Motor Starts

Is the "starts per hour" rating of a motor a factor when you're not starting a motor across the line? Say for example ramping up to 60Hz with a VFD?


TIA,

Greg

We start and stop 1HP motors 15-20 times a minute on six of our little lines (indexing conveyors). Is it a factor? I guess it is if you don't have the VFD parameters right (or the Ramp Up begins before the Ramp Down is complete which is a PLC/Control issue). We have done this constant up down thing for 2 of the 4 years I've worked here.

When we put in our new siemens softstart panels for our 600hp pump motors the technician wanted to keep the starts and stops under 4 per hour. The plc setup that we have and other setpoints will not let this occur but his main reason was the heating on the windings. I suppose a lot depends on the loads and voltage that you run. Our motors are of 1958 vintage and would be costly to replace.

George

Starts per hour is a factor, but usually not a problem with VFDs.

On a motor started across the line the inrush and starting current are several times the full load current. This causes the motor to heat up during starting. Cumulative thermal damage will destroy the insulation and the motor. The typical starts per hour is based on sufficient running time and idle time to allow the motor to cool between starts. Smaller motors cool faster, so they can be started more frequently.

A conventional reduced voltage starter, such as an autotransformer type, cuts down on inrush currents but they are still quite high. The number of starts per hour can be increased compared to across the line, but there is still a limit.

A Reduced Voltage Solid State starter or a VFD have adjustable limits on inrush currents, and current during starting can be only slightly higher than full load amps if the starting torque isn't severe. That limits motor heating, so starts per hour are usually not a problem.

I've done multiple starts at 2 or 3 minute intervals with VFDs during control system testing on easy starting centrifugal blower loads at several hundred horsepower, and motor temperature didn't change significantly.

On large motors I do recommend winding RTDs to monitor actual motor temp, so you know exactly when a problem is developing.

In addition, if you're planning on using a larger motor with a VFD, you need external cooling (separately powered blower) for low speed operations. If you're just starting it, and bringing it to speed where you intend to leave it for a while, then you don't necessarily need the blower.

However, if you plan to leave the motor at a low speed for a significant amount of time, you'll need the blower. The shaft mounted blower just won't provide the cooling needed at lower speed, especially higher torque, applications.

With a properly designed VFD, the number of starts per hour is, for all practical purposes, nearly infinite. You aren't getting the inrush damage as others have noted.