Re: [CAD_CAM_EDM_DRO] Working voltage for Servo Motors

Bernard R wrote:

>Hi All,
>
>What is the reason for the recommendations I have read that the
>supply voltage for servo motors should not exceed the nominal working
>volts by more than a few volts? In contrast the recommended voltage
>for stepper motors is 15 to 25 times nameplate voltage.
>
>Bottom line, how much over nameplate can you apply?
>
>

Quite a bit, in servo motors. the reason is the amp limits the current
to the peak maximum rating of the motor. Applying more than rated
voltage while current is limited will cause the motor to run above rated
speed, which is probably the softest of the limits on the motor. A motor
rated at 3000 RPM will not fly apart at 3500. However, a motor with a peak
current rating of 15 Amps may be ruined at 16 A when the magnets are
depolarized.

Now, on a stepper motor, the voltage rating is artificially low, as
these ratings are at motor standstill. Modern stepper drivers use a
PWM (chopping) technique to limit current, but the higher voltage
is needed to reverse the current in the windings quickly, as well as
supply the back EMF of the motor. That's why these motors often
have ratings of 2 V @ 4 A or something. This rating is obviously
low, because 2 V * 4 A * 2 phases = 16 W = 0.02 Hp, which will barely
move the hands on a clock. When a stepper is moving fast, it takes
a lot more voltage to run it. So, the dynamic voltage rating of a stepper
would be far higher than the standstill rating, which is what is actually
printed on the motor.

If you had a 24 V motor of normal construction, and an ordinary PWM
servo amp with no output filter, you could probably get away with
running it from a 48 V supply, and a 36 V supply would almost always
be safe. The insulation of the motor would certainly stand the overvoltage,
and the extra dissipation caused by the PWM waveform would only become
a little higher. With an output filter on the servo amp, you could probably
push it much higher, but there might not be a whole lot of benefit to that.

Jon