This document provides information about understanding AC/DC motors and generators theory. It outlines the objectives, safety requirements, risk level, and evaluation criteria for a block of instruction. The instruction will cover fundamentals of rotating machines, different types of DC motors and generators, induction motors, and motor theory concepts. Students will be evaluated on an examination and must score 80% or higher to pass. Lab exercises will use Labvolt trainers to cover the topics.
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ACTION: Understand AC/DC Motors and Generators Theory
CONDITIONS: Given a laboratory environment, Labvolt trainers,
regulatory guidance, and applicable personal protective equipment.
STANDARD: Understand AC/DC Motors and Generators Theory In
order to aid the electrical design process.
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• Safety Requirements: None
• Risk Assessment Level: Low
• Environmental Considerations: None
• Evaluation: Students will be evaluated on this
block of instruction during the Electrical Systems
and Design Examination 3. Students must
receive a score of 80 percent or above to receive
a GO.
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• Fundamentals of Rotating Machines (1-3)
• DC Motors and Generators
• Separately excited DC Motor (2-1)
• Series, shunt, and compound DC motors (2-2)
• Special Characteristics of DC Motors
• The Universal Motor (3-2)
• AC Induction Motors
• Squirrel-cage motors (4-1)
• Single-phase Induction Motors (4-4)
S3D03-5
Lesson Objectives
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Everyone should be familiar with some type of electric motor.
Typical examples include:
• Small DC motors in toys
• Fans
• Car starter motors
• Dryers
• Washers
• Well pumps
• HVAC fans/compressors
These common devices all operate using the same simple principal.
Motors use the reaction of two magnetic fields to create motion.
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This is a simple image of how two magnets can interact and create
motion. If magnet A is turned, magnet B will follow, and vice-versa.
• The reaction of two
magnetic fields
• Attraction results in motion
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Instead of just simply using 2 magnets, one magnet can be replaced
with a coil of wire wrapped around an iron core. The ends of the coil
are connected to a DC power source and results in current flowing
through the coil. This creates a north and south pole on the coil;
thus, creating an electromagnet.
https://www.youtube.com/watch?v=
mdZo_keUoEs
https://www.youtube.com/watch?v=
wzXRFp0DDrU
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Electromagnets still require some initial motion to start the spinning.
One way to overcome this is to enable some type of directional
current switching.
• Current switching changes
polarity, and will simulate
rotating the magnet
• The operating principle of
all motors is based on
producing the electrical
equivalent to a rotating
magnet
• This avoids manual rotation
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The Generator Principle
• Based on Faraday’s Law
• Voltage is induced between terminals of a wire loop if the magnetic fluxl
inking the loop varies as a function of time
• The value of the induced voltage is proportional to the rate of change of
magnetic flux.
The induced voltage is given by the following equation
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Work
A mechanical work is done whenever a force F moves through an
object over distance d, and work is defined by the following
equation:
Work is expressed in joules (J) when the force (F) is in Newtons (N)
and d is in meters (m).
𝑾 = 𝑭 ∗ 𝒅c
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Torque
Now consider that the block is moved over the same distance using
a pulley that has a radius r.
A twisting force must be applied to turn the pully so that the rope
pulls the block with a force F.
The twisting force is known as torque, and defined by the following
equation:
The torque T is expressed in Newton-meters (N∙m) when the force F
and the radius r are expressed in Newtons (N) and in meters (m),
respectively.
T= 𝑭 ∗ 𝒓c
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Torque
At the end of each complete rotation of the pulley, the block has
advanced a distance of 2(pi)r meters (inches), meaning that 2(pi)r*F
joules (pound force-inches) of work has been done. Since torque
equals F*r, the work may be expressed as 2(pi)T joules per
revolution.
