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Advanced Diploma of Mechanical
Engineering Technology
(DME - 52884WA)
DMEEMD605: AC Electrical Motors and Drives
Webinar 1
Fundamental Principles of AC Motors

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Fundamentals of Electric Motors:
• Production of torque in electric motors
• Construction and control of ac motors.
Motor Selection, Troubleshooting and Speed Control:
• The winding connected to the ac supply creates the magnetic field. The
other is simply a shorted winding.
• The current induced in the shorted winding interacts with the magnetic
field and sets up a torque.
Module Structure

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Fundamental Principles of AC Motors

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How torque is produced in an electrical motor
• Examine and discuss the basic relationship between magnetism, electric
current and force.
• Explain motor parameters such as torque, inertia, efficiency, and
power.
• Explain how an electric motor produces torque.
• List the different types of motors and explain their suitability in
different applications.
Torque in an Electrical Motor

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• Electricity is obtained by conversion from other forms of energy.
• Example: A generator driven by a prime mover (engine, steam turbine,
gas turbine, hydro-turbine).
Energy Conversion

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• Motors convert electrical energy obtained from a power system to
mechanical energy in the form of motion (kinetic energy)
• Motors account for more than 60% of electrical power used today.
Motors
Motor generator sets can be a cost effective method of providing
“clean” power to sensitive equipment

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Rotational movement:
• Very common
• Mechanical equipment often uses rotational movement to perform
work e.g. pumps, compressors, crushers.
Linear movement:
• Can be obtained by mechanical conversion of rotating motion
– Ex: Actuators using electric motors, hydraulic/pneumatic systems and
wheel drives
• Can be obtained directly by a linear electrical drive: Certain types of
monorail electric trains.
Types of Mechanical Output

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• Motors of rotary type will be discussed here.
• Such motors give their mechanical output in the form of a torque.
• Some energy is naturally lost in the process of conversion (electrical
losses) and after the conversion (mechanical losses).
The energy relationships can be expressed as:
• Mechanical Output = Electrical Input – Electrical Losses
• Useful Mechanical Output = Motor Output – Mechanical Losses.
Motors

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Motor Efficiency

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Motor Efficiency
The rated output of the motor is 1,5 kW, i.e. the
motor is able to supply a shaft output of at least 1,5
kW if connected to the mains supply as indicated.

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Torque and Power
• Torque = Force x moment arm
• Energy = Force x distance travelled
• In the case of a rotating equipment, distance of travel is 2π x radius x
no. of revolutions
• Energy/Work = Force x 2 π x radius x no. of revolutions
• Power = Force x 2 π x radius x no. of revolutions/ second
• The radius here is the moment arm
• So Power = Torque x 2 π x RPS = Torque x 2 π x (RPM/60)
• When torque unit is Newton meter, power is in watts
• The output power of motors is expressed in kW (1000 Watts)
Torque and Power

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• Previous slide: Power = Torque x 2 π x RPS = Torque x 2 π x (RPM/60)
• Power (kW) = Torque*2π*(RPM/60x 1000) = (2*π*RPM*Torque)/60000
• If a motor produces a shaft output torque of 50 N.m when rotating at
1470 RPM, what is its output power in kW?
P=2*π*N*T/60000 =7.7kW
• If the efficiency is 90% what is the input power?
• Input power = 7.7/0.9 = 8.55 kW
Power - Torque Equation

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• The torque produced by a motor 𝑇𝑀 is used towards supplying:
– Torque required by the load 𝑇𝐿.
– Torque required to overcome friction 𝑇𝐹.
– Torque required to accelerate the load 𝑇𝐴.
• When 𝑇𝑀 = 𝑇𝐿 + 𝑇𝐹 the motor speed is constant.
• When 𝑇𝑀 > 𝑇𝐿 + 𝑇𝐹 then the excess torque (𝑇𝐴) available is used to
accelerate the motor.
• The relationship is 𝑇𝐴 = M x Angular Acceleration where M is the
moment of inertia of the rotating components.
• When 𝑇𝑀 < 𝑇𝐿 + 𝑇𝐹 then the machine will decelerate.
Torque, Acceleration and Inertia

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Acceleration Torque

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• Moment of inertia is the measure of an object's resistance to changes
in its rotation rate.
• When an object is stationary, the moment of inertia is 0.
• The units of inertia are commonly used in two ways, o oz-in2 and oz-in-
sec2. The former includes gravity, the latter only mass.
• Motor inertia is given by the manufacturer, while the load inertia is
calculated by adding the inertia of all rotating parts, which typically
includes the actuator or drive (belt, ball screw, rack and pinion), the
external load, and the coupling.
• JL = inertia of load reflected to motor
JD = inertia of actuator or drive (ball screw, belt, rack & pinion)
JE = inertia of external (moved) load
JC = inertia of coupling
Moment of Inertia

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A motor has two main components.
1. A stator which is fixed and carries some form of electrical windings.
2. A rotor which rotates and also carries some form of electrical windings.
• The conductors on the rotor develop a force and thus create a torque
around the rotor axis.
• The rotor is rigidly fixed to a shaft which transmits the torque output to
a mechanical device (load).
How is Torque produced in a Motor?

