrifugal switch which disconnects the Capacitor
start winding and the capacitor when the motor reaches
about 75% of the rated speed. Rotor
Since the capacitor is in series with the start circuit, it
creates more starting torque, typically 200% to 400% of
the rated torque. And the starting current, usually 450%
to 575% of the rated current, is much lower than the Input
Power Main
split-phase due to the larger wire in the start circuit. Winding
Refer to Figure 9 for torque-speed curve.
A modified version of the capacitor start motor is the Start Winding
resistance start motor. In this motor type, the starting
capacitor is replaced by a resistor. The resistance start Permanent split-capacitor motors have a wide variety
motor is used in applications where the starting torque of applications depending on the design. These include
requirement is less than that provided by the capacitor fans, blowers with low starting torque needs and inter-
start motor. Apart from the cost, this motor does not offer mittent cycling uses, such as adjusting mechanisms,
any major advantage over the capacitor start motor. gate operators and garage door openers.
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16. AN887
Capacitor Start/Capacitor Run AC Shaded-Pole AC Induction Motor
Induction Motor Shaded-pole motors have only one main winding and
This motor has a start type capacitor in series with the no start winding. Starting is by means of a design that
auxiliary winding like the capacitor start motor for high rings a continuous copper loop around a small portion
starting torque. Like a PSC motor, it also has a run type of each of the motor poles. This “shades” that portion of
capacitor that is in series with the auxiliary winding after the pole, causing the magnetic field in the shaded area
the start capacitor is switched out of the circuit. This to lag behind the field in the unshaded area. The
allows high overload torque. reaction of the two fields gets the shaft rotating.
Because the shaded-pole motor lacks a start winding,
FIGURE 7: TYPICAL CAPACITOR starting switch or capacitor, it is electrically simple and
START/RUN INDUCTION inexpensive. Also, the speed can be controlled merely
MOTOR by varying voltage, or through a multi-tap winding.
Mechanically, the shaded-pole motor construction
Start Cap Centrifugal Switch allows high-volume production. In fact, these are usu-
ally considered as “disposable” motors, meaning they
Run Cap are much cheaper to replace than to repair.
Rotor FIGURE 8: TYPICAL SHADED-POLE
INDUCTION MOTOR
Shaded Portion of Pole
Copper Ring
Input
Power Main
Winding
Start Winding
This type of motor can be designed for lower full-load
currents and higher efficiency (see Figure 9 for torque-
speed curve). This motor is costly due to start and run Supply Line
capacitors and centrifugal switch.
Unshaded Portion of Pole
It is able to handle applications too demanding for any
other kind of single-phase motor. These include wood-
working machinery, air compressors, high-pressure The shaded-pole motor has many positive features but
water pumps, vacuum pumps and other high torque it also has several disadvantages. It’s low starting
applications requiring 1 to 10 hp. torque is typically 25% to 75% of the rated torque. It is
a high slip motor with a running speed 7% to 10%
below the synchronous speed. Generally, efficiency of
this motor type is very low (below 20%).
The low initial cost suits the shaded-pole motors to low
horsepower or light duty applications. Perhaps their larg-
est use is in multi-speed fans for household use. But the
low torque, low efficiency and less sturdy mechanical
features make shaded-pole motors impractical for most
industrial or commercial use, where higher cycle rates or
continuous duty are the norm.
Figure 9 shows the torque-speed curves of various
kinds of single-phase AC induction motors.
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17. AN887
FIGURE 9: TORQUE-SPEED CURVES OF DIFFERENT TYPES OF SINGLE-PHASE
INDUCTION MOTORS
Capacitor Start and Run
500
Changeover of Centrifugal Switch
Torque (% of Full-Load Torque)
Capacitor Start
400
Split-Phase
300
PSC
200
Shaded-Pole
100
20 40 60 80 100
Speed (%)
THREE-PHASE AC INDUCTION Wound-Rotor Motor
MOTOR The slip-ring motor or wound-rotor motor is a variation
Three-phase AC induction motors are widely used in of the squirrel cage induction motor. While the stator is
industrial and commercial applications. They are the same as that of the squirrel cage motor, it has a set
classified either as squirrel cage or wound-rotor of windings on the rotor which are not short-circuited,
motors. but are terminated to a set of slip rings. These are
helpful in adding external resistors and contactors.