T= 𝑭 ∗ 𝒓c
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Power
Power is the rate of doing work, and given by the following equation:
P=
𝑾
𝒕
c
𝑷 𝒊𝒔 𝒕𝒉𝒆 𝒑𝒐𝒘𝒆𝒓 𝒊𝒏 𝒘𝒂𝒕𝒕𝒔 𝑾
W is the work in joules (J)
t is time taken to do the work, in seconds (S)
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Power
Since power is work done per unit of time, the power of a motor
turning at speed n, can be found using the following equation when
torque is expressed in newton-meters
P= 𝟐 ∗ 𝒑𝒊 ∗ 𝑻 ∗ 𝒏 ∗
𝟏
𝟔𝟎
c
P=
𝒏∗𝒕
𝟗.𝟓𝟓
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Motor power, losses, and efficiency
• Torque was earlier defined as a twisting force that causes an
object to rotate.
• In electric motors, this twisting force comes from the interaction
of magnetic fields, and its value is related to the current flowing in
the motor.
• Since the magnetic forces in the rotor of a dc motor are produced
by current flowing in a wire loop, increasing the current will
increase the strength of the magnetic forces.
• The motor will therefore produce more torque, meaning increased
motor power, and it will consume more electric power.
• The prime mover that you have been using is actually a dc motor
and it converts electrical power to mechanical power.
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Motor power, losses, and efficiency
• Electric motor efficiency is expressed as the ratio of its
mechanical output power to its electrical input power, Pm/Pin.
• The mechanical output power of a motor depends on its speed
and torque, and can be determined using one of the following two
formulas, depending on whether torque is expressed in N∙m or
lbf∙in:
P 𝐦 =
𝒏∗𝒕
𝟗.𝟓𝟓
t in N*m P 𝐦 =
𝒏∗𝒕
𝟖𝟒.𝟓𝟏
𝒕 𝒊𝒏 𝒍𝒃𝒔 ∗ 𝒊𝒏
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Motor power, losses, and efficiency
• Efficiency for a motor is usually shown in the form of a graph of
efficiency versus mechanical output power, although a specific
value at the nominal power rating is sometimes given.
• Rotating machine losses come from bearing friction, brush
friction, as well as windage and cooling-fan friction.
• These losses vary somewhat as speed increases from zero to its
nominal value, but remain fairly constant over the normal
operating range between no-load and full-load
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Motor power, losses, and efficiency
• Electrical losses are classed as copper losses, brushes losses,
and iron losses.
• Copper losses (I*I*R) result from the resistance of the wire used in
the machine, are dissipated as heat, and depend on the value of
current in the machine.
• Brushes losses are usually very small, and are due to the contact
resistance of the brush which causes a typical voltage drop
between 0.8 V and 1.3 V.
• Finally, iron losses come from hysteresis and eddy currents in the
machine, and depend on the magnetic flux density, the speed of
rotation or frequency, the kind of steel and the size of the motor.
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• Motors turn due to an interaction between two magnetic fields
• The basic principle of a DC motor is the creation of a rotating magnet
inside of the mobile portion of the motor known as a ROTOR
• Commutator makes AC
needed for creation of
rotating magnet
• Contact to commutator
and external DC source
made through brushes
• Rotor of DC motor also
known as armature
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• The brushes make contact with
segments A and B of the
commutator and current flows in
wire loop A-B. No current flows in
the other wire loop (C-D). This
creates an electromagnet A-B with
north and south.
• As the rotor continues to rotate
clockwise, a time comes where a
commutation occurs, i.e., the
brushes make contact with
segments C and D instead of
segments A and B. As a result,
current now flows in wire loop C-D
instead of flowing in wire loop A-B.
This creates an electromagnet C-
D with north and south poles.
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• By comparing Figure 2-2b and
Figure 2-2c, you can see that the
magnetic north and south poles
rotate 90° counterclockwise at the
commutation.
• As the rotor continues to rotate
clockwise, the same phenomenon
repeats every 90° angle of
rotation, as shown in Figure 2-3a,
Figure 2-3b, and Figure 2-3c.
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• As the rotor turns, north and south
poles of the electromagnet
oscillate 90 degrees
• N and S poles are stationary, and
do not turn as the rotor turns
• Equivalent to having
electromagnet in the rotor that
rotates as the same speed as the
rotor, but in an opposite direction
• If rotor is placed next to a fixed
permanent magnet stator, opposite
polarity poles attract each other
• Direction of rotation depends on
polarity of applied voltage to
brushes of motor
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• Remember that a change in magnetic flux over a change in time can
induce a voltage on the ends of a coil wire……
• If a wire loop is placed between
two magnets and rotated,
magnetic lines of force are cut
and a voltage “e" is induced in
the loop.
