An electric motor is an electromechanical
device that converts electrical energy to
The mechanical energy can be used to
perform work such as rotating a pump
impeller, fan, blower, driving a compressor,
lifting materials etc.
TYPES OF MOTOR LOADS
Output power varies
but torque is
Torque varies with
square of operation
inversely with speed
CLASSIFICATION OF MOTORS
Alternating Current (AC)
Direct Current (DC)
TYPES OF AC MOTORS
* Electrical current reverses direction
* Two parts: stator and rotor
Stator: stationary electrical component
Rotor: rotates the motor shaft
* Speed difficult to control
* Two types
AC MOTOR: INDUCTION MOTOR
Most common motors in industry
High power to weight ratio
Easy to maintain
Direct connection to AC power source
COMPONENTS OF INDUCTION
A 3-phase induction motor has two main parts:
• A stator – consisting of a steel frame that supports a
hollow, cylindrical core of stacked laminations. Slots on
the internal circumference of the stator house the stator
• A rotor – also composed of punched laminations, with
rotor slots for the rotor winding.
COMPONENTS OF INDUCTION
There are two-types of rotor windings:
• Squirrel-cage windings, which produce a
squirrel-cage induction motor (most common)
• Conventional 3-phase windings made of
insulated wire, which produce a wound-rotor
induction motor (special characteristics)
Induction Motor: Operating
Operation of 3-phase induction motors is based upon the
application of Faraday’s Law and the Lorentz Force on a
Consider a series of conductors (length L) whose
extremities are shorted by bars A and B. A permanent
magnet moves at a speed v, so that its magnetic field
sweeps across the conductors.
Operating Principle Contd…
The following sequence of events takes place:
1. A voltage E = BLv is induced in each conductor while it
is being cut by the flux (Faraday’s Law)
2. The induced voltage produces currents which circulate
in a loop around the conductors (through the bars).
3. Since the current-carrying conductors lie in a magnetic
field, they experience a mechanical force (Lorentz
4. The force always acts in a direction to drag the
conductor along with the magnetic field.
Now close the ladder upon itself to form a squirrel
cage, and place it in a rotating magnetic field – an
induction motor is formed!
Induction Motor: Rotating Field
Consider a simple stator with 6 salient poles -
windings AN, BN, CN.
The windings are mechanically spaced at 120° from
The windings are connected to a 3-phase source.
AC currents Ia, Ib and Ic will flow in the windings, but
will be displaced in time by 120°.
Each winding produces its own MMF,which creates a
flux across the hollow interior of the stator.
The 3 fluxes combine to produce a magnetic field that
rotates at the same frequency as the supply.
Induction Motor: Stator Winding
In practice, induction motors have internal diameters
that are smooth, instead of having salient poles.
In this case, each pole covers 180° of the inner
circumference of the rotor (pole pitch = 180°).
Also, instead of a single coil per pole, many coils are
lodged in adjacent slots.
The staggered coils are connected in series to form a
Spreading the coil in this manner creates a sinusoidal
flux distribution per pole, which improves
performance and makes the motor less noisy.
INDUCTION MOTOR : SLIP
The difference between the synchronous speed and
rotor speed can be expressed as a percentage of
synchronous speed, known as the slip.
s = (Ns – N)
Where s = slip, Ns = synchronous speed (rpm), N = rotor
• At no-load, the slip is nearly zero (<0.1%).
• At full load, the slip for large motors rarely exceeds
0.5%. For small motors at full load, it rarely exceeds
• The slip is 100% for locked rotor.
Induction Motor: Frequency
induced in the rotor
The frequency induced in the rotor depends
on the slip:
fR = s f
fR = frequency of voltage and current in the
f = frequency of the supply and stator field
s = slip
Induction Motor: Active Power Flow
Efficiency – by definition, is the ratio of output
/ input power: η = PL / Pe
Rotor copper losses: PJr = s Pr
Mechanical power: Pm = ( 1-s)Pr
Motor torque: Tm = 30Pr
Where: Pe = active power to stator
Pr = active power supplied to rotor
PL = Shaft Power
Induction Motor: Relationship
between Load, Speed
At full speed: torque
and stator current
At start: high
current and low
At start: high
At 80% of full