The advent of frequency converters has revolutionized industrial automation control and motor energy conservation. Frequency converters are almost indispensable in industrial production, and even in daily life, elevators and variable frequency air conditioners have become essential components. Frequency converters have begun to permeate every corner of production and daily life. However, frequency converters have also brought many unprecedented problems, among which motor damage is one of the most typical phenomena.
Many people have noticed the damage that frequency converters can cause to motors. For example, a water pump factory has experienced frequent reports from its customers of pumps failing within the warranty period over the past two years. Previously, this pump factory's products were of very reliable quality. Investigation revealed that these damaged pumps were all driven by frequency converters. The damage to motors caused by frequency converters includes two aspects: damage to the stator windings and damage to the bearings, as shown in Figure 1. This damage generally occurs within a few weeks to a dozen months, with the specific timing depending on many factors such as the brand of the frequency converter, the brand of the motor, the motor's power, the carrier frequency of the frequency converter, the length of the cable between the frequency converter and the motor, and the ambient temperature. Early unexpected motor damage causes significant economic losses to enterprises. This loss includes not only the costs of motor repair and replacement, but more importantly, the economic losses from unexpected production downtime. Therefore, when using frequency converters to drive motors, sufficient attention must be paid to the issue of motor damage.
To understand why power frequency motors are more prone to damage under inverter drive conditions, we must first understand the difference between the voltage used to drive the motor by the inverter and the power frequency voltage. Then, we need to understand how this difference adversely affects the motor.
The basic structure of a frequency converter is shown in Figure 2, consisting of a rectifier circuit and an inverter circuit. The rectifier circuit is a DC voltage output circuit composed of ordinary diodes and filter capacitors. The inverter circuit converts the DC voltage into a pulse-width modulated voltage waveform (PWM voltage). Therefore, the voltage waveform of the motor driven by the frequency converter is a pulse waveform with varying pulse width, rather than a sinusoidal voltage waveform. Using a pulse voltage to drive the motor is the root cause of the motor's susceptibility to damage.
Whether it's small devices like mobile phones and watches, medium-sized devices like fans and water pumps, or large equipment like trains and ships, motors are present and play a significant role. Among various types of motors, three-phase motors are the most widely used.
As the name suggests, three-phase motors require three-phase electricity. A power supply is essential for the motor to operate smoothly. While this significantly increases production capacity, it also leads to considerable electricity waste. Motors typically cannot adjust their speed or control their starting voltage on their own, resulting in both high power consumption and potential damage. This is where a device comes in, providing energy savings and protection through real-time speed adjustment and soft starting – the frequency converter (VFD). Simply put, a VFD acts like a remote control for the motor, offering advantages such as energy efficiency, protection, ease of operation, and wide applicability.
After installing a frequency converter, the motor speed becomes more stable, its lifespan increases, and the number of malfunctions decreases. The benefits are energy saving and cost savings, making it very useful.
The working principle of a frequency converter is explained below:
As the name suggests, a frequency converter changes the frequency of voltage. Normally, motors use alternating current (AC) with a constant frequency that cannot be adjusted. A frequency converter can convert this AC current into AC with adjustable frequency and voltage.
The specific process involves installing a frequency converter between the power supply and the motor. This prevents current from directly entering the motor. Once the current enters the frequency converter, the rectifier module converts the incoming AC to DC, which then flows through a capacitor to regulate and stabilize the voltage. Finally, the inverter converts the DC back to AC. This AC is different from the initial input and is controllable. It can then be output to the motor. To control the motor speed by changing the voltage frequency, adjustments can be made at the inverter output.
The relationship between frequency converters and motors is more than just control. Because of frequency converters, motors operate more stably and efficiently, and are also safe and energy-saving. Therefore, frequency converters also play a positive role in promoting motor performance.