• The polarity of the induced
voltage “e" depends on the
direction in which the wire loop
moves as it cuts the magnetic
lines of force.
• Since the wire loop cuts
magnetic lines of force in both
directions within a full
revolution, the induced voltage
is an ac voltage
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• If a commutator such as that shown in Figure 2-1 is used, it will act as a
rectifier and convert the induced ac voltage into a dc voltage (with
ripple), as shown in Figure 2-6. Direct current will therefore be
produced at the output of the generator. The faster the rotor turns, the
more lines of force that are cut and the higher the output voltage. Also,
the stronger the stator magnet, the more lines of force that are present,
and therefore, the higher the output voltage.
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• Many DC motors use an electromagnet for the stator, such as seen in
the figure below. When the power for the stator electromagnet is
supplied from an external source, it is referred to as separately excited.
• The current flowing in the stator is often referred to as field current.
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• Motor speed and armature voltage are a linear relationship.
• Induced voltage is proportional to motor speed.
• Because of this, DC motors are considered linear voltage-to-speed
converters.
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• Also considered linear current-to-torque converter
• When armature current increases, the
voltage drop Era across the armature
resistor increases, and cannot be
neglected.
• At that point, armature voltage is equal to
the sum of induced voltage and Era.
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Lab 2-1 Wrap-up
• The speed of the separately-excited dc motor is proportional to
armature voltage applied.
• The torque of a separately-excited dc motor is proportional to
armature current.
• The speed decreases with armature current increases if the
armature voltage is fixed.
• This is due to an increased voltage drop across the armature
resistor.
• Speed vs. voltage and current vs. torque relationships do not
change based off of reversed polarity.
• Also, the direction of rotation is reversed when polarity of
armature voltage is reversed.
• Complete 2-1 quiz on blackboard!!
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• It is possible to change the characteristics of a separately-excited dc
motor by changing the strength of the fixed magnetic field produced by
the stator electromagnet. This can be carried out by changing the
current that flows in the stator electromagnet. This current is usually
referred to as the field current because it is used to produce the fixed
magnetic field in the dc motor.
• A rheostat connected in series with the electromagnet winding can be
used to vary the field current.
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• Electromagnet is a series winding connected in series with armature.
• Speed decreases non-linearly as the torque increases
• Series motors provide strong starting torque
• Wide range of speeds
• Difficult to operate at constant speed as load fluctuates
• MUST NEVER run without load because of speed increase.
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• Electromagnet is a shunt winding connected in parallel with armature.
• Difficult to change the speed by changing armature voltage
• Changes in field current oppose changes to speed
• Only requires a single fixed voltage
• Speed doesn’t vary as load shifts
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• the characteristic of a cumulative
compound motor is a compromise of the
series and shunt motor characteristics.
• It provides the compound motor with a
fairly wide range of operating speed, but
the speed does not vary linearly as the
torque varies.
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Lab 2-2 Wrap-up• Decreasing field current below its nominal value increases k1 but
decreases K2.
• Allows motor to operate at higher speeds without exceeding
nominal armature voltage.
• Reduces torque without exceeding armature current.
• It is possible to increase field current above rated value for short
periods of time to increase starting torque.
• The speed vs. torque characteristic of a series motor is such that
the speed of a series motor decreases more rapidly than a
separately excited motor as torque increases.
• Series motor- n vs. T in non-linear
• Separately-excited- n vs. T is linear
• Shunt- similar to separately-excited
• Compound- a compromise of both
• Complete 2-2 quiz on blackboard!!
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• Remember that an Armature winding creates a rotating magnetic field
that rotates at the same speed as the motor but in the opposite
direction.
• The poles of the rotor electromagnet remain fixed at 90 degrees to the
stator electromagnet.
• If the polarity of the stator electromagnet or rotor is reversed, the
direction of rotation reverses. Remember Flemings Rule!
