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How to select an appropriate motor?

API motor standards series for form-wound, medium voltage ac motors (541, 547, and 546) all have the same requirement: 5000 DOL starts for the life of the machine. Obviously the greater the frequency of starting, the lower calendar life. Starting a centrifugal pump motor once a quarter will last a long time. Starting something 4 times a day won't last as long.

Just as a light bulb: coil life is less dependent upon running hours than the jolt of inrush current.

API requirements are for robust, oil & gas industry machines. General Purpose machines don't measure up to these requirements, and many companies aren't willing to pay for API compliant machines.

I'd look at the specifics of power or torque requirements that you have in your application and work with the local sales office to select an appropriate motor. Once you have a specific motor, it's been my experience, that you can determine the limitation of line starts per unit time. Again remember this is a thermal limit in large part, so oversizing or selecting a motor with a larger frame may be required. If this is not a viable solution, and your application can tolerate a subtle ramp, a VFD might be a way of minimizing the motor size (frame).

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