Re: [CAD_CAM_EDM_DRO] stepper drives and motor size

"Carey L. Culpepper" wrote:

> I got my new Warner drives today.I spent some time reading through the
>
> instruction book. It has a caution: Do not use larger frame size motor
>
> than those listed, or the drive may be damaged. If a larger frame size
>
> motor must be used, consult the factory for recommendations.
> Well, I made that phone call. The recommendation was for a different,
> very expensive(over $2000) drive. The problem with the NEMA 42
> motors,he
> said, was a voltage increase in the buss when the motors are stopped
> quickly as in a high friction situation,which I certainly
> have(Bridgeport conversion).

I think you got the scenario wrong. Probably what he was saying is that
when the motors
are decelerating heavy inertial loads, energy flows back to the driver,
and pumps up
the DC supply bus. If this voltage goes too high, transistors will
fail. Stopping the
motors mechanically when they are moving is not as likely to cause
damage.
There are a couple ways to prevent this problem. The simplest is to put
a large
capacitor bank in the power supply, and arrange the normal DC voltage
such that
rapidly decelerating all motors at once does not raise the DC voltage
above the
spec.

This may require an excessive capacitor bank, depending on how much
energy
is drawn back from the motors. You can check this with a voltmeter and
one
motor connected to the heaviest load (probably the Y axis on a knee-type
mill).
Start with gentle decelerations, and increase the deceleration rate in
small
increments and see how much the DC voltage pumps up. If it gets close
to
75 V, then you need to reduce the normal DC voltage of the supply, add
more
capacitors, or go to "plan B".

Plan B is to add a braking resistor. The resistor is simple, the
control is a bit
more complicated. What you need is a sensor to tell when the DC voltage

is too high, and connect the braking resistor to drain it off. A 100 W
wirewound
resistor makes a good braking resistor. P = V^2 / R, therefore R =
V^2/P.
So, for a 75 V system, a 50 Ohm, 100 W resistor will do, and will bleed
off 1.5 A when turned on. Depending on the current flowing back from
the motors, that may or may not be enough.

There are a couple of simple control circuits that should work. A 75 V
zener
diode connected to the base of an NPN power transistor, with a modest
current
limiting resistor of about 100 Ohms in series should work.

You could also use a power FET, with the same 75 V zener connected to
the gate of an N-channel FET. But, there might be some need for a bit
of protective circuitry to keep the FET's gate from being blown.

Finally, these big power resistors are quite inductive, so a frewheeling

diode would be needed to prevent the inductive voltage spike created
when the transistor turns off from destroying the transistor.

>
> These drives are rated at 75Vdc and7A. I will be using only 5A and
> probably 56Vdc.

So, that gives you a margin of 19 Volts. If you have a 100,000 uF
capacitor bank,
a charging current of 1 amp for 1 second will raise the capacitor
voltage 10 Volts.

> Anything that I can do to protect these drives while running NEMA 42
> motors? Would microstep resolution make a difference as far as induced
>
> voltage ?

no.

> Anything that I can add to the cicuit to the motors for
> protection?

No, adding circuits to the motors won't help, it is the DC voltage that
normally goes
INTO the drives that is the problem.

If you need more help with this, I can send you some schematics.
I just hate to try to explain better with "ASCII art".

Jon