Measuring the quality of an inverter with a multimeter
During the design process of electronic circuits, engineers inevitably need multimeters to measure some measuring instruments. Engineers all know that a multimeter can measure DC current, AC voltage, and DC voltage. The frequency converter is a device that controls an AC motor by modifying the operating power frequency of the motor. This article will explain to you how to use a multimeter to measure the quality of the inverter.
It should be noted that for personal safety, the machine must be powered off and the input power wires R, S, T and output wires U, V, W of the inverter must be removed before operation! First, set the multimeter to the "secondary tube" setting, and then use the red and black test leads of the multimeter to test according to the following steps:
The black test lead contacts the negative pole P(+) of the DC bus, and the red test lead contacts R, S, and T in sequence, and records the displayed value on the multimeter. Then touch the red test lead to N(-), and the black test lead to R, S, and T in sequence to record the displayed value of the multimeter. If the six displayed values are basically balanced, it means that there is no problem with the diode rectifier or soft-start resistor of the frequency converter. Otherwise, the rectifier module or soft-start resistor at the corresponding position is damaged, and the phenomenon is: no display.
The red test lead contacts the negative pole P(+) of the DC bus, and the black test lead contacts U, V, and W in sequence, and records the displayed value on the multimeter. Then touch the black test lead to N(-), and the red test lead to U, V, and W in sequence to record the displayed value of the multimeter. If the six displayed values are basically balanced, it indicates that there is no problem with the IGBT inverter module of the frequency converter. Otherwise, the IGBT inverter module at the corresponding position is damaged, and the phenomenon is: no output or a fault.
Use a frequency converter to drive an asynchronous motor with matching power to run without load on site, adjust the frequency f, and start to decrease from 50Hz to the lowest frequency.
During this process, use an ammeter to detect the no-load current of the motor. If the no-load current is stable during the frequency decrease process and remains basically unchanged, then it is a good inverter.
The minimum frequency can be calculated as follows, (synchronous speed - rated speed) × number of pole pairs p÷60. For example, a 4-pole motor has a rated speed of 1470 rpm and a minimum frequency = (1500-1470)×2÷60=1Hz.
There is no problem with the soft-start resistor. Otherwise, the rectifier module or soft-start resistor at the corresponding position is damaged. Phenomenon: no display.
The red test lead contacts the negative pole P(+) of the DC bus, and the black test lead contacts U, V, and W in sequence, and records the displayed value on the multimeter. Then touch the black test lead to N(-), and the red test lead to U, V, and W in sequence to record the displayed value of the multimeter. If the six displayed values are basically balanced, it indicates that there is no problem with the IGBT inverter module of the frequency converter. Otherwise, the IGBT inverter module at the corresponding position is damaged, and the phenomenon is: no output or a fault.
Use a frequency converter to drive an asynchronous motor with matching power to run without load on site, adjust the frequency f, and start to decrease from 50Hz to the lowest frequency.
During this process, use an ammeter to detect the no-load current of the motor. If the no-load current is stable during the frequency decrease process and remains basically unchanged, then it is a good inverter.
The minimum frequency can be calculated as follows, (synchronous speed - rated speed) × number of pole pairs p÷60. For example, a 4-pole motor has a rated speed of 1470 rpm and a minimum frequency = (1500-1470)×2÷60=1Hz.
Identification of AC and DC solid-state relays: Usually, next to the input and output terminals of the DC solid-state relay shell, there are "+" and "-" symbols, and the words "Dc input" and "DC output" are marked. However, AC solid-state relays can only be marked with "+" and "-" symbols on the input terminal, and the output terminal is not divided into positive and negative.
Identification of input terminal and output terminal: Unmarked solid-state relay, multimeter R×10k range, and distinguish the input terminal and output terminal by measuring the forward and reverse resistance values of each pin respectively. When the measured forward resistance of two pins is small and the reverse resistance is infinite, these two pins are the input terminals, and the remaining two pins are the output terminals. In a measurement with a smaller resistance, the black test lead is connected to the positive input terminal and the red test lead is connected to the negative input terminal.
If the measured forward and reverse resistances of a certain two pins are both 0, it means that the solid-state relay has been broken down and damaged. If the forward and reverse resistance values of each pin of the solid-state relay are measured to be infinite, it means that the solid-state relay has been damaged by an open circuit.