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• When a conductor carrying current is placed in a magnetic field with a
particular orientation, a force is exerted on the conductor.
• The force depends on the current, magnetic field strength and the
length of the conductor.
• The physical relationships of direction of current , magnetic field and
force is given in the next slide.
• Magnetic field direction is conventionally from a North pole to a South
pole.
Force on a Conductor

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Fleming’s Left Hand Rule
The force depends on:
the current,
magnetic field strength and
the length of the conductor

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F = B x I x L
Where:
• F is the force in Newton
• B is the magnetic flux density in Tesla
• I is the current in amperes
• And L is the conductor length in metres.
What is the force produced on a conductor of 0.6m length carrying 100A
in a field of 0.05 Tesla?
Ans: F = B x I x L = 0.05T x 100A x 0.6m = 3Nm
Force on a Conductor in a Magnetic Field

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• Electric motors make use of a magnetic field to produce torque.
• In very small motors the field is obtained using permanent magnets.
• Larger machines use electromagnetism to generate a magnetic field
also refer to as a electromagnet.
• Depending on the design, the magnetic field can be produced by the
rotor winding OR the stator winding.
– DC motor – stator or field winding
• The other winding should have a current flowing through it to generate
torque.
Electric Motors

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• Motors are classified into different types depending on the type of
electrical supply:
– DC motor and AC motor.
• AC motors can be of single or 3-phase types.
• Single phase motors are of lower capacity compared to three phase
motors.
• Single phase motors can be of two types:
1. Commutator or UNIVERSAL motors.
2. SPLIT PHASE motors.
• 3-phase motors are of SYNCHRONOUS type or INDUCTION type.
Types of Motors

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• In an AC system current reverses 50 times a second (50 Hz supply).
• Producing unidirectional torque poses challenges and Universal motors
are limited in rating/application.
This is solved using more than one coil to create the required magnetic
field such as:
• Split winding in single phase machines
• 3 coils distributed in space and supplied from a three-phase ac system
• Both arrangements create a magnetic field which rotates in space.
AC motors on Single Phase Supply

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• The stationery part has windings which create the required magnetic
field when fed from a single phase AC or DC supply.
• The winding on the rotating part carries a current which is connected to
the same supply source through a commutator which does not exist in
other AC motors.
• The interaction produces a torque.
• With single phase AC both field direction and armature current reverse
together the machine produces torque in the same direction.
• Efficiency is not high.
• FHP applications sometimes use universal motors as they are self
starting unlike a conventional single phase motor.
Universal Motor

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• Domestic appliances (blenders, vacuum cleaners, hair dryers etc.)
• Portable power tools (grinders, drills, sanders, jig saws etc.)
• IC Starter motors (DC)
• Rail Traction (historic).
Universal Motor - Applications

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• A universal motor is a single-phase series motor, which is able to run on
either alternating current (ac) or direct current (dc) and the
characteristics are similar for both ac and dc. The field windings of a
series motors are connected in series with the armature windings
Universal Motor - Operation

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• A single phase induction motor can be considered as a stator with a
single coil.
• When energised from an ac source, it produces a magnetic field which
pulsates but does not rotate.
• Thus it does not produce a torque on the rotor and need a special
arrangement to start.
Single Phase Induction Motor

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• The single coil of a single phase induction motor does not produce a
rotating magnetic field, but a pulsating field.
• Single-phase induction motors are not self-starting without an auxiliary
stator winding driven by an out of phase current of near 90°. Once
started the auxiliary winding is optional.
• An alternative is the shaded-pole motor which has only one main
winding and no start winding.
• A continuous copper loop around a small portion of the motor pole
“shades” that portion of the pole, causing the magnetic field in the
ringed area to lag the field in the un-ringed portion and starts the shaft
rotating.
Starting Single Phase Induction Motors

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Starting Single Phase Induction Motors

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Single Phase Split Configurations
Split Phase Motor

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Single Phase Split Configurations
Split Phase Induction Motor Capacitor Start Induction Motor

