BLOWING FUSES DURING MACHINE SETUP

PROBLEM: "I work for a major machine tool manufacturer. We use Myron Zucker, Inc. Calmount^® brand capacitors on each of our motors. While we are setting up the machine to test, we sometimes have a problem with blowing one or more of the fuses in the capacitor unit. Are we doing something wrong?

ANSWER: What you are experiencing is not uncommon in the machine tool industry.

First, we will break down what is happening when you are setting up your machine for testing. More than likely, you are checking for the rotation of your motor. You are probably energizing the motor circuit very quickly once or twice to see which way the motor is rotating. When you do this, the capacitor is being charged and recharged very rapidly. The capacitor has not had time to fully discharge before being recharged.

During this process, the capacitor is being energized at one point of the voltage sine wave, and then, with no time to discharge, the capacitor is recharged at another point on the voltage sine wave. This creates a very large inrush current to the capacitor.

The following formula will help you determine how much current will pass through the fuse:

Let's look at a 10 kVAR capacitor at 480 volts when this happens:

In the example above, there is a very large amount of current that will pass through the fuse, and the fuse will not be able to stay on the line. This can also happen if you have a starter or contactor and you get contact bounce when it is energized or jogging (inching) the machine through its normal operating cycle.

So, the best way to avoid this type of fuse blowing is to remove the capacitor fusing or disconnect the capacitor wiring when you are setting up the machine and re-install them before final commission.