Hazards of Switching 3Phase 480VAC motors?

[tishalane 0]

So today while I was visiting a customer about a conveyor project that we are working on for him, I mentioned that it would be much better if we could turn off the conveyor in one of his "dumb" ovens. He was super against the idea because he said the motor couldn't handle the switching. We would honestly only have to switch it on and off at a rate larger than once per minute. I've never came across an application where this is a problem. This lead me to want to find out what REALLY are the hazards of switching a motor like this (480VAC, 3 Phase) and when the limits come into play. I am still pretty young in my career and maybe I somehow missed this lesson. Anyone have any input?

[BobLfoot 301]

Unfortunately your questions has more holes than one can answer. You don't state whether this is an across-the-line, soft-start or variable-frequency-drive application. Based on your customers response I am guessing ATL motor. In that case the startup in-ruch current is 8x to 11x the FLA. This can really affect the overall thermal load the motor sees. And of course the bigger the motor the bigger an issue this is. If you can provide more details like motor size and on/off frequency then others can provide a better response than mine I am sure.

[tishalane 0]

Thanks for the response. Lets just assume its a standard induction motor ran straight with motor contactors and overloads in the range of 3/4 HP. I understand the in rush current but how often do you have to turn something like this on and off before the heat becomes a problem? What about a 5HP motor? 10HP?

[Ken Roach 194]

The engineering answer to every question is: "it depends" ! Fortunately, industry experience in these sorts of things has distilled this sort of knowledge down to tables and standards, over the years. NEMA has tables on this sort of thing, and every motor manufacturer is going to give you similar information for each model. You'll need to learn a little about the application, including basic details about the horsepower rating, number of motor poles, and cooling method, as well as the basic information about the conveyor drivetrain and the load (moment of inertia) that's being driven. Here's a nice summary that includes a sample NEMA table for starts-per-hour for various motor frame sizes within a typical load range. http://heaa.org/downloads/MotorMaintenanceGuide.pdf

[ControlsGirl 2]

Thanks. Digging through this should be helpful. If you happen to have any rule of thumb advice, or know someone that does, please let me know before I get too buried. :)

[BobLfoot 301]

Dang Ken you come up with the sweetest toys :)