These motors are self-starting and use no capacitor,
start winding, centrifugal switch or other starting The slip necessary to generate the maximum torque
device. (pull-out torque) is directly proportional to the rotor
resistance. In the slip-ring motor, the effective rotor
They produce medium to high degrees of starting resistance is increased by adding external resistance
torque. The power capabilities and efficiency in these through the slip rings. Thus, it is possible to get higher
motors range from medium to high compared to their slip and hence, the pull-out torque at a lower speed.
single-phase counterparts. Popular applications
include grinders, lathes, drill presses, pumps, A particularly high resistance can result in the pull-out
compressors, conveyors, also printing equipment, farm torque occurring at almost zero speed, providing a very
equipment, electronic cooling and other mechanical high pull-out torque at a low starting current. As the
duty applications. motor accelerates, the value of the resistance can be
reduced, altering the motor characteristic to suit the
load requirement. Once the motor reaches the base
Squirrel Cage Motor speed, external resistors are removed from the rotor.
Almost 90% of the three-phase AC Induction motors This means that now the motor is working as the
are of this type. Here, the rotor is of the squirrel cage standard induction motor.
type and it works as explained earlier. The power This motor type is ideal for very high inertia loads,
ratings range from one-third to several hundred horse- where it is required to generate the pull-out torque at
power in the three-phase motors. Motors of this type, almost zero speed and accelerate to full speed in the
rated one horsepower or larger, cost less and can start minimum time with minimum current draw.
heavier loads than their single-phase counterparts.
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18. AN887
FIGURE 10: TYPICAL WOUND-ROTOR TORQUE EQUATION GOVERNING
INDUCTION MOTOR MOTOR OPERATION
Wound Rotor The motor load system can be described by a
fundamental torque equation.
Brush
EQUATION 3:
dω m dJ
T – T l = J ----------- + ω m -----
- -
dt dt
External Rotor where:
Slip Ring Resistance T = the instantaneous value of the
developed motor torque (N-m or lb-inch)
Tl = the instantaneous value of the load torque
(N-m or lb-inch)
The downside of the slip ring motor is that slip rings and
ωm = the instantaneous angular
brush assemblies need regular maintenance, which is
velocity of the motor shaft (rad/sec)
a cost not applicable to the standard cage motor. If the
J = the moment of inertia of the motor –
rotor windings are shorted and a start is attempted (i.e.,
load system (kg-m2 or lb-inch2)
the motor is converted to a standard induction motor),
it will exhibit an extremely high locked rotor current –
typically as high as 1400% and a very low locked rotor For drives with constant inertia, (dJ/dt) = 0. Therefore,
torque, perhaps as low as 60%. In most applications, the equation would be:
this is not an option.
Modifying the speed torque curve by altering the rotor EQUATION 4:
resistors, the speed at which the motor will drive a dω m
T = T l + J -----------
-
particular load can be altered. At full load, you can dt
reduce the speed effectively to about 50% of the motor
synchronous speed, particularly when driving variable This shows that the torque developed by the motor is
torque/variable speed loads, such as printing presses counter balanced by a load torque, Tl and a dynamic
or compressors. Reducing the speed below 50% torque, J(dωm/dt). The torque component, J(dω/dt), is
results in very low efficiency due to higher power called the dynamic torque because it is present only
dissipation in the rotor resistances. This type of motor during the transient operations. The drive accelerates
is used in applications for driving variable torque/ or decelerates depending on whether T is greater or
variable speed loads, such as in printing presses, less than Tl. During acceleration, the motor should sup-
compressors, conveyer belts, hoists and elevators. ply not only the load torque, but an additional torque
component, J(dωm/dt), in order to overcome the drive
inertia. In drives with large inertia, such as electric
trains, the motor torque must exceed the load torque by
a large amount in order to get adequate acceleration.
