The detrimental effects of harmonics on motors and generators are usually taken for granted due to complacency or lack of knowledge. For example, the increase application of variable frequency drives (VFD) has subjected motors to considerably higher harmonic levels compared to when it was still using traditional controllers. As a consequence, the machine efficiency and torque developed are significantly affected.
Harmonic currents can give rise to excessive audible noise emission and vibration due to the difference between time harmonics and frequencies. In addition, IEEE 519 has mentioned that harmonics produce a resultant flux distribution in the air gap, which can cause cogging (refusal to start smoothly) or crawling (very high slip) in induction motors. However, this post will only elaborate the major effects of harmonics on motors and generators: overheating because of increased losses and mechanical oscillations due to the so-called pulsating torque.
Iron losses such as eddy current and hysteresis losses, are produced in the core of motors and generators due to the alternating magnetic field. The amount of eddy current loss varies as the square of the frequency, while hysteresis loss is directly proportional to frequency. Thus, higher frequency voltage components (i.e. harmonic voltages) generate additional losses, which result to higher operating temperature of the core and the surrounding windings.
Nevertheless, winding losses are of more concern than iron losses. Basically, I2R losses in the machine windings vary as the square of the RMS current. Therefore, an increase in the RMS current due to harmonics should be minimized since it will lead to higher winding losses. Moreover, actual losses would be slightly higher than calculated values because of skin effect.
Furthermore, stray losses - winding eddy current losses, high frequency rotor and stator surface losses, and tooth pulsation losses will increase due to harmonic voltages and currents.
Core and stray losses may become significant for an induction motor with skewed rotors. Single-phase motors are the most affected.
The heating effect of nonlinear loads on generators is greater compared to transformers. This is because a generator has higher reactance and impedance, that when paired with high frequency flux changes could cause stator heating. Also, high frequency currents will induce currents in the pole faces and hunting winding and hence cause rotor heating. Subsequently, generators supplying nonlinear loads should be derated based on the generator reactance.
Related Content: Effects of Harmonics on Transformers
Mechanical (Torsional) Oscillations
Mechanical or torsional oscillation in a turbine-generator combination or motor-load system is often disregarded by plant personnel. Harmonic pairs, such as the 5th and 7th harmonics, have the potential for creating mechanical oscillations. This phenomenon results when pulsating torques, caused by interaction between harmonic currents and the fundamental frequency magnetic field, excite a mechanical resonant frequency. For example, the 5th and 7th harmonics can combine to produce a torsional stimulus on a generator rotor at the 6th harmonic frequency. Mechanical oscillations can cause increased vibrations, shaft fatigue and accelerated aging of the shaft and connected mechanical parts. In the worst case, if the frequency of a mechanical resonance exists close to the frequency of electrical stimulus, vibrations are amplified and severe damage to the rotary machine may occur.
IEEE 519 published the table below, which defines the characteristic harmonic orders derived from a six-pulse converter and implies the effect when applied to the terminals of a rotating machine.
|Six-Pulse Converter Harmonic|
To explain further, the flow of each current in the stator will produce a magnetomotive force in the air gap that will induce current flow in the rotor of the machine. As each harmonic can be described as positive or negative sequence, the rotation of that harmonic will be either forward or backward with respect to rotor rotation.
For example, the 5th harmonic will rotate in a backward direction (negative sequence), so a harmonic current will be induced in the rotor with a frequency corresponding to the net rotational difference between the fundamental air gap frequency and the 5th – the 5th plus one, or the 6th harmonic. Also, the 7th harmonic will rotate in a forward direction (positive sequence), then, a harmonic current will be induced in the rotor with a frequency corresponding to the net rotational difference between the 7th and the fundamental air gap frequency - the 7th minus one or the 6th harmonic.
Additionally, mechanical oscillations can affect product quality where motor loads are sensitive to such variations such as in some synthetic fiber spinning or some metal working applications. In cases in which considerable inertia is coupled to the rotor shaft (e.g. motor generator or engine-generator), the electrical harmonics can excite a mechanical resonance.
To sum up, the net effect of harmonics to motors and generators is a decrease in machine life and efficiency. Neither reduction is pronounced for normally encountered harmonic content, but the harmonic heating usually reduces performance to 90-95% of that which would be experienced with perfect sinusoidal waves applied.
IEEE 519-1992. Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems
Sankaran, C. (1999). Effects of Harmonics on Power Systems 1