Re: [CAD_CAM_EDM_DRO] Dead servo

Vladimir Huzevka RUTEX.COM wrote:

>Hi Jon,
>
>
>
>I love to test one of your drives.
>
>
>
>Strictly speaking, it does not matter if the current sensing is in the
>high-side or low-side as long as the power supply is floating. It does
>exactly the same job. The Regulations of many countries require power
>supply to be floating - that's why most of the OEMs prefer to use floating
>power supply. And then, simple ground leakage detector can detect short to
>the ground in the motor before the drive is damaged.
>

Well, because of the computer interface, my servo amp is not floating.
I could make it float with just one more opto-isolator, and maybe I should
do that.

>
>
>
>Even if you put a current sensing in high-side as well as in low side (or in
>each leg of the motor) in the drive delivering kilowatts of peak power at
>voltages close to 100V or above, then the drive still might not handle a
>solid short circuit.
>

Well, my amp has a filter, with a series inductor before each motor
terminal.
But, I had a shorted low-side transistor, and the amp simply shut down,
protecting the high side transistor. The inductor wasn't even in the
current
path. So, I think it is pretty solid. It uses FETs, not IGBTs, which
are much
more sensitive to short circuit conditions.

>
>
>Having an inductor in series with motor might not be a best choice as well.
>A small inductor (in micro-henries) only decreases the RFI, although the
>closely twisted motor wires do about the same job.
>

It is 56 uH per leg. This gives a reasonable time constant for the
overcurrent trip
to respond. At 50 KHz, with the motors I'm using, the time constant is
about
one cycle (20 uS).

> Large enough inductor (in
>hundreds of micro-henries) dramatically increases the electrical time
>constant of the motors and even worse - it stores the energy and keeps
>driving the motor when the drive is not. It can totally upset the PID
>filter of the drive. Secondly, and more importantly, the inductor can
>quickly saturate at high currents. You would have to use physically very
>large transformer-like inductor - not a PCB mount ferrite coil.
>

These are small toriods. In a previous servo amp, I used 2 150 uH inductors
in each leg, in a 2-pole filter. This still had a very short electrical
time
constant, and I get full power bandwidth to 3 KHz on that amp.

Jon