Re: [CAD_CAM_EDM_DRO] How to test a servo motor without servo drive?

Bill Kichman wrote:

>I think your worries are unwarranted. There is nothing wrong with applying
>the full DC voltage to the motor assuming you stay within the motor's
>nameplate volts rating.
>

Absolutely wrong when applied to servo motors! And, easy to prove!
Take a particular servo motor, measure the DC resistance. Compute the
current at rated motor voltage, and compare to the rated PEAK current.
Many servo motors, especially high performance ones, will have a peak
current rating well below the possible peak current surge when started at
full rated voltage.

This applies ONLY to servo motors. Most other PM DC motors are designed
explicitly for across the line starting, and will not demagnetize the
permanent
magnets even in the locked-rotor condition. High performance servo motors
may have extremely low series resistance, thus very high currents would
develop. They are designed expecting a servo amp to limit the current.

Read the manufacturer's data sheet literature on a typical servo motor
if you don't believe me. Most have a specific warranty disclaimer that
if you over-current them, the warranty is void.

> Just clamp it down :-). As soon as it begins to
>spin up, it generates its own back-emf, opposing the heavy starting current,
>and settles into a low load running current, probably around 10-15% of full
>load current.
>

Yes, this applies to general-purpose DC motors, and may apply to some
servo motors with high enough internal resistance to limit current. that
resistance severely limits performance, as the resistance produces heat.
In an effort to get the most performance from a given size motor, cutting
the resistance is one of the biggest improvements. But, it gets you into
this motor safety problem.

>
>

Jon