What is the impact of the inverter on the motor

Knowledge about frequency converters and their impact on motors requires a clear understanding of the motor's efficiency and temperature rise, the motor's insulation strength, and the impact of harmonic electromagnetic noise and vibration, as well as the cooling issue at low speeds. Let's take a closer look at this below.

The impact of frequency converter on motor

1. The problem of motor efficiency and temperature rise

Regardless of the form of frequency converter, it will generate harmonic voltage and current of varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current.

Taking the commonly used sinusoidal wave PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components that are about twice the carrier frequency are: 2u+1 (u is the modulation ratio).

High-order harmonics will cause an increase in the stator copper loss, rotor copper (aluminum) loss, iron loss and additional losses of the motor, the most significant of which is the rotor copper (aluminum) loss.

Because the asynchronous motor rotates at a synchronous speed close to the fundamental frequency, high-order harmonic voltages cut the rotor bars with a large slip, resulting in large rotor losses.

In addition, the additional copper loss caused by the skin effect needs to be considered. These losses will cause the motor to heat up, reduce efficiency, and reduce output power. If an ordinary three-phase asynchronous motor is operated under the non-sinusoidal power supply output by the inverter, its temperature rise will generally increase by 10%~20%.

2. Motor insulation strength problem

Currently, many small and medium-sized inverters use PWM control, and their carrier frequency is about several thousand to tens of kilohertz, which makes the stator winding of the motor withstand a very high voltage rise rate, which is equivalent to applying a very steep impact voltage to the motor, making the inter-turn insulation of the motor undergo a more severe test.

In addition, the rectangular chopping impulse voltage generated by the PWM inverter is superimposed on the motor operating voltage, posing a threat to the motor's insulation to ground. The insulation to ground will age faster under repeated impacts of high voltage.

3. Harmonic electromagnetic noise and vibration

When ordinary asynchronous motors are powered by frequency converters, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors become more complicated.

The time harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is consistent with or close to the inherent vibration frequency of the motor body, resonance will occur, thereby increasing the noise.

Since the operating frequency range of the motor is wide and the speed variation range is large, the frequencies of various electromagnetic force waves are difficult to avoid the inherent vibration frequencies of the motor components.

4. The motor's adaptability to frequent starting and braking

Because the motor is powered by a frequency converter, it can be started at a very low frequency and voltage without impact current, and can be quickly braked using various braking methods provided by the frequency converter, creating conditions for frequent starting and braking.

Therefore, the mechanical system and electromagnetic system of the motor are under the action of cyclic alternating forces, which causes fatigue and accelerated aging problems to the mechanical structure and insulation structure.

5. Cooling problem at low speed

First, the impedance of the asynchronous motor is not ideal. When the power supply frequency is low, the loss caused by high-order harmonics in the power supply is large.

Secondly, when the speed of an ordinary asynchronous motor decreases, the cooling air volume decreases in proportion to the cube of the speed, causing the low-speed cooling condition of the motor to deteriorate and the temperature rise to increase sharply, making it difficult to achieve constant torque output.