RE: stepper power supplies

Tim Goldstein wrote:

> > 2) I jumped on the tail end of a thread about using a scope to
> determine
> > overvoltage for stepper drive. I never found out the procedure. I
> only
> > caught the peripheral discussions about everyones' scopes.
>
> I think this is the message you want. I seem to remember someone added
> more,
> but this is a start:
> From: Walter McCarthy <wmccarthy@...>
>
>
> > Where can I get supplies to work with my 20 amp motors?
> >
> > How much power saving do you think I can have by changing power
> supplies?
> >
> > I am confused on how to pick a power supply. The motors are rated
> at
> less than 2 volts,
> but are 12.7 amp and the larger ones are rated at 20 amps. I think
> the
> Ah-ha supplies are 48
> volt. Can I send 48 volt to a motor rated at 2 volts?
>
> The over voltage you are supplying is supplied to shorten the turn on
> time of the motors.
>
> As the motors are only rated at 2 volts, 40 volts supplies 20 times
> the
> rated voltage of the coils.
>
> If you can hook your motors to an occilliscope, you can visually see
> the
> turn on time of the motors you are working with. Watching the rise
> time
> on the scope eliminates all the calculations and variables you don't
> know in the design of the motor.

I really don't agree with this description. It doesn't take into
account how a switching
stepper driver works. You DON'T need to know anything about the motor
except
the rated current, assuming you have already bought it. You only need
to know the
maximum current and voltage capability of the stepper driver. The
driver must be capable
of delivering the motor's rated current. Then, you need a DC power
supply that can
deliver a voltage close to the rated voltage of the driver. So, if the
driver is rated for
80 Volts, you probably would want to use a power supply of 48-60 Volts,
leaving
a safety margin. You then set the current adjustment on the stepper
driver for the
motor's rated current. No scopes, calculations, etc.

The only place the motor voltage comes in is to calculate the current
capacity of
the power supply needed. Assume a 4 V 8A stepper (it has two phases, so
that
really is 16 Amps for both). So, this motor dissipates 64 Watts idling,
and
somewhat less when moving. Lets say you have a 64 Volt power supply.
The current from the power supply at 64 Volts would be 1 Amp, to supply
64 Watts to the motor. (Now, actually, there are additional losses in
the motor
and driver, so it might be 10 - 15% higher).

So, you do NOT need an 8 (or 16) Amp power supply to run this motor.

Finally, you need to determine how fast it can run. You either need the

manufacturer's data sheets on the motor, or run tests on your actual
machine
to find out what the limit is. A higher voltage DC source should
improve
the torque at higher speeds.

Jon