If you are an electrical engineer or technician then you must know about VFD (Variable Frequency Drive). Many Engineers and technicians are facing difficulties while checking and troubleshooting the VFD. That’s why I will write in detail on How to test VFD with multimeter? and share my personal experience with you. Also, I am writing on different testing techniques with a multimeter. Have a look at those too.
Before we get into the topic, first of all, we will discuss a little the VFD and multimeter.
Variable Frequency Drive (VFD) is a device used to control the speed of the induction motor. The motor speed can be changed by regulating the input power.
VFD is also used to save energy because it will decrease the torque of the motor.
How does VFD works?
Speed Control and Application of AC Induction Motor using Variable Frequency Drive
Variable Frequency Drive commonly known as VFD has AC to DC converter and DC to AC inverter. These converters and inverters will regulate the current and frequency. Every VFD has its ratings.
What is Multimeter?
A Multimeter is a tool for measuring electrical quantities like resistance, voltage, and current. All these meters (ohmmeter, voltmeter, and ammeter) that measure those quantities are combined to a single meter are known as multimeters. There are two types of multimeters:
Analog multimeters work on the principle of moving coil meters. These meters use the deflection of an indicator needle to indicate the level of the measurement on an analog display.
2. Digital Multimeters
Digital multimeter is used to measure two or more electrical values like voltage (volts), current (amps) and resistance (ohms). Because of digital display its easier to read and chances of error will be less.
Why we need to check the VFD?
Variable frequency drive is very important because this is using in many industries. It is important to test the VFD to ensure that it is operating properly. The testing includes
Input supply
Output of voltage
Frequency
Phase sequence.
Parts of Variable Frequency Drive
For testing it is very important that we should have idea about different parts of VFD or variable frequency drive.
Controller : Main part of VFD that will perform all functions.
Rectifier Diode: Converts the AC voltage to DC voltage
Capacitor Bank: Absorb excess voltage from motor when the VFD changes the frequency
IGBT: Convert DC voltage to AC voltage
How to Test VFD with Multimeter?
Follow this VFD testing procedure with meter
For VFD it is necessary that input and output voltage, frequency, and phase sequence are consistent. The performance of VFD is based on
Input frequency and input voltage
Output frequency and output voltage
Current at maximum speed
We can test the VFD either live or in off condition. But it is recommended that always test the VFD is off condition. Because we can not open while it is live and chances of electric shock is always there.
A. Testing VFD in live condition
While testing the VFD with multimeter you have to be careful because you can get shock or your VFD can be damage.
Check the DC bus Voltage
Rotate the multimeter knob to DC voltage and measure the dc bus voltage.
If the voltage is between 450 to 750 then it’s fine.
But if the voltage is below 450V then there is issue with dc capacitors
Check Terminals
If there is variation in power then check terminals.
Because in VFD most of the faults occur due to lose connections.
Check control and power cable
Now, Check all control cables and power cables connections.
Check Cooling Fan
Check for temperature if it is under 85 degrees Celsius then fan is working properly.
Like wise if the temperature is above 85 degrees Celsius then fan is faulty.
B. Test VFD with multimeter in off condition
To test the VFD in off condition with multimeter. Follow these steps
Disconnect the input supply.
Now, Remove the cover of the VFD.
Check for input and output terminals.
Input resistance and continuity test
Rotate the multimeter to continuity and and attach the red probe of multimeter with input phase and the black probe on the VFD body.
If there is minute beep then it means the drive is fine.
Like wise, If beep is loud then bridge rectifier is faulty or short circuit at inputs.
Output resistance and continuity test
To test the resistance and continuity of output side
Connect motor output terminals of variable frequency drive.
Output side terminal are indicated as U, V, W or U2, V2, W2.
When the motor will be connected at the output of VFD. Outputs will show continuity because of motor windings.
It will show infinite resistance If the the motor is not connected.
Replace the VFD : In case of unbalance current at output its better to replace the CT.
Check the IGBTS: If you are technician or engineer then check the IGBTS with multimeter. But I suggest if you are not professional then don’t do so.
Loose connections are most common faults in VFD . There can be other error in VFD. If you see any of these don’t use it.
Current unbalance
Faded display
DC bus over voltage
DC bus under voltage
Colling Fan failure
Short circuit in the inverter section
How do you troubleshoot and diagnose a non working VFD?
For trouble shooting and diagnosing of non working VFD follow these steps.
1. Testing VFD in Live condition
Check the DC bus Voltage :If the voltage is between 450 to 750 then it’s fine. Below 450V means issue with dc capacitors
Terminals Check : In case of variation in power then check terminals.
Check control and power cable :Check all control cables and power cables connections.
Check Cooling Fan: If temperature is under 85 degrees Celsius then fan is working properly. Above 85 degrees Celsius mean fan is not working properly.
Replace the VFD : In case of unbalance current at output its better to replace the CT.
2. TestingVFDinoffcondition
Input Continuity Test: Connect red probe of multimeter with input supply and black to body.
Minute beep means VFD is fine. Other wise loud beep means rectifier is faulty or short circuit at inputs.
Out put Continuity test: If motor is connected at the output. Outputs will show continuity because of motor windings. other wise it shows infinite resistance.
How do you test a VFD capacitor?
To test the VFD capacitor follow these steps
Measure the DC bus voltage.
If the voltages are between 450 to 750 then it’s ok.
Below 450V means there is issue with dc capacitors.
HowtotestABBVFDwithamultimeter?
To test the ABB VFD with a multimeter. Follow the blog post and apply all the test given.
How to check IGBT in VFD?
To check the IGBT in VFD, remove it from circuit. Basically there are two test we performed on IGBT.
1.Collector-emitter junction test
Short the gate with emitter.(these pins located on side of IGBT)
Rotate the multimeter knob to Continuity or Diode check.
Connect the positive probe of multimeter with emitter.
Like wise, connect the negative lead of multimeter with collector.
This must show small value.
Now, swap the leads like positive with collector and negative with emitter.
If you check the resistance between Gate to collector and Gate to emitter it must show OL.
If it shows any value, this indicates leakage resistance or a short circuit.
The best method to test VFD with multimeter. We have tried to cover each and every thing and look for more and add in next update. If you have any suggestions and questions let us know in comment section below.
Inhamullah Jadoon is an Electrical Engineer and Co Founder of geekyengineers.com. He has vast amount of experience in the field of Engineering and in assisting Engineers on Software and Hardware issues.
