Time and space harmonics in induction motors can negatively impact performance. Time harmonics are present in the input supply and include 3rd, 5th, and 7th order harmonics. Space harmonics are caused by the distribution of windings and slot openings/closings. Effects include increased heating and torque modulation. Cogging occurs when the number of rotor and stator slots are equal, causing the motor to not start. Crawling is stable running at speeds near 1/7 of synchronous speed. To mitigate these issues, the number of rotor and stator slots should be unequal, their difference should avoid certain values, and external resistance can be added to the squirrel cage induction motor. Filters can also be used to

1. HORMONICS IN INDUCTION MOTOR
By
YASA PRAKASH REDDY
172113211

2. contents:
Introduction
Harmonics in induction motor
Time harmonics
Space harmonics
Cogging
Crawling
Remedies

3. Introduction:
 With increasing the number of harmonics generating devices
in power systems, the problem of their impact on the
performance of system components like induction motors
needs further consideration.
 In attempting to understand the performance of an induction
machine, we consider that the air-gap flux wave is purely
sinusoidal.
 It is from that assumption the analysis of induced emf,
sinusoidal currents, the expressions for generated torque etc.
proceed. In practice, there are deviations from this idealistic
picture.

4. Types of harmonics in induction
motors:
1. Time harmonics
2. Space harmonics

5. Time harmonics:
• The first non-ideality is the presence of harmonics in the
input supply given to the three phase machine. The
source may contain 3rd , 5th , 7th. . . harmonics.
• Vr = V1m sin(ω1t + φ1) + V3m sin(3ω1t + φ3) +V5m
sin(5ω1t + φ5) + V7m sin(7ω1t + φ7) + − − −
• Vr3 = V3m sin(3ω1t + φ3)
• Vr5 = V5m sin(5ω1t + φ5) and so on…

6. Effects of time harmonics
• The actual effect of these harmonics on the induction machine
would depend on the reactance of the machine since at high
frequencies, it is the reactance component that dominates the
resistance.
• Excitation voltage waveforms with considerable harmonic content
may result when induction machines are controlled through
inverters.
• Apart from the effects on torque, these harmonics cause
considerable heating in the machine and are hence a cause for
concern.

7. Space harmonics:
• These harmonics are due to mechanical construction of 3-
phase induction motor.
• Space harmonics are created due to distribution of the
winding, hence called ‘phase belt harmonics’.
• Harmonics created due to slot opening and closing are called
‘slot harmonics’.
• These harmonics will lead to misbehaviour of induction motor.

8. Cogging and Crawling
• Proper numbers of rotor slots are to be selected
in relation to number of stator slots otherwise
undesirable effects will be found at the starting of
the motor.
• Cogging and Crawling are the two phenomena
which are observed due to wrong combination of
number of rotor and stator slots.

9. Cogging
• Induction motors have a series of slots in the
stator and in the rotor.
• These slots should not be equal in number
because if they are, there is a good chance that
the motor will not start at all due to a characteristic
known as cogging.
• The slots will align like a stepper motor.

10. Cogging
• For this reason, there are an unequal number of
slots in the rotor and in the stator, but there can
still be situations where the slot frequencies
coincide with harmonic frequencies and this can
cause torque modulations.
• The slots are skewed to keep an overlap on all
slots to reduce this problem.

11. Crawling
• Another characteristic of induction motors, is
crawling. There are harmonic fluxes developed in
the gap due to the magnetics of the motor. These
harmonics create additional torque fields. It has
been observed that in induction motors especially
the squirrel cage type motors, there is a tendency
of the motor to run stably as speeds which are
nearly one-seventh of the synchronous speed and
this phenomenon is known as crawling.

12. FIG: EFFECT OF SPACE HARMONICS ON PERFORMANCE OF IM

13. To avoid cogging and crawling
• (i) To avoid cogging and crawling: (a)Ss ≠ Sr (b) Ss - Sr
≠ ±3P (c) adding external resistance to SCIM.
• (ii) To avoid synchronous hooks and cusps in torque speed
characteristics
Ss-Sr≠ ±P, ±2P, ±5P.
• To avoid undesirable heating effect due to time
harmonics supply should be balanced.
• We can place some filters to reduce harmonic content
in the supply.
Remedies: