left: current carrying wire F=BIL pair of force produces torque - spins the rotor
right: electromagnet with metal core wrapped by wire coils coil creates N and S poles - becomes attracted to S and N poles on stator, respectively the idea, is then how to create a dynamically changing magnetic flux to keep the rotor spinning constantly faraday&apos;s law concerning generators: generated emf = rate of change of magnetic flux
2 pole DC electric motor Direct Current a better picture of rotation/commutation next slide
important to note that with this simple 2 pole motor, when rotor rotates 90 degrees from this picture, there will be 0 torque. Unable to start from rest at that 90deg position in practice, a real DC motor use more than 2 poles to eliminate - zero torque zone, and shorting of battery
mechanical brushes could be metallic or carbon
under no load conditions, motor will rotate at a speed such that the back emf equals the applied voltage plus voltage drop across armature
generally highest torque at zero speed, zero torque at max speed increase current to increase torque increase voltage to increase speed
shunt wound, series wound DC motors: Here, the stator is an electromagnet instead of permanent magnet.
shunt has stator and armature connected in parallel. series has stators and armature connected in series. Has different loading characteristics
series wound DC is also known as universal motor and can run on both AC and DC because both stator and rotor polarity can be switched
Brushed DC motor - &apos;conventional&apos;/&apos;inrunner&apos; configuration: flipped inside out - stator is now coil, rotor is permanent magnet that spins on the inside typically less torque, but high RPM &apos;outrunner&apos; configuration - rotor spins on the outside around stator. typically high torque but lower RPM
Energize the stator electromagnet coils sequentially (very much like a stepper motor) to make the rotor rotate
How to know when to energize coils? cannot do this in open loop like stepper due to smaller number of poles on stator; needs feedback
2 ways to sense rotor position: -hall effect sensor (detects magnetic fields) -sensorless (back emf on the un-energized coils) -photo transistors (encoders, lab3 slot and detector)
left diagram (delta): sequentially energize each of the 3 leads to make rotor turn if more poles/windings on stator, typically still arranged into 3 groups - hence still 3 leads
delta, wye in AC transformers - neutral wires - phase to neutral voltages available for wye. only phase to phase voltage available for delta
DC Motors (Brushed and Brushless)
Electric Motor Basic Principles
Interaction between magnetic field and current
carrying wire produces a force
Opposite of a generator
Conventional (Brushed) DC Motors
for outer stator
Rotating coils for inner
performed with metal
contact brushes and
contacts designed to
reverse the polarity of
the rotor as it reaches
2 pole brushed DC motor commutation
Conventional (Brushed) DC Motors
Small/cheap devices such as toys, electric tooth
brushes, small drills
Easy to control - speed is governed by the voltage and
torque by the current through the armature
Mechanical brushes - electrical noise, arcing, sparking,
friction, wear, inefficient, shorting
DC Motor considerations
Back EMF - every motor is also a generator
More current = more torque; more voltage = more speed
Load, torque, speed characteristics
Shunt-wound, series-wound (aka universal motor), compound
Brushless DC Motors
Essential difference - commutation is performed
electronically with controller rather than
mechanically with brushes
Brushless DC Motor Commutation
Commutation is performed electronically using a
controller (e.g. HCS12 or logic circuit)
Similarity with stepper motor, but with less #
Needs rotor positional closed loop feedback: hall
effect sensors, back EMF, photo transistors
BLDC (3-Pole) Motor Connections
Has 3 leads instead of 2 like brushed DC
Delta (greater speed) and Wye (greater torque)
Brushless DC Motors
CPU cooling fans
Pros (compared to brushed DC)
Longer lifespan, low maintenance
Clean, fast, no sparking/issues with brushed contacts
More complex circuitry and requires a controller
A stepper motor is an electromechanical device
which converts electrical pulses into discrete
mechanical movements. The shaft or spindle of a
stepper motor rotates in discrete step increments
when electrical command pulses are applied to it in
the proper sequence.
The sequence of the applied pulses is directly
related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly
related to the frequency of the input pulses.
The length of rotation is directly related to the
number of input pulses applied.
Stepper Motor Characteristics
The motors response to digital input pulses provides open-loop
control, making the motor simpler and less costly to control.
Very reliable since there are no contact brushes in the motor.
Therefore the life of the motor is simply dependant on the life of
The rotation angle of the motor is proportional to the input pulse.
Speed increases -> torque decreases
Torque vs. Speed
Torque varies inversely with
Current is proportional to
Torque ∞ means Current ∞,→ →
which leads to motor damage.
