Re: Working voltage for Servo Motors

Jon,

Thanks for the explanation. The reason for the question is that I'm
building a 4th. axis and the motor is much smaller than those
diriving the other axes and if possible I wanted to avoid building
another supply. I don't mind bending the rules a little as long as I
know the consequences; in this case I will be applying about 1-1/2
times nameplate voltage but will current limit to well within the
motor rating. Speed is not an issue.

Thanks again,

Bernard



Jon Elson <elson@p...> wrote:

>
>
> 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