How To Test VFD With Multimeter

The smooth operation of various industrial processes requires variable-speed drives, and variable-speed drives must be kept in good working order. When a fault indicator flashes, signaling a problem, it’s critical to identify the issue and quickly fix it.

If you are an electrical engineer or industrial technician, then you must understand VFDs (variable frequency drives). While inspecting and troubleshooting the VFD, many engineers and technicians are having problems. For this reason, I will go into detail on how to test a VFD with a multimeter.

The following points are on the list for the key no-power checks we offer here:

• An input test, similar to a diode test
• Checking the DC bus visually
• An output test, similar to a diode test
• Analyzing the checks
• A variable Frequency Drive (VFD) is a device used to regulate the speed of an induction motor.

Motors may be protected, controlled, and, in certain cases, even offered energy savings via VFDs

VFDs help in energy conservation, lengthen the useful life of machinery, match other rotating speeds, and lessen machine stress, all of which contribute to increased productivity.

At our feed industry facility, we also have a lot of variable frequency drives (VFDs) and a few soft starts, so we have expertise using multimeters to deal with and test VFDs.

Why do we need to check the VFD?

The application of variable-frequency drives in several sectors makes them highly significant. To make sure the VFD is functioning correctly, it is crucial to test it. The testing consists of

• Input supply
• Output of voltage
• Frequency
• Phase sequence

How to Test a VFD with a Multimeter?

While troubleshooting a VFD or motor powered by a VFD, the first step should be to check the supply voltage of the VFD and the voltage supplied to the motor via the VFD.

Follow the actions listed below to test them.

Check the level of voltage supply coming to the VFD first to see whether it is within acceptable limits.For VFDs, consistency in input and output voltage, frequency, and phase order is required.

VFD’s performance is determined by

• Input voltage and frequency
• Output voltage and output frequency
• Current at its fastest

The VFD may be tested both on and off the power. However, it is advised to test the VFD in the off position at all times. There is always a danger of receiving an electric shock if we attempt to open it while it is active.

Testing the VFD in live condition

You must take precautions while using a multimeter to test the VFD because you can get shocked or your VFD can be damaged.

     B.Checking DC bus voltage

• Now test the dc bus voltage by connecting the multimeter’s negative and positive terminals, and then monitor the screen for the results.
• There is a formula that is 1.44 * line voltage for determining the DC bus voltage. For instance, the DC bus voltage should be 605 v dc if the line-to-line voltage is 420 v.

 C.Testing current of vfd

• For testing the current of VFDs, you should need a fluke clamp meter or fluke motor drive analyzer.
• When doing this test, make sure the motor is operating at full load. Clamp each cable exiting the motor terminal and record the current readings.
• Compare amp values next to one another; there should not be a significant variation between the present measurements.
• At any load, the motor current imbalance shouldn’t exceed 10%.

D. Voltage measurement for vFD I/O

• You may check the voltage level by connecting your multimeter to the VFD’s motor terminals. Record the measurement.
• Now compare your reading with the voltage reading on the VFD display.
• Since the projected difference between these two values shouldn’t be greater than 3%, there should be a minimal difference between the two.
• Compare the results after performing this while the motor is under full and minimal load.

How to test a VFD’s input/output diodes

For this test, you must take out the power supply testing going to the VFD. All I/O wires attached to the VFD should be disconnected. To continue, switch your digital multimeter to the diode testing mode.

Step 1

• Now connect the meter’s positive or red lead to the negative terminal of the voltage-controlled device and the common lead to the device’s I/O terminals, accordingly, and record the results.
• Comparing the data now, you should see a voltage drop of between 0.3 and 0.6 v dc assuming the terminals are in excellent condition.

Step 2

• Now connect the comm or negative meter lead terminal to the DC VFD terminal.
• Place the multimeter’s positive testing lead on the VFD input and output terminals one at a time, noting readings as you go.
• If the terminals are functioning properly, the voltage drop from 0.3 to 0.6 should be the same.

Step 3

• The negative lead should be connected to the I/O terminals and the positive lead to the positive DC terminal of the VFD.
• If the VFD is working properly, your meter should indicate an above limit or open.

Step 4

• Connect the – terminal of the VFD to the multimeter’s negative or common lead.
• Now confirm the findings by sequentially connecting the positive lead of the meter to the negative and positive VFD terminals.
• To ensure that the VFD terminals are working properly, you should obtain an OL reading (over the limit).

Checks on the input of a variable frequency drive
Step	(+) Multimeter lead	(-) Multimeter lead	Multimeter reading (diode check)
1	R/L1, S/L2, T/L3	(+) Terminal	0.5 Vdc (approximately)
2	(+) Terminal	R/L1, S/L2, T/L3	OL
3	(-) Terminal	R/L1, S/L2, T/L3	0.5 Vdc (approximately)
4	R/L1, S/L2, T/L3	(-) Terminal	OL

Checks on the output of a variable frequency drive
Step	(+) Multimeter lead	(-) Multimeter lead	Multimeter reading (diode check)
1	U/T1, V/T2, W/T3	(+) Terminal	0.5 Vdc (approximately)
2	(+) Terminal	U/T1, V/T2, W/T3	OL
3	(-) Terminal	U/T1, V/T2, W/T3	0.5 Vdc (approximately)
4	U/T1, V/T2, W/T3	(-) Terminal	OL

Conclusion

It is necessary to conduct tests and measurements in a systematic manner at critical system points in order to identify the underlying cause of motor drive failures. When it comes to power supply, in particular, distinguish between power input and output and use various measurement techniques and evaluation criteria for each. With a little understanding, such metrics can help troubleshoot, pinpoint the real cause of the issue, and speed up system recovery.

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