How to test Vfd with Multimeter

What is a Vfd?

Vfd or “variable frequency drive” is a device that is used to run motors at different speeds/rpm by controlling the frequency and voltage of their power supply. They are also called speed controlling devices because their main principle of action is motor speed adjustment.

If you are working in industry, you should know how to test vfd with multimeter well. Because they are so much important when we have to run elevators, mixers, shellers and some other machinery at dedicated rpm, controlled by programable logic controllers.

VFDs help us to save energy, enhance machinery life, match other rotary speeds and reduce stress on machines which ultimately improves their efficiency.

We also have many vfds(variable frequency drives) and some soft starters at our feed industry plant, so we have experience in working and testing vfd with multimeters.

How to test vfd with Multimeter

1) How to test input and output voltage of a VFD

While troubleshooting VFD or motor powered by vfd, the first step should be to check supply voltage of vfd and voltage supplied to motor through vfd. For testing them, follow the steps below

• First of all check the voltage supply level coming to vfd, is it at permissible level or not?

2) Testing dc bus voltage

• Now test dc bus voltage, attach negative and positive terminal of multimeter with it and watch result on the screen.
• For verifying dc bus voltage there is a formula which is 1.44*line voltage. For example, if line to line voltage is 420v then dc bus voltage should be = 605v dc.

3) Testing current of vfd

• For testing current of VFDs you should have a fluke clamp meter or fluke motor drive analyzer.
• This test should be done when the motor is on full load, clamp each cable coming out of motor terminal and note current values.
• After that compare amp values with one another, there should be not a big difference among current readings.
• Motor current unbalance should not be more than 10 percent at any load.

4) Measuring vfd I/O voltage

• Test voltage level by connecting your multimeter to motor terminals of vfd and note reading.
• Now check voltage reading on vfd display and compare them with your reading.
• There should be a minimum difference between these two, as an estimated difference between these two readings should not be above 3 % max.
• Repeat this while the motor is on full load and least load and compare results.

5) How to test Input/output diodes of a vfd

• For this test, we have to remove the power supply coming to vfd.
• Disconnect all types of I/O wires connected to vfd.
• Now set your digital multimeter on diode testing mode to move further.

Step 1

• Now connect positive or red lead of meter with negative terminal of vfd and common lead to its I/O terminals respectively and note results.
• Now compare values, if terminals are good in condition, you should have a voltage drop value between 0.3 to 0.6 v dc.

Step 2

• Now connect comm or negative terminal of meter lead with dc terminal of vfd.
• Put positive testing lead of multimeter on input and output terminal of vfd one by one and note readings.
• You should have the same level of voltage drop from 0.3 to 0.6 if terminals are working well.

Step 3

• Connect positive lead with positive dc terminal of vfd and negative lead to I/O terminals.
• You should get a over limit or open reading on meter if vfd is working well.

Step 4

• Connect negative or common lead of multimeter with – terminal of vfd.
• Now check results by connecting positive lead of meter with both terminals of vfd( negative and positive) one by one.
• You should get an OL reading(over limit) to make sure that vfd terminals are fine.