Induction motors are considered the most well-known prime movers as they are simple in construction and quite easy to operate and maintain. Nearly 80% of all the electric drives are induction motors. For the best performance from an induction motor, it is necessary to feed the induction motor with the perfect sinusoidal voltage.

The most commonly used drives in industrial installation are variable frequency drives (VFDs). The reason for VFDs’ popularity is that they facilitate easy and accurate speed control features.

For converting the input voltage into DC and again into regulated variable voltage/frequency, semiconductors are being used in VFDs. Semiconductor devices such as diodes and insulated gate bipolar transistors (IGBTs) are used in the inverter and converter section of VFDs. Non-linear characteristics have been exhibited by the semi-conductor devices in applied voltage and drawn current. In the electric network, harmonics current is introduced by the non-linear behavior of current with applied voltage. The performance of the induction motor is highly affected by the harmonic current.

Performance parameters of induction motors

The following are the most significant performance parameters of the induction motor:

Torque-speed characteristics

The load should be accelerated by the motor and then continued to the mechanical power as per equipment demand.

Reliability

Reliable operation of the induction motor is dependent upon the life of the insulating material being used. With the increase in temperature, the insulation resistance decreases. The harmonics current possesses higher frequency, which is due to increases in the eddy current. Likewise, hysteresis losses occur, subsequently raising the core temperature. At higher order harmonic current, due to increased stray capacitance, shaft voltage may also be set up. This higher shaft current/voltage may fail the bearings. Bearings at the non-driving end (NDE) must be insulated, or, using carbon brushes, the shaft must be grounded.

Effects of harmonic distortion on induction motors

Torque

Dependence of the machine torque is on the input power of the rotor.

Just like in transformers, harmonic distortion produces increased losses in motors as well. Though the production of harmonic generated fields results in the additional losses. There are three sequences in each harmonic and they are positive (+), negative (-) and zero (0). These sequences suggest the direction of the rotor that would result if the sequence were to be directed to the induction motor regarding the fundamentals.

A stationary field is generated by the zero (0) harmonics or the third and multiples of third. But the frequencies of the harmonic field are higher, due to which magnetic losses increase and result in harmonic energy dissipation as heat.

The counter-rotating field is produced due to the negative sequence, which causes reduced torque. A forward rotating field is generated by the positive sequence harmonics that result in increased torque. Torque components of both positive and negative sequence harmonics produce vibration and shorten the service life of the induction motor.

Efficiency and temperature rise

At harmonic frequencies, one of the major impacts of the harmonic current and voltage is increased heat because of the increased iron and copper losses. It causes an increase in temperature and machine efficiency is affected. Both copper and iron losses increase due to the harmonic distortion as they are frequency dependent.

Cogging and crawling

A resultant flux distortion is produced in the air gap by the fifth and seventh harmonics order current, which can result in a phenomenon called cogging. Cogging can be described as the refusal to start smoothly or crawl (run at higher slip).

The motor has a different synchronous speed for different order harmonics. During motor acceleration, it is necessary to control the base synchronous speed of the motor with the synchronous speed of harmonics. For example, when the synchronous torque of seventh order harmonic will combine with the load torque, then the motor speed will increase by one-seventh of the base speed, which is known as crawling.

Noise and vibration

At the start, the noise produced by both synchronous and asynchronous is most obvious. If the number of pairs of the higher harmonic field poles in rotor and stator has a difference of 1, then vibratory forces are produced that tend to make the rotor-stator vibrate by setting up a magnetic pull in a specific direction that travels around the machine. If the pole pairs have a difference of more than 1, then many unbalanced radial forces will produce noise and vibration. The audible noise is produced in the motor by the current of higher-order harmonics.

Increased resistance due to skin effect

The current is likely to flow at the outer surface of the conductor with an increased frequency. This phenomenon is called skin effect.

Consequently, the effective area is decreased and the conductor's resistance is increased due to the inverse relation resistance has to the cross-section area. The increase in the resistance results in higher copper losses (I2xR) in the motor.

Conclusion

Considering the above points, it can be concluded that harmonic distortion has a negative impact on the performance of induction motors, potentially reducing life and efficiency.