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• Single phase motors are used where three-phase power is unavailable
or impractical and are used for lower power general duty applications
up around 15kW eg. machine-tools (drills, lathes, mills); smaller fans
and blowers; material handling (pumps, screw conveyors, short belt
conveyors, etc.).
• Starting torque is low and they are not suitable for high-inertia
applications that require extended starting time. (Large fans,
centrifuges, long conveyor belts, etc.)
• They are also not suitable for variable speed operation.
• Efficiency is lower than three-phase motors.
• Have all the low maintenance advantages of induction motors.
Single Phase IM Applications

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Synchronous motor:
• A fixed direction magnetic field created using a dc supply interacts with
the winding supplied from ac mains.
Induction motor:
• The winding connected to the ac supply creates the magnetic field. The
other is simply a shorted winding.
• The current induced in the shorted winding interacts with the magnetic
field and sets up a torque.
3-phase AC Motor Types

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• Consider 3 pairs of terminals each with an ac supply of the same
voltage magnitude (A-B-C).
• But the AC waveforms are shifted in time - 120°.
• The shift between A and B is 1/3rd of the cycle time period; and
between B and C.
• It means that A (Phase 1) attains maximum value first followed by B
followed by C (Phase sequence A-B-C).
• Reverse sequence is A-C-B.
3-phase AC Supply

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3 phase Waveform R-Y-B
• U =V stands for voltage.
• The phase separation is shown in degrees instead of time (1 cycle
period=360 degrees. This applies to any value of frequency. For 50 Hz
the cycle time is 20 m. sec)

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• The AC motor stator has 3 windings connected to phases A, B and C.
• Each winding is located physically at equal angles within the stator inner
bore (120 spatial degrees).
• When supply is given to the windings they set up a magnetic field which
rotates in space at the speed of ac supply frequency (50 RPS).
• This is the 2-pole arrangement.
Rotating Magnetic Field

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AC mains supply is always given to the winding on the fixed part (called
stator):
• True for both synchronous and induction types.
The rotating part (rotor) carries either:
• The shorted winding in the case of induction motors or;
• The dc field winding in the case of synchronous machines.
• This winding is connected through a pair of slip rings so that the current to
the winding is always in the same direction in spite of the rotation.
3-phase Motor-Stator and Rotor

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• The stator winding sets up a rotating magnetic field.
• The rotating field induces current in the shorted rotor winding.
• The current interacts with the stator field and generates a torque.
• The rotor turns in a direction that will reduce the induced current (that
is along the direction of the rotating field).
3-phase Induction Motor

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Three Phase IM

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Operate from a three phase AC supply and are extensively used for various
industrial applications from sizes of kWs up many MW (workhorse of
industry) to due to following characteristics:
• Very simple and rugged construction
• Very reliable and low cost
• High efficiency and good power factor at full load
• Require minimum maintenance required
• Self starting, reversible and can operate at fixed speed or variable speed
(from a VSD).
Three Phase IM Applications

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• The rotation speed of the shaft is synchronized with the frequency of
the supply current.
• Shaft either has a permanent magnet (small motors) or a DC
electromagnet.
• The stator is connected to AC and this rotating magnetic field interacts
with the stator magnetic field producing “synchronous” operation.
Synchronous Motors

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• Magnetic poles on the rotor.
• The rotating field of the stator and the field of the rotor interact.
• The rotor must rotate at the speed of the rotating field of the stator to
remain in synchronism.
• The torque produced is f1 x f2 x sin a where f1 and f2 and are the
stator and rotor magnetic fields and a is the angle between the two
fields.
• As the torque increases the rotor position Wrt the stator falls back a
little although it rotates at the same speed.
• This type of motor can produce a torque only when rotating at its
synchronous speed.
3-phase Synchronous Motor

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• When supply to a synchronous motor is switched on to both stator and
rotor, the stator field rotates at synchronous speed but rotor is at rest
• That means the average torque over a cycle is 0 (sum of instantaneous
values of torque over a cycle)
• Thus a synchronous motor does not start by itself.
• If the rotor is brought to synchronous speed first and then the field
supply is switched, the rotor will align itself with the rotating field of the
stator
• This action is called synchronising.
3-phase Synchronous Motor at Rest

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• Synchronous motors use a cage winding (damper winding)on the rotor
in addition to the dc field winding .
• The induction action causes initial rotation.
• Dc is supplied to rotor near the synchronous speed.
• Then the rotor poles align with the magnetic field of the stator.
• This is achieved through automatic schemes provided in the field supply
(excitation) equipment.
• Alternatively ‘Pony’ motors coupled to the rotor shaft can be used for
bringing the rotor to rated speed.
Starting a Synchronous Motor

