First, let me state that a multimeter can only roughly test the main power circuit and output inverter of a frequency converter , and its accuracy cannot be guaranteed. If there are complex control issues with the frequency converter, it is best to find a professional frequency converter repair technician for the safest approach.
When designing electronic circuits, you will inevitably encounter various testing and measuring instruments, and a multimeter is one of them. Multimeters are generally used to measure DC current, DC voltage, and AC voltage. A frequency converter is a device that modifies the operating frequency of a motor's power supply to control an AC motor. With the widespread adoption of industrialization, frequency converters are increasingly used in modern electronics. This article will introduce how to use a multimeter to test the condition of a frequency converter.
I. Precautions before testing
For personal safety, the machine must be powered off. After the power indicator light goes out, disconnect the inverter's input power lines R, S, T and output lines U, V, W before proceeding with any operation!
II. Testing the rectifier module
To test the rectifier circuit, locate the P and N terminals of the DC power supply inside the inverter (Note: the P terminal of the inverter is the positive terminal after rectification by the rectifier bridge, and the N terminal is the negative terminal after rectification; PB is the terminal connected to the braking resistor). Set the multimeter to diode mode, connect the red probe to the P terminal, and connect the black probe to R, S, and T respectively. There should be a resistance of approximately tens of ohms, and the circuit should be basically balanced.
Conversely, connect the black probe to terminal P and the red probe to terminals R, S, and T in sequence. A near-infinite resistance value will be observed (the multimeter will display 1). Connect the red probe to terminal N and repeat the above steps; the same result should be obtained.
If the following results are observed, it can be determined that the circuit has malfunctioned:
A. An imbalance in the resistance values of the three phases indicates a rectifier bridge fault.
B. When the red probe is connected to terminal P, the resistance is infinite, which indicates that the rectifier bridge or the starting resistor is faulty.
III. Testing the Inverter Module
When testing the inverter circuit, connect the red probe to terminal P and the black probes to terminals U, V, and W respectively. The resistance should be several tens of ohms, with each phase having a roughly the same resistance. The resistance in the reverse phase should be infinite.
Connect the black probe to the N terminal and the red probe to the U, V, and W terminals respectively. Repeating the above steps should yield the same result; otherwise, it can be determined that the inverter module is faulty.
IV. Testing the Inverter's Functionality
Note: This method requires the frequency converter to be powered on.
Use a frequency converter to drive an asynchronous motor with matching power under no-load conditions, and adjust its frequency f from 50Hz down to the lowest frequency.
Note: The minimum frequency is calculated as follows: (synchronous speed - rated speed) × number of pole pairs p ÷ 60. For example, for a 4-pole motor with a rated speed of 1470 rpm, the minimum frequency is (1500 - 1470) × 2 ÷ 60 = 1 Hz.
During this process, the no-load current of the motor is detected by an ammeter. If the no-load current is stable and remains basically unchanged during the frequency drop, it indicates that it is a good frequency converter.
