Variable Speed Drive Fault Finding

Quite often you will come across an installation that has a variable speed drive in the panel. When you need to find a fault here, check the display on the drive and act on it. It is very easy to ignore the message and assume the drive is faulty. Fact is, normally the drive has just saved you a lot of time in fault finding.

Read the error code on the drive and if you are not sure what it means, check the manual. If you do not have a manual on site, the internet becomes a very useful tool as you can search the drive and get access to the manual. Just a quick tip, if you installed the Variable Speed Drive and it displays the earth fault, check your earth connection to the drive. Once you secured the earth connection, the fault will be cleared unless you have an issue downstream in which case proceed with the following:

Let’s assume it indicated that you have a ground fault. This will immediately point to the motor or cable connected to it. We can determine very quickly which it is but: Proceed with Caution! Power the drive down and isolate the main switch. As usual, make sure you follow the correct lock out and tagging procedure. Once you have allowed the drive enough time to de-energise, open the terminal and disconnect the motor cable from the drive. This is very important. Never use an insulation tester to test the motor with the drive still connected. This will cause serious damage to the drives electronic components and more than likely, destroy the drive.

Once you have the cable disconnected, you can perform an insulation resistance test as follows:

• Connect earth lead to ground
• Make sure your tester is set to 500V
• Touch the red lead to ground to make sure your tester works. This should return a zero reading
• Now test each of the 3 leads of the motor cable
• If you get a reading down to earth it indicates you have a fault on one of the downstream components
• There should be an isolating switch at the motor. Turn this switch to the off position and repeat the test
• If this test indicates no fault, we have just proven the cable to be good up to the isolating switch
• Open the terminal cover on the motor and disconnect the cable.
• Now test the motor  as per the previous post on motor test
• 95% of the time this will be where the fault is. If however the motor does test clear, test the cable between the motor and the isolating switch

Point to Please Make Sure of:

Ensure you follow the correct testing procedure before touching/opening any switches or motors to make sure there is no power to any piece of equipment you are going to work on. You may think it is silly that I keep saying this but I know what it is like when the pressure is on. Your mind is racing ahead and already working on the fault and possible scenarios because you want production running again. The time it takes to work safe is no longer at all. In fact, that peace of mind you have would allow you to concentrate on the actual fault finding and get it done quicker

Should everything downstream test clear, you will no doubt have a faulty drive unit and you will need to replace it. Unfortunately it does happen and the drives on the market today range from quick and easy to install through to very complicated. You will need the manual for the drive to follow the setup of all the different parameters. If you have a good relation with the supplier, they may be able to assist you with this. I have had the expert on the phone with me whilst setting the parameters on a number of occasions and this saves you a lot of time. These guys are experts in their field and they know their product. I would suggest you have a look on the site you work at and note the brands of VSD’s they use. Make contact with the suppliers and see what courses they have available. They often run free courses that will help you a lot and could save you a lot of headaches.

That covers in principle the earth fault. What if it was an overcurrent fault? Again, if you refer to an earlier post on Mechanical or Electrical, this will give you some guidelines to work with. My first step is to do a visual inspection of the machine and in particular the section driven by this motor. It may be the fault is due to a seized bearing or something similar. It could also very well be motor bearings that failed. Either way, if you can’t find it with the visual, remove the mechanical load from the motor and run it on its own providing you can do so safely! With the motor running, listen to the bearings and take the current reading of the motor. This will quickly indicate whether the fault is on the motor or mechanical equipment after it. What I also do is to check for any play on the motor shaft. This is a dead giveaway for a failed bearing.

Hope this could be of use to you. Please feel free to leave comments and/or any questions.