You probably know that motors require more current to start than they do to run. What this means is motors have more heat generated in them when they start than when they are running. It takes time to dissipate the extra heat.
The heat can dissipate as the motor runs (the fan in a fan-cooled motor helps dissipate this heat) or as the motor sits in a non-running condition. The important thing is to dissipate the starting heat rather than increase the motor temperature significantly through repeated starts placed too close together. If the cycle of start/stop/start is too short, we call that short-cycling. And it’s deadly to motors.
Sometimes, the controls for motor-driven equipment will state the minimum time between starts. Sometimes, the controls will actually limit this time so that no matter what the operator does the motor will not start until the preset time has passed.
Typically, a running motor will cool faster than one that’s sitting. So the recommended waiting time between starts depends on whether the motor was running (and just now stopped) or has been sitting since the last start.
For this reason, your motor data may show two different starting frequencies for a given motor. As you’ve no doubt guessed, the shorter one is for a running motor and the longer one is for a motor that has not run since the last attempted start. The ratio between these is usually about 3 to 1.
It’s possible to be unsure of which one to apply. For example, the motor started, but then stopped. This can happen if the motor is trying to start while under load. The classic example of the jammed scrap grinder illustrates this well. The motor doesn’t get up to running speed before overloads kick it out. In such cases, use the longer of the two times to be safe.
Note also, this is the minimum time between starts, and it assumes “typical” conditions. If the motor is on a mezzanine above a hot process and the ambient temperature makes you dripping wet, you’ll need to allow more time.
If you suspect short-cycling has been plaguing your motors, determine the minimum times and take corrective actions. For example, at the controls post a sign that states the minimum time. Or install a timer that locks the controls out until the minimum time has expired.
