Induction motor

An induction or asynchronous motor is an AC motor in which all electromagnetic energy is transferred by inductive coupling from a primary winding to a secondary winding, the two windings being separated by an air gap.

In three-phase induction motors, that are inherently self-starting, energy transfer is usually from the stator to either a wound rotor or a short-circuited squirrel cage rotor.

Three-phase cage rotor induction motors are widely used in industrial drives because they are rugged, reliable and economical.

Single-phase induction motors are also used extensively for smaller loads.

Although most AC motors have long been used in fixed-speed load drive service, they are increasingly being used in variable-frequency drive (VFD) service, variable-torque centrifugal fan, pump and compressor loads being by far the most important energy saving applications for VFD service.

Squirrel cage induction motors are most commonly used in both fixed-speed and VFD applications.

Squirrel cage rotor

Principle of operation

A three-phase power supply provides a rotating magnetic field in an induction motor.

In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic field that rotates in time with the AC oscillations.

Whereas a synchronous motor's rotor turns at the same rate as the stator field, an induction motor's rotor rotates at a slower speed than the stator field.

The induction motor stator's magnetic field is therefore changing or rotating relative to the rotor.

This induces an opposing current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited or closed through an external impedance.

The rotating magnetic flux induces currents in the windings of the rotor; in a manner similar to currents induced in transformer's secondary windings.

These currents in turn create magnetic fields in the rotor that react against the stator field.

Due to Lenz's Law, the direction of the magnetic field created will be such as to oppose the change in current through the windings.

The cause of induced current in the rotor is the rotating stator magnetic field, so to oppose this the rotor will start to rotate in the direction of the rotating stator magnetic field.

The rotor accelerates until the magnitude of induced rotor current and torque balances the applied load.

Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates slower than synchronous speed.

The difference between actual and synchronous speed or slip varies from about 0.5 to 5% for standard Design torque curve induction motors.

The induction machine's essential character is that it is created solely by induction.

For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( ), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced.

As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque.

The ratio between the rotation rate of the magnetic field as seen by the rotor (slip speed) and the rotation rate of the stator's rotating field is called slip.

Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load.

For this reason, induction motors are sometimes referred to as asynchronous motors. Under normal load, Induction motors run very close to synchronous speed. Because they never actually run st synchronous speed, they called Asynchronous machines .

Induction Motor Drive• Why induction motor (IM)?

– Robust; No brushes. No contacts on rotor shaft– High Power/Weight, Lower Cost/Power ratios– Easy to manufacture– Almost maintenance-free, except for bearing and other

“external” mechanical parts

• Disadvantages– Essentially a “fixed-speed” machine– Speed is determined by the supply frequency– To vary its speed need a variable frequency supply

• Motivation for variable-speed AC drives– Inverter configuration improved– Fast switching, high power switches– Sophisticated control strategy– Microprocessor/DSP implementation

• Applications– Conveyer line (belt) drives, Roller table, Paper mills,

Traction, Electric vehicles, Elevators, pulleys, Air-conditioning and any industrial process that requires variable-speed operation.

• The state-of-the-art in IM drives is such that most of the DC drives will be replaced with IM in very near future.

SPEED

The speed of a rotating fields depends upon the frequency of the source and the number of poles on the stator.

Synchronous speed is given by:

ns = 120f / p

where:

ns = synchronous speed (r/min)

f = frequency of the source (Hz)

p = number of poles

This equation shows that the synchronous speed

a) increase with frequency and b) decreases with the number of poles

Example: Calculate the synchronous speed of a 3 phase induction motor having 10 poles when it is connected to a 60 Hz source: Solution:

ns = 120f / p

=120 X 60 / 10 = 720 r/min

Slip and Slip Speed

The slip s of induction motor is the difference between the synchronous speed and the rotor speed.

Expressed in per unit:

s = 𝑛𝑠− 𝑛𝑛𝑠

where:

s = slip

ns = synchronous speed (r/min)

n= rotor speed (r/min)

The slip is practically zero at no load

And equal to 1 when the rotor is locked.

Example:

A 0.5 hp , 6-pole induction motor is excited by 3 phase, 60 Hz source. If the full load speed is 1140 r/min, calculate the slip.

Solution

The synchronous speed is

ns = 120f / p

= 120 X 60

6

= 1200 r/min

Slip s = 𝑛𝑠− 𝑛𝑛𝑠

= 1200 – 1140

1200

= 0.05 or 5%

(Power Electronics)

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(Power Electronics)

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(Power lectronics)

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(Power Electronics)

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(Power Electronic)

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(Power Electronics)

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(Power Electronics)

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EFFICIENCY

(Power Electronics)

Single frequency characteristic

• As slip is increased from zero (synchronous), the torque rapidly reaches the maximum. Then it decreases to standstill when the slip is unity.

• At synchronous speed, torque is almost zero.

• At standstill, torque is not too high, but the current is very high. Thus the VA requirement of the IM is several times than the full load. Not economic to operate at this condition.

• Only at “low slip”, the motor current is low and efficiency and power factor are high.

(Power Electronics)

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BLOCK DIAGRAM

Question 1:

A 0.5 hp, 6 pole induction motor is excited by 3-phase, 60 Hz. Calculate the frequency of the rotor current under the following conditions:

a. At stand still b. Motor running at 500 r/min in the same direction as the

revolving field c. Motor turning at 500 r/min in the opposite direction to the

revolving field d. Motor turning at 2000 r/min in the same direction as the

revolving field

Solution 1:

a) At standstill, the motor speed n=0

S =(ns-n)/ns

=(1200 -0)/1200

= 1

Frequency f2 = s f

= 1 x 60 = 60 Hz

b) Motor turns in the same direction as the field, the motor

speed n is positive. The slip is S = (ns – n) / ns

= (1200 – 500) / 1200 = 700/1200 = 0.583

Frequency f2 = s f = 0.583 x 60 = 35 Hz

c) Motor turns in the opposite direction to the field, motor speed is negative. Thus n = -500 Slip s = (ns – n) / ns

= (1200- (-500)) / 1200 = 1700 / 1200 = 1.417 The slip S greater than 1 implies motor is operating as a brake Frequency f2 = s f

= 1.417 x 60 = 85 Hz

d) n = 2000 s = (ns – n) / ns

= (1200- 2000) / 1200 = -800 / 1200 = - 0.667 Frequency f2 = s f

= -0.667 x 60 = -40 Hz

Motor operating as a generator as Slip s = negative