If you're an industrial technician then you must know about variable frequency drive, Converter, and inverter. Being an electronics engineer, troubleshooting VFD problems was always a challenge for me. So, after a lot of research and experience, here I will tell you how to test VFD with a digital multimeter. I have also made a list of 5 best multimeter for testing electronics for your ease. But before VFD troubleshooting, you must know about variable frequency drive.
What is a Variable Frequency Drive (VFD)?
"A Variable Frequency Drive (VFD) is a device used to control the speed of an electric motor. It is also used for energy savings because it reduces the motor torque. You can change the motor frequency by regulating its input power supply. A VFD has an inverter and converter.
It has an AC to DC converter and DC to AC inverter. These converters and inverters are used as regulators to maintain the flow of current and frequency. Before buying any VFD you must know about its rated power otherwise you will face drive failure issues.
Why Test a VFD?
A VFD is a critical piece of equipment in many processes and industries. It is important that a VFD is properly tested to ensure that it is operating properly. The process of testing a VFD includes verifying the correct input and output of voltage, frequency, and phase sequence.
Parts of VFD
Before testing a VFD, it is important to understand the different parts of the drive. The following is a list of the most common parts of a VFD:
The controller of the VFD is the main part of the VFD. It regulates all of the functions of the VFD. Drive- The drive is the part of the VFD that converts the AC voltage to DC voltage. It also converts the DC voltage back to AC voltage. rectifier-
The rectifier diode is the part of the VFD that converts the AC voltage to DC voltage.
A capacitor bank is used to absorb excess voltage from the motor when the VFD changes the frequency. The capacitors store the charge until it is needed by the drive. They're also used as filters on DC bus, which provides clean DC voltage.
The IGBT is a semiconductor device for power conversion. It has three terminals- Collector, Base, and Emitter. In VFD, it is used to convert DC voltage to AC voltage.
How to test VFD with multimeter?
VFDs are rated with certain maximum frequencies, voltages, and amperage. A VFD's performance is measured by the following:
Input Frequency: The input frequency is the rate at which an AC voltage is applied to the VFD.
Input Voltage: The input voltage is the voltage that is applied to the VFD.
Output Frequency: The output frequency is the rate at which an AC voltage is output from the VFD.
Output Voltage: The output voltage is the voltage that is output from the VFD.
Amperage: The amperage is the maximum amperage that can be output by a VFD at maximum speed. A lower amperage may cause the motor to overheat.
When testing a VFD, it is important to make sure that the input and output voltage, frequency, and phase sequence are consistent. For example, if the input and output voltage is not in the same range (eg: 3 phase VFD running on a single-phase input), the VFD could be destroyed.
Tools Required for troubleshooting VFD problems
Steps by step guide of variable frequency drives testing
Troubleshooting and Testing Drive in Energize/live condition
To test drive in the live condition you must have to be very careful. You have to be a bit conscious when the power supply is connected.
- The first step before checking any drive or motor is to measure the dc bus voltage. If the voltages are in-between 450 to 750 then it's ok. But if the voltage drop below 450V then there might be an issue with your dc capacitors.
- If you see any variation in power then must check terminals. Because in VFD most of the faults occur due to losing of connection. If you're facing issues in power supply testing then you can visit my blog.
- You can not open a VFD in live condition. So, check all control cables and power cables connections. Hopefully, it can solve the problem. Check the cooling fan is working properly if yes then note the temperature it must be under 85 degrees celsius. I have also compiled a list of best multimeters to check temperature. If the issue still prolong then you have to shut down the drive and open it.
Troubleshooting and Testing Drive in Idle condition
To test a VFD for input and output voltage, frequency, and phase sequence:
- Disconnect the input power to the VFD.
- Remove the cover of the VFD.
- Locate the drive input and output rails of the VFD.
- Now connect motor at drives output terminals.
- Set the multimeter to continuity function and place the red lead of multimeter on each phase of input and the black lead on the VFD body. If there is minute resistance or interpreted beep then it means the drive is fine. If it beeps strongly higher then it means the diode/bridge rectifier is faulty or any short circuit at inputs. You can also perform a diode check test by diode check function of your digital multimeter.
- After measuring the resistance and continuity of input components you should test terminals of the output side. Normally Output side is indicated as U, V, W or U2, V2, W2. If the motor is connected to drive then all the outputs will show continuity due to motor windings. Otherwise, it will show infinite resistance.
- If you face any current unbalance then immediately stop the drive and find the root cause. Sometimes C.T at the output side of VFD also gives you current unbalance results. So, it's better to replace the C.T or its driving card.
- Caution this is only for professionals: If you haven't found the fault yet then all you have to do is open the drive mark the tags take photos of all connections. After taking photos take out the IGBT and Diode from the drive and do a bench test. If you want to know about bench testing of diode and IGBTs then it's very simple. Hopefully one of them will be faulty. Now replace it.
- If the IGBTs and Diodes are good then please call your customer care provider for further help. Because the problem might be in the program or in the control card.
How do I know if my VFD is bad?
The motor drives on VFD will stop functioning or run in an abnormal manner. You can also check the output voltage of the drive to check its status. There can be any fault in your VFD but the following are the most common faults of VFDs.
- shorted IGBT,
- Current unbalance
- terminal scaling
- meter reads OL (overload)
- Faded display
- Short circuit in the inverter section
- DC bus overvoltage
- DC bus under voltage
- Colling Fan failure
Almost every VFD manufacturer provides customer support and an extensive user manual to operate it. In the user manual, you will find some common wiring diagrams, Alarm indications, and fault codes with descriptions.
Things to do
- Never touch the dc bus voltage with the AC range. The multimeter will explode.
- Always use an auto-ranging multimeter with CAT-II standard test leads to check dc bus voltage.
- Your multimeter should have a dc current measurement function. If you're not sure which multimeter should you buy then I have made a complete guide about it.
- Always wait minimum 5 minutes before opening terminal connections
Things no to do
- Never connect full load motor with respected drive.
- Troubleshooting variable frequency drives and motors
- Never touch input or output terminals while testing drive.
Frequently Asked Questions-FAQs
What is the aim of dc voltage at dc bus in VFD? How to use them?
The aim of DC bus connection at terminals is to check the status of the diode. You can also discharge the capacitors by these terminals. Secondly no you can not use them for DC load purposes.
How to test ABB VFD with a multimeter?
You can check any drive including ABB VFD with a multimeter. The only thing you have to do is read the blog and follow the instructions thoroughly. If you still can't find a solution then you consult an ABB VFD expert. (firstname.lastname@example.org)
If you want to check the power supply of a VFD with a multimeter then I have written a complete blog on power supplies testing.