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• SM are rarely used below 40kW output due to their higher cost
compared with IM. SM also need DC excitation source and starting and
control devices which add cost.
• For applications that involve high kW output and low speed the SM has
advantages over the IM.
Applications for SM are classified as:
• Power factor correction (with or without mechanical load)
• Voltage regulation (applications at the end of long transmission lines)
• Constant speed constant load drives (fans, blowers, dc generators, line
shafts, centrifugal pumps, compressors, reciprocating pumps, rubber
and paper mills).
SM Applications

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Examine the construction and control of AC motors
• Identify the important components of a motor.
• Examine the relation between supply frequency, number of poles and
speed of an AC motor.
• Explain the reversal of direction of a 3-phase AC motor and the basic
principle.
• Examine the efficiency of an AC motor and the types of losses involved.
AC motors Construction and Control

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AC Motor Components

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• In some cases, the rotor conductors are of die-cast aluminium with a
shorting ring.
• This is the squirrel cage rotor.
• Squirrel cage induction motor is the most common motor around the
world.
• An alternative design uses windings shorted outside the rotor (using
slip rings).
• This type is the wound rotor induction motor.
Rotor Winding

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Squirrel Cage Rotor

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Wound Rotor IM

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In its simplest form:
• 3-phase Stator windings connected to power supply
• Flux completes one rotation for every cycle of mains
• On 50Hz, the stator flux rotates at 50 revs per second
• Rotor turns at 50 x 60 = 3,000 revs per minute.
• Called a 2 pole motor (2 poles 1-North, 1-South).
The design of the Stator windings can be changed to be suitable for 4-pole
operation:
• Therefore rotates at half the speed ... 1,500 rev/min
• Called a 4 pole motor (4 poles 2-North, 2-South).
AC Motor Shaft Speed

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• AC motors can be designed and manufactured with the number of
stator windings to suit speed requirements:
– 3-phase Stator windings connected to power supply
– Flux completes one rotation for every cycle of mains
– On 50Hz, the stator flux rotates at 50 revs per second
– Rotor turns at 50 x 60 = 3,000 revs per minute.
– Called a 2 pole motor (2 poles 1-North, 1-South).
• Speed of Stator Flux is called Synchronous Speed:
Fixed Speed AC Motors
min
rev/
p/2
60
x
f
=
pairs
-
pole
60
x
f
=
no min
rev/
p
120
x
f
=
no

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When Rotor Speed approaches synchronous speed:
• Magnitude and frequency of rotor voltage becomes small
• If rotor reached synchronous speed, the rotor windings would be
moving at the same speed as the rotating flux
• Induced voltage (and current) in the rotor would be zero
• Without rotor current .... no rotor field and no Torque.
To produce Torque:
• Rotor must rotate at a slower (or faster) speed
• So, the rotor settles at a speed less than rotating flux called the Slip
Speed
• The difference in actual speed to synchronous speed is called the Slip.
IM Actual Rotor Speed

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• If there is no load, or conversion loss the rotor will rotate at the speed
of the rotating magnetic field created by the stator.
• In a real-life system, the load makes the rotor rotate at a slightly lower
speed:
– Also called as Asynchronous motor.
• The speed difference is called slip .
• Higher the load torque, higher is the slip.
• E.g.: The synchronous speed (Ns) of an induction motor is 1500 RPM
and at full load it has a slip (s) of 3%.
• Rated speed = Ns x (1-s) = 1500 x (1-0.03) = 1455rpm
Speed of an Induction Motor

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• Direct On line (full voltage application, high current)
• Reduced voltage (NB! torque reduced) – star delta / autotransformer /
soft starter
• Wound rotor motors can be started using a resistance in the rotor:
– Lower starting current but high torque.
• Variable voltage and frequency (low current, high torque).
Starting an Induction Motor

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• In AC 3-phase machines, the direction of rotation is governed by the
rotation of the magnetic field created by the stator.
• This can be reversed by reversing any two of the three supply
connections.
• The normal sequence A-B-C now becomes A-C-B.
• This is called phase reversal and the motor will rotate in the reverse
direction.
Reversal of Rotation

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Reversing a Three - Phase Motor

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Losses in an Induction Motor

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Calculation of Motor Efficiency

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Efficiency vs Load / Rating

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• Pioneering technology leader ABB achieves almost 100% energy
efficiency with synchronous motor.
• Tests carried out on a 44 megawatt 6-pole synchronous ABB motor
shortly before delivery showed an efficiency 0.25% greater than the
98.8% stipulated in the contract, resulting in the world record for
electric motor efficiency.
• This efficiency improvement could save approximately $500,000 in
electrical energy costs over the course of a 20-year lifetime for each
motor.
World Record Electric Motor Efficiency

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Efficiency vs Output Power

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End of Webinar
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