Re: [CAD_CAM_EDM_DRO] Re: Power Supply for Gecko 320 and servo motors for 3 axis router

Ed Gilbert wrote:

>Hi Gregory,
>
>What type of machine are you running them on? When you de-accelerate an axis
>the motor produces voltage back into the power supply, know as back EMF. A
>DC motor whether servo on not is a generator when being turned by the load.
>A switching supply will not like it when voltage is back induced into its
>output. Makes smoke in most cases! (:/
>
>

This is not exactly true. The motor, on its own, can never generate a
higher
voltage than the DC power source, unless it is actually being turned faster
than the motor would spin if just connected to the DC source. But, what
happens, especially with high-inductance motors, is that a PWM servo driver
can build up flux in that inductance, and then generate a short pulse of
voltage
higer than the DC supply. The key, here, is that it will be a very
short pulse
in most cases. So, even it the motor is being braked at 10 A, say, the time
the voltage exceeds the DC supply voltage might be 10% of the total PWM
cycle. Therefore, the current flowing back to the DC supply would AVERAGE
only 1 Amp, not counting the current draw during the rest of the cycle.
So, except in cases of HUGE masses being braked, you don't see much rise in
the DC supply voltage when a decent filter capacitor is present. (I see
from 1 to 2 Volts rise on my Bridgeport, for instance.)

Many switching supplies have a "crowbar", a failsafe overvoltage protector
that will use a large SCR to short out the power supplies output if the
output
voltage exceeds 10 or 15% of nominal voltage. If this crowbar circuit is
triggered, it will cause a huge current, and would likely cause the
crowbar's
SCR to fail shorted. But, I think it is fairly unlikely to happen if
sufficient
external capacitor bank is supplied. It sure could happen if a
switching supply
was used with no external filter caps or an overvoltage drain circuit,
however.

Jon