In drives requiring fast transient response, the motor
torque should be maintained at the highest value and
the motor load system should be designed with the low-
est possible inertia. The energy associated with the
dynamic torque, J(dωm/dt), is stored in the form of
kinetic energy (KE) given by, J(ω2m/2). During deceler-
ation, the dynamic torque, J(dωm/dt), has a negative
sign. Therefore, it assists the motor developed torque T
and maintains the drive motion by extracting energy
from the stored kinetic energy.
To summarize, in order to get steady state rotation of
the motor, the torque developed by the motor (T)
should always be equal to the torque requirement of
the load (Tl).
The torque-speed curve of the typical three-phase
induction motor is shown in Figure 11.
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19. AN887
FIGURE 11: TYPICAL TORQUE-SPEED CURVE OF 3-PHASE AC INDUCTION MOTOR
Pull-out Torque
7 x FLC Full Voltage Stator Current
Current (% of Motor Full-Load Current)
Torque (% of Motor Full-Load Torque)
LRC
6 x FLC 2 x FLT
5 x FLC
4 x FLC
Full Voltage Start Torque
LRT
3 x FLC 1 x FLT
2 x FLC Pull-up Torque
1 x FLC Sample Load Torque Curve
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Rotor Speed (% of Full Speed)
STARTING CHARACTERISTIC The LRT of an induction motor can vary from as low as
60% of FLT to as high as 350% of FLT. The pull-up
Induction motors, at rest, appear just like a short cir- torque can be as low as 40% of FLT and the breakdown
cuited transformer and if connected to the full supply torque can be as high as 350% of FLT. Typically, LRTs
voltage, draw a very high current known as the “Locked for medium to large motors are in the order of 120% of
Rotor Current.” They also produce torque which is FLT to 280% of FLT. The PF of the motor at start is
known as the “Locked Rotor Torque”. The Locked typically 0.1-0.25, rising to a maximum as the motor
Rotor Torque (LRT) and the Locked Rotor Current accelerates and then falling again as the motor
(LRC) are a function of the terminal voltage of the motor approaches full speed.
and the motor design. As the motor accelerates, both
the torque and the current will tend to alter with rotor
speed if the voltage is maintained constant. RUNNING CHARACTERISTIC
The starting current of a motor with a fixed voltage will Once the motor is up to speed, it operates at a low slip,
drop very slowly as the motor accelerates and will only at a speed determined by the number of the stator
begin to fall significantly when the motor has reached poles. Typically, the full-load slip for the squirrel cage
at least 80% of the full speed. The actual curves for the induction motor is less than 5%. The actual full-load slip
induction motors can vary considerably between of a particular motor is dependant on the motor design.
designs but the general trend is for a high current until The typical base speed of the four pole induction motor
the motor has almost reached full speed. The LRC of a varies between 1420 and 1480 RPM at 50 Hz, while the
motor can range from 500% of Full-Load Current (FLC) synchronous speed is 1500 RPM at 50 Hz.
to as high as 1400% of FLC. Typically, good motors fall The current drawn by the induction motor has two com-
in the range of 550% to 750% of FLC. ponents: reactive component (magnetizing current)
The starting torque of an induction motor starting with a and active component (working current). The magne-
fixed voltage will drop a little to the minimum torque, tizing current is independent of the load but is depen-
known as the pull-up torque, as the motor accelerates dant on the design of the stator and the stator voltage.
and then rises to a maximum torque, known as the The actual magnetizing current of the induction motor
breakdown or pull-out torque, at almost full speed and can vary, from as low as 20% of FLC for the large two
then drop to zero at the synchronous speed. The curve pole machine, to as high as 60% for the small eight pole
of the start torque against the rotor speed is dependant machine. The working current of the motor is directly
on the terminal voltage and the rotor design. proportional to the load.
