VFD Harmonics increase motor losses

The induction motor, when under Pulse Width Modulation (PWM) voltage coming from the variable frequency drive (VFD), is subjected to voltage harmonics (frequency components above the fundamental frequency). Depending on the type of PWM employed, the switching frequency and other peculiarities of the control, the motor may present efficiency decrease and losses, temperature, noise and vibration levels increase.

Furthermore other effects may appear when induction motors are fed by variable frequency drives. Insulation system dielectric stress and shaft voltages allied with potentially damaging bearing currents are well known side effects. Although not produced specifically by harmonics but by other matters that will soon be approached, these are important effects and should not be neglected. The motor current and voltage waveforms when under PWM supply are illustrated below.

PWM voltage at the VFD output	VFD fed motor current

Then the motor fed by VFD sees a pulsating (PWM) voltage and a practically sinusoidal current, so that the voltage harmonics generally present higher magnitudes than the current harmonics.

There are basically the following solutions to mitigate the harmonics generated by a PWM VFD:

Methods of reduction of harmonics	Solution characteristics
Installation of output passive filters (L, LC (sinusoidal), dV/dt)	Installation costs increase
	Restrictions for vector control operation
	Voltage drop (motor horsepower reduction)
Use of multi-level variable frequency drives	Costs increase
	Variable frequency drive reliability decrease
	Control complexity increase
Pulse Width Modulation quality improvement (optimization of pulse patterns)	Space Vector Modulation (SVM)*
	Do not increase costs
	Voltage control upgrade
	Higher system (variable frequency drive + motor) efficiency
Switching frequency increase	Variable frequency drive efficiency decrease (higher switching losses)
	Common mode leakage current flow increase

* All VFDs manufactured by Gozuk employ Space Vector Modulation.

Normative considerations about the variable frequency drive output harmonics
There is no international standardization defining maximum acceptable values for voltage and current harmonic distortion. However, the international standards do consider the increase of motor losses due to the non-sinusoidal supply.

IEC 60034-17 provides an example of motor losses increase owing to PWM supply. Motor info: 315 IEC frame, rated torque and speed values.

Losses caused by fundamental frequency
A – Stator winding losses
B – Rotor winding losses
C – Iron losses
D – Additional load losses
E – Frictional losses

Losses caused by harmonics
F – Stator winding losses
G – Rotor winding losses
H – Iron losses
I – Additional load losses
J – Commutation losses

IEC 60034-25 illustrates the motor losses increase due to PWM supply by means of the following curves:


NEMA MG1 – Part 30 considers a derating factor (torque reduction) to avoid excessive overheating of a general purpose motor fed by variable frequency drive, compensating for the circulation of harmonic currents due to the PWM voltage harmonic content:

HVF = √(∑^∞_n=5 (V_n/n)²)

Where
n: order of the odd harmonic, not including those divisible by three
V_n: per unit magnitude of the voltage at the nth harmonic frequency