Torque thus needs to be limited
to rated value of motor.
Disadvantages of stepper motors
There are two main disadvantages of stepper motors:
Resonance can occur if not properly controlled.
This can be seen as a sudden loss or drop in torque at certain
speeds which can result in missed steps or loss of synchronism. It
occurs when the input step pulse rate coincides with the natural
oscillation frequency of the rotor. Resonance can be minimised by
using half stepping or microstepping.
Not easy to operate at extremely high speeds.
Stepper motors consist of a permanent magnet
rotating shaft, called the rotor, and electromagnets
on the stationary portion that surrounds the motor,
called the stator.
When a phase winding of a stepper
motor is energized with current, a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
At position 1, the rotor is
beginning at the upper
electromagnet, which is
currently active (has voltage
applied to it).
To move the rotor clockwise
(CW), the upper
electromagnet is deactivated
and the right electromagnet is
activated, causing the rotor to
move 90 degrees CW, aligning
itself with the active magnet.
This process is repeated in the
same manner at the south and
west electromagnets until we
once again reach the starting
Resolution is the number of degrees rotated per step.
Step angle = 360/(NPh * Ph) = 360/N
NPh = Number of equivalent poles per phase =
number of rotor poles.
Ph = Number of phases.
N = Total number of poles for all phases together.
Example: for a three winding motor with a rotor
having 4 teeth, the resolution is 30 degrees.
Two phase stepper motors
There are two basic winding
arrangements for the electromagnetic
coils in a two phase stepper motor:
bipolar and unipolar.
A unipolar stepper motor has two windings
per phase, one for each direction of
magnetic field. In this arrangement a
magnetic pole can be reversed without
switching the direction of current.
Bipolar motors have a single winding per
phase. The current in a winding needs to be
reversed in order to reverse a magnetic
Bipolar motors have higher torque but need
more complex driver circuits.
Wave Drive (1 phase on)
A1 – B2 – A2 – B1
(25% of unipolar windings , 50% of bipolar)
Full Step Drive (2 phases on)
A1B2 – B2A2 – A2B1 – B1A1
(50% of unipolar windings , full bipolar
Half Step Drive (1 & 2 phases on)
A1B2 – B2 – B2A2 – A2 ----
varying motor currents)
A microstep driver may split a full step into as many as 256 microsteps.
Types of Stepper Motors
There are three main types of stepper motors:
Variable Reluctance stepper motor
Permanent Magnet stepper motor
Hybrid Synchronous stepper motor
This type of motor consists of a soft iron multi-toothed
rotor and a wound stator.
When the stator windings are energized
with DC Current, the poles become magnetized.
Rotation occurs when the rotor teeth
are attracted to the energized stator
Variable Reluctance motor
Permanent Magnet motor
The rotor no longer has teeth as with
the VR motor.
Instead the rotor is
magnetized with alternating north
and south poles situated in a straight
line parallel to the rotor shaft.
These magnetized rotor poles provide an increased
magnetic flux intensity and because of this
the PM motor exhibits improved torque characteristics
when compared with the VR type.
Hybrid Synchronous motor
The rotor is multi-toothed like the VR motor and
contains an axially magnetized concentric
magnet around its shaft.
The teeth on the rotor provide an even
better path which helps guide the
magnetic flux to preferred locations in
the air gap.
Stepper motors can be a good choice whenever
controlled movement is required.
They can be used to advantage in applications
where you need to control rotation angle, speed,
position and synchronism.
automotive and scientific equipment etc.
3phase induction motor-Not a variable speed motor
1 Phase induction motor-Not self starting, poor power factor,
Common single phase commutator motors are
4.Repulsion –induction motors
If we connect normal dc series motor to ac what happens?
1. Torque developed is not constant Magnitude
2. Alternating flux induce eddy currents causing heat and there by
3. No inductive coupling between armature and field since they
are placed in quadrature
4. Sparking in brushes is more due to transformer emf induced
5. Due to large voltage drop speed reduced
6. Starting Torque is low, low pf
Modification needed for ac series motor.
1. To reduce eddy current loss-Laminations used
2. To reduce reactance-series field should contains less number of
3. To improve torque no. of armature conductors should be large
4. To reduce reactance compensating winding should be used
5. Operating voltage kept low to reduce inductance
6. Reduce frequency to reduce inductance
7. Interpoles to reduce armature resistance leads
Changes should be employed to work in both ac and dc
When motor is connected to an a.c. supply, the same alternating
through the field and armature windings.