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20. AN887
The tendency for the large machines and high-speed In most drives, the electrical time constant of the motor
machines is to exhibit a low magnetizing current, while is negligible as compared to its mechanical time con-
for the low-speed machines and small machines the stant. Therefore, during transient operation, the motor
tendency is to exhibit a high magnetizing current. A can be assumed to be in an electrical equilibrium,
typical medium sized four pole machine has a implying that the steady state torque-speed curve is
magnetizing current of about 33% of FLC. also applicable to the transient operation.
A low magnetizing current indicates a low iron loss, As an example, Figure 12 shows torque-speed curves
while a high magnetizing current indicates an increase of the motor with two different loads. The system can
in iron loss and a resultant reduction in the operating be termed as stable, when the operation will be
efficiency. restored after a small departure from it, due to a
Typically, the operating efficiency of the induction motor disturbance in the motor or load.
is highest at 3/4 load and varies from less than 60% for For example, disturbance causes a reduction of ∆ωm in
small low-speed motors to greater than 92% for large speed. In the first case, at a new speed, the motor
high-speed motors. The operating PF and efficiencies torque (T) is greater than the load torque (Tl). Conse-
are generally quoted on the motor data sheets. quently, the motor will accelerate and the operation will
be restored to X. Similarly, an increase of ∆ωm in the
speed, caused by a disturbance, will make the load
LOAD CHARACTERISTIC torque (Tl) greater than the motor torque (T), resulting
In real applications, various kinds of loads exist with in a deceleration and restoration of the point of
different torque-speed curves. For example, Constant operation to X. Hence, at point X, the system is stable.
Torque, Variable Speed Load (screw compressors, In the second case, a decrease in the speed causes
conveyors, feeders), Variable Torque, Variable Speed the load torque (Tl) to become greater than the motor
Load (fan, pump), Constant Power Load (traction torque (T), the drive decelerates and the operating
drives), Constant Power, Constant Torque Load (coiler point moves away from Y. Similarly, an increase in the
drive) and High Starting/Breakaway Torque followed by speed will make the motor torque (T) greater than the
Constant Torque Load (extruders, screw pumps). load torque (Tl), which will move the operating point
The motor load system is said to be stable when the further away from Y. Thus, at point Y, the system is
developed motor torque is equal to the load torque unstable.
requirement. The motor will operate in a steady state at This shows that, while in the first case, the motor
a fixed speed. The response of the motor to any selection for driving the given load is the right one; in
disturbance gives us an idea about the stability of the the second case, the selected motor is not the right
motor load system. This concept helps us in quickly choice and requires changing for driving the given load.
evaluating the selection of a motor for driving a
particular load. The typical existing loads with their torque-speed
curves are described in the following sections.
FIGURE 12: TORQUE-SPEED CURVE – SAME MOTOR WITH TWO DIFFERENT LOADS
ωm T Tl ωm T
X Y
Tl
0 0
Torque Torque
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21. AN887
Constant Torque, Variable Speed Loads FIGURE 15: CONSTANT POWER
LOADS
The torque required by this type of load is constant
regardless of the speed. In contrast, the power is
linearly proportional to the speed. Equipment, such as Torque
screw compressors, conveyors and feeders, have this
type of characteristic.
Power
FIGURE 13: CONSTANT TORQUE,
VARIABLE SPEED LOADS
Speed
Torque
Constant Power, Constant Torque Loads
This is common in the paper industry. In this type of
Power load, as speed increases, the torque is constant with
the power linearly increasing. When the torque starts to
Speed decrease, the power then remains constant.
