Re: 10 amps to drive steppers

> From: "Mo" <mo@...>
>
> Any of you guys integrating hi power stepper drive units such as Dans or
> others with custom power supplies, should consider 2 areas of disconnection
> which will cause problems,
> 1 Driver board driving the phases without a motor connected - can happen
> inadvertantly - I've been there

A properly designed stepper driver shouldn't be troubled by operating
with no load. Unplugging or plugging in the motor while the driver is
active could cause some arcing that might cause damage, but just running
the driver with no motor connected shouldn't be a problem.

> 2 Reconnecting a driver board with the motor voltage present - the high
> value smoothing capacitor(s) on your power supply is charged to the full
> supply voltage and carries a serious amount of charge - unless you take
> action to get rid of it it will stay charged for a very long time.

Yes, connecting a discharged capacitor to a charged one can make some
big sparks.

> Both of these situations seriously endanger your valuable boards.
> To protect against the driver driving without a motor connected, a good idea
> is to use a plug and socket connection for the motor with a couple of pins
> more than you need - so if you need 4 pins for your 4 wire motor get a 6pin
> plug and socket. Use the 2 unused pins as a loopback - shorting them
> together in the plug which is connected to the motor and the socket which is
> mounted on the case can then use this short to enable the board. You could
> have your DC supply voltage going into the socket via one pin, looping back
> in thro the plug when it is connected and then taken from the second pin of
> the socket and sent to your board, no motor connected = no DC voltage to
> board....the boards I use have an output inhibit pin which must be held low
> 0v for the board to drive the motors - a high or no connection will disable
> the boaard from driving.
> I use the above method to feed a ground wire into the socket that then loops
> back via the motor plug and then back onto that pin.

This could still cause a spark when plugging in with the power on.

> With the other issue of main motor suopply capacitors remaining charged
> after switch off, you should use a bleed resistor connected across the motor
> supply voltage - eg across the capacitor (s).
> The trick here is to get a value of resistor which is low enough to allow
> the capacitor to discharge quickly thro it but without drawing too much
> current from the capacitors and getting too hot.

Here's what I did, although it is on a servo system, but the problems are
similar. The E-stop signal on my system releases a pair of relays (one
a signal size, the other a power contactor). When these relays are
released, they connect a power resistor to the servo amps power
input, discharging the power supplies. When the relays are engaged,
first the signal relay closes, connecting the same power resistor in
series with the DC supply and the servo amps. Another contact on
this relay connects power through to a time delay circuit. After
a 1 second delay, giving time for the resistor to ramp up the
voltage in the servo amps, the power contactor closes, delivering direct
power to the servo amps. When this relay closes, it also connects
the enable input to the servo amps to +12, turning them on.
This gets rid of all the high DV/DT transients when powering things
up and down, and makes sure that whenever there is a fault, everything
is powered down in a controlled manner.

This also sees to it that there are no hot resistors smoldering away
anywhere, and that the voltages around the servo amps are cut off
in a fraction of a second when need be.

Jon