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• When the field and armature electromagnet of a DC motor are powered
by the same source, like series and shunt motors, reversing polarity
reverses direction of currents .
• Shunt and series DC motors WILL rotate when connected to an AC
source because both armature and field currents flip-flop together, and
directional torque remains the same.
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• Due to higher impedance in windings due to alternating current, a shunt
winding motor will not provide desired results.
• The shunt winding has a larger number of turns in the coil, and
produces higher impedances.
• Conversely, a series motor winding has very few windings and has a
lower impedance with AC.
• However, when a COMPENSATING winding is added to a series motor
winding, you can achieve greater results.
• These results can be seen using a universal motor with series and
compensating windings.
• Let’s watch some videos!!
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Lab 3-2 Wrap-up
• DC series motors and universal motors without compensating
windings have similar operations with both AC and DC sources.
• The direction of rotation of these types depends on polarity of
armature and field currents.
• These specific types of motors operate poorly on AC sources
because of higher impedance.
• Universal motors operate much better when a compensating
winding is added to reduce overall impedance.
• Complete 3-2 quiz on blackboard!!
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• Voltage is induced between 2 ends of a wire loop when magnetic flux
varies as a function of time.
• If the ends of the loop are shorted together, current flows in the closed
loop.
• As currents flow through the loops of the conductor ladder, they create
magnetic fields.
• When the magnetic fields interact, a force moves the closed ladder in
the direction of the moving magnet.
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• Because of the pulling force of the magnetic field on the ladder, the
ladder must always move at a slower speed than the moving magnet.
• The greater the difference in speed, the larger variation of flux; thus,
greater magnetic force acting on the ladder.
• When the ladder of an asynchronous induction motor is closed like the
image below, it looks like a squirrel cage; thus, the name squirrel cage
motor.
• It sounded better than hamster wheel,
So smart folks just went with it.
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• The stator of the squirrel-cage motor acts as a rotating electromagnet.
• If it is started with no load, it turns almost the same speed as the
rotating magnetic field (synchronous speed).
• Changing the phase sequence changes direction of rotation.
• Motor line currents increases as mechanical load increases.
• This means squirrel cage motors require more power for heavier
loads.
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• No-load speed is just less than synchronous speed.
• As torque increases, speed decreases.
• Full load torque corresponds to full load speed.
• Too much torque creates instability (breakdown torque)
• When torque is zero (locked rotor torque), the current is very high and
the amount of power consumed is higher than normal operations.
• Squirrel-cage motors draw reactive power from the AC source.
• Under no-load conditions, reactive power is higher than active power.
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• A Three phase squirrel cage induction motor starts and runs close to
normal if only using 2 phases.
• With only one phase connected, there is no rotating magnetic field to
pull the motor and start motion.
• In order to start a single phase induction motor, you must add an
auxiliary winding with a capacitor.
• The result is two currents that are 90 degrees out of phase from one
another, and allowed a rotating magnetic field to get it started.
• These motors typically use a centrifugal switch to isolate the auxiliary
windings once the motor is at a significant speed.
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• In order to start a single phase induction motor, you must add an
auxiliary winding with a capacitor.
• The result is two currents that are 90 degrees out of phase from one
another, and allowed a rotating magnetic field to get it started.
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Lab 4-4 Wrap-up
• A Three phase squirrel cage induction motor starts and runs close to
normal if only using 2 phases.
• With only one phase connected, there is no rotating magnetic field to
pull the motor and start motion.
• In order to start a single phase induction motor, you must add an
auxiliary winding with a capacitor.
• The result is two currents that are 90 degrees out of phase from one
another, and allowed a rotating magnetic field to get it started.
• These motors typically use a centrifugal switch to isolate the auxiliary
windings once the motor is at a significant speed.
• Complete 4-4 quiz on blackboard!!
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• Fundamentals of Rotating Machines (1-3)
• DC Motors and Generators
• Separately excited DC Motor (2-1)
• Series, shunt, and compound DC motors (2-2)
• Special Characteristics of DC Motors
• The Universal Motor (3-2)
• AC Induction Motors
• Squirrel-cage motors (4-1)
• Single-phase Induction Motors (4-4)
S3D03-57
Summary