FIGURE 16: CONSTANT POWER,
Variable Torque, Variable Speed Loads CONSTANT TORQUE
This is most commonly found in the industry and LOADS
sometimes is known as a quadratic torque load. The
torque is the square of the speed, while the power is the
cube of the speed. This is the typical torque-speed Torque
characteristic of a fan or a pump. Power
FIGURE 14: VARIABLE TORQUE,
VARIABLE SPEED LOADS
Speed
High Starting/Breakaway Torque
Torque Followed by Constant Torque
Power This type of load is characterized by very high torque at
relatively low frequencies. Typical applications include
Speed extruders and screw pumps.
FIGURE 17: HIGH STARTING/
Constant Power Loads
BREAKAWAY TORQUE
This type of load is rare but is sometimes found in the FOLLOWED BY
industry. The power remains constant while the torque CONSTANT TORQUE
varies. The torque is inversely proportional to the
speed, which theoretically means infinite torque at zero
speed and zero torque at infinite speed. In practice,
there is always a finite value to the breakaway torque
required. This type of load is characteristic of the trac-
tion drives, which require high torque at low speeds for Torque
the initial acceleration and then a much reduced torque
when at running speed.
Speed
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22. AN887
MOTOR STANDARDS • Design A has normal starting torque (typically
150-170% of rated) and relatively high starting
Worldwide, various standards exist which specify vari- current. The breakdown torque is the highest of all
ous operating and constructional parameters of a the NEMA types. It can handle heavy overloads
motor. The two most widely used parameters are the for a short duration. The slip is = 5%. A typical
National Electrical Manufacturers Association (NEMA) application is the powering of injection molding
and the International Electrotechnical Commission machines.
(IEC). • Design B is the most common type of AC
induction motor sold. It has a normal starting
NEMA torque, similar to Design A, but offers low starting
current. The locked rotor torque is good enough to
NEMA sets standards for a wide range of electrical
start many loads encountered in the industrial
products, including motors. NEMA is primarily associ-
applications. The slip is = 5%. The motor effi-
ated with motors used in North America. The standards
ciency and full-load PF are comparatively high,
developed represent the general industry practices and
contributing to the popularity of the design. The
are supported by manufacturers of electrical equip-
typical applications include pumps, fans and
ment. These standards can be found in the NEMA
machine tools.
Standard Publication No. MG 1. Some large AC motors
may not fall under NEMA standards. They are built to • Design C has high starting torque (greater than
meet the requirements of a specific application. They the previous two designs, say 200%), useful for
are referred to as above NEMA motors. driving heavy breakaway loads like conveyors,
crushers, stirring machines, agitators, reciprocat-
ing pumps, compressors, etc. These motors are
IEC
intended for operation near full speed without
IEC is a European-based organization that publishes great overloads. The starting current is low. The
and promotes worldwide, the mechanical and electrical slip is = 5%.
standards for motors, among other things. In simple • Design D has high starting torque (higher than all
terms, it can be said that the IEC is the international the NEMA motor types). The starting current and
counterpart of the NEMA. The IEC standards are full-load speed are low. The high slip values
associated with motors used in many countries. These (5-13%) make this motor suitable for applications
standards can be found in the IEC 34-1-16. The motors with changing loads and subsequent sharp
which meet or exceed these standards are referred to changes in the motor speed, such as in
as IEC motors. machinery with energy storage flywheels, punch
The NEMA standards mainly specify four design types presses, shears, elevators, extractors, winches,
for AC induction motors – Design A, B, C and D. Their hoists, oil-well pumping, wire-drawing machines,
typical torque-speed curves are shown in Figure 18. etc. The speed regulation is poor, making the
design suitable only for punch presses, cranes,
elevators and oil well pumps. This motor type is
usually considered a “special order” item.
FIGURE 18: TORQUE-SPEED CURVES OF DIFFERENT NEMA STANDARD MOTORS
Design A
Torque (% of Full-Load Torque)
300 Design D
Design C
200
Design B
100
20 40 60 80 100
Speed (%)
 2003 Microchip Technology Inc. DS00887A-page 11