The field winding produces an alternating flux that reacts with the
current flowing in the armature to produce a torque.
Since both armature current and flux reverse simultaneously, the
torque always acts in the same direction.
It may be noted that no rotating flux is produced in this type of
machines; the principle of operation is the same as that of a d.c. series
The operating characteristics of an a.c. series motor are similar to those
of a d.c. series motor.
(i) The speed increases to a high value with a decrease in load. In very
small series motors, the losses are usually large enough at no load that
limit the speed to a definite value (1500 - 15,000 r.p.m.).
(ii) The motor torque is high for large armature currents, thus giving a
(iii) At full-load, the power factor is about 90%. However, at starting or
carrying an overload, the power factor is lower
The fractional horsepower a.c. series motors have high-speed (and
corresponding small size) and large starting torque. They can,
therefore, be used
(a) high-speed vacuum cleaners (b) sewing machines
(c) electric shavers (d) drills
(e) machine tools etc.
A repulsion motor is similar to an a.c. series motor except that:
(i) brushes are not connected to supply but are short-circuited -
currents are induced in the armature conductors by transformer
(ii) the field structure has non-salient pole construction.
By adjusting the position of short-circuited brushes on the
commutator, the starting torque can be developed in the motor
The field of stator winding is wound like the main winding of a split-phase
motor and is connected directly to a single-phase source.
The armature or rotor is similar to a d.c. motor armature with drum type
winding connected to a commutator
However, the brushes are not connected to supply but are connected to
each other or short-circuited.
Short-circuiting the brushes effectively makes the rotor into a type of
The major difficulty with an ordinary single-phase induction motor is the
low starting torque.
It has also better power factor than the conventional single-phase motor.
The total armature torque in a repulsion motor can be shown to be
Ta = sin 2α
where α = angle between brush axis and stator field axis
For maximum torque, 2α = 90° or α = 45°
Thus adjusting α to 45° at starting, maximum torque can be obtained
during the starting period. However, α has to be adjusted to give a
suitable running speed.
(i) The repulsion motor has characteristics very similar to those of an
a.c. series motor i.e., it has a high starting torque and a high speed at
(ii) The speed which the repulsion motor develops for any given load
will depend upon the position of the brushes.
(iii) In comparison with other single-phase motors, the repulsion
motor has a high starring torque and relatively low starting current.
Sometimes the action of a repulsion motor is combined with that of a
single phase induction motor to produce repulsion-start induction-run
motor (also called repulsion-start motor).
The machine is started as a repulsion motor with a corresponding high
At some predetermined speed, a centrifugal device short-circuits the
commutator so that the machine then operates as a single-phase
This motor has the same general construction of a repulsion motor.
The only difference is that it is equipped with a centrifugal device fitted
on the armature shaft.
When the motor reaches 75% of its full pinning speed, the centrifugal
device forces a short-circuiting ring to come in contact with the inner
surface of the commutator.
This short-circuits all the commutator bars.
The rotor then resembles squirrel-cage type and the motor runs as a
single-phase induction motor.
At the same time, the centrifugal device raises the brushes from the
commutator which reduces the wear of the brushes and commutator as
well as makes the operation quiet.
(i) The starting torque is 2.5 to 4.5 times the full-load torque and the
current is 3.75 times the full-load value.
(ii) Due to their high starting torque, repulsion-motors were used to
devices such as refrigerators, pumps, compressors etc.
However, they posed a serious problem of maintenance of brushes,
arid the centrifugal device.
The repulsion-induction motor produces a high starting torque entirely
due to repulsion motor action. When running, it functions through a
combination of induction-motor and repulsion motor action.
It consists of a stator and a rotor (or armature).
(i) The stator carries a single distributed winding fed from single-phase supply.
(ii) The rotor is provided with two independent windings placed one inside the
The inner winding is a squirrel-cage winding with rotor bars
permanently short-circuited. Placed over the squirrel cage winding is a
repulsion commutator armature winding.
The repulsion winding is connected to a commutator on which ride short-
circuited brushes. There is no centrifugal device and the repulsion winding
functions at all times.
(i) When single-phase supply is given to the stator winding, the repulsion
winding (i.e., outer winding) is active. Consequently, the motor starts as a
repulsion motor with a corresponding high starting torque.
(ii) As the motor speed increases, the current shifts from the outer to
winding due to the decreasing impedance of the inner winding with
increasing speed. Consequently, at running speed, the squirrel cage
winding carries the greater part of rotor current.
This shifting of repulsion motor action to induction-motor action is thus
achieved without any switching arrangement.
(i) The no-load speed of a repulsion-induction motor is somewhat above
synchronous speed because of the effect of repulsion winding. However,
the speed at full-load is slightly less than the synchronous speed as in an
(ii) The speed regulation of the motor is about 6%.
(iii) The starting torque is 2.25 to 3 times the full-load torque; the lower
being for large motors. The starting current is 3 to 4 times the full-load
This type of motor is used for applications requiring a high starting
Very small single-phase motors have been developed which run at true
synchronous speed. They do not require d.c. excitation for the rotor.
Because of these characteristics, they are called unexcited single-phase
The most commonly used types are:
(i) Reluctance motors (ii) Hysteresis motors
The efficiency and torque-developing ability of these motors is low; The
output of most of the commercial motors is only a few watts
It is a single-phase synchronous motor which does not require d.c.
excitation to the rotor.
Its operation is based upon the following principle:
Whenever a piece of ferromagnetic material is located in a magnetic field;
a force is exerted on the material, tending to align the material so that
reluctance of the magnetic path that passes through the material is
(i) a stator carrying a single-phase winding along with an auxiliary
winding to produce a synchronous-revolving magnetic field.
(ii) a squirrel-cage rotor having unsymmetrical magnetic construction.
This is achieved by symmetrically removing some of the teeth from the
squirrel cage rotor to produce salient poles on the rotor.
The salient poles created on the rotor must be equal to the poles on the
Note that rotor salient poles offer low reluctance to the stator flux and,
therefore, become strongly magnetized.
(i) When single-phase stator having an auxiliary winding is energized, a
synchronously-revolving field is produced. The motor starts as a standard
squirrel-cage induction motor and will accelerate to near its synchronous
(ii) As the rotor approaches synchronous speed, the rotating stator flux
exert reluctance torque on the rotor poles tending to align the salient-
axis with the axis of the rotating field. The rotor assumes a position where
its salient poles lock with the poles of the revolving field
(ii)) Consequently, the motor will continue to run at the speed of
revolving flux i.e., at the synchronous speed.
(iii) When we apply a mechanical load, the rotor poles fall slightly behind
stator poles, while continuing to turn at synchronous speed.
As the load on the motor is increased, the mechanical angle between the
poles increases progressively.
Nevertheless, magnetic attraction keeps the rotor locked to the rotating
flux. If the load is increased beyond the amount under which the
reluctance torque can maintain synchronous speed
(i) These motors have poor torque, power factor and efficiency.
(ii) These motors cannot accelerate high-inertia loads to synchronous
(iii)The pull-in and pull-out torques of such motors are weak.
Despite the above drawbacks, the reluctance motor is cheaper than any
type of synchronous motor. They are widely used for constant-speed
applications such as timing devices, signaling devices etc
It is a single-phase motor whose operation depends upon the hysteresis
effect i.e., magnetization produced in a ferromagnetic material lags
behind the magnetizing force.
(i) a stator designed to produce a synchronously-revolving field from a
single-phase supply. This is accomplished by using permanent-split
capacitor type construction. Consequently, both the windings (i.e.,
as well as main winding) remain connected in the circuit during running
operation as well as at starting. The value of capacitance is so adjusted as
result in a flux revolving at synchronous speed.
(ii) a rotor consisting of a smooth cylinder of magnetically hard steel,
without winding or teeth.
(i) When the stator is energized from a single-phase supply, a
synchronously revolving field (assumed in anti-clockwise direction) is
produced due to split-phase operation.
(ii) The revolving stator flux magnetizes the rotor. Due to hysteresis
effect, the axis of magnetization of rotor will lag behind the axis of stator
hysteresis lag angle ,the rotor and stator poles are locked. If the rotor is
stationary, the starting torque produced is given by:
From now onwards, the rotor accelerates to synchronous speed with a
(iii) After reaching synchronism, the motor continues to run at
synchronous speed and adjusts its torque angle so as to develop the
torque required by the load.
(i) A hysteresis motor can synchronize any load which it can accelerate,
matter how great the inertia. It is because the torque is uniform from
standstill to synchronous speed.
(ii) Since the rotor has no teeth or salient poles or winding, a hysteresis
is inherently quiet and produces smooth rotation of the load.
(iii) The rotor takes on the same number of poles as the stator field. Thus
changing the number of stator poles through pole-changing connections,
we can get a set of synchronous speeds for the motor.
Due to their quiet operation and ability to drive high-inertia toads,
motors are particularly well suited for driving
(iv)from-tables and other precision audio-equipment.