Re: [CAD_CAM_EDM_DRO] Re: capacitor

Mike Snodgrass wrote:

> Could I use the variac as input to the transformer?

Yes, this will work fine.

> I need wide control over output voltage.

Why? The Gecko drives do the regulating themselves. All you
need is to supply a voltage within their acceptable input range,
and the control circuits in the drive take it from there. You will
get best performance somewhere between 50 - 70 Volts with
most motors.

> I thought that a variac was a variable transformer?

It is a variable AUTOtransformer. There is only one winding, with
a sliding tap. There is no isolation between input and output.

> Either way I would
> still have to know the cap value. I have tons of large caps
> but the highest V rating is 40V, I would need 80v with
> headroom.

What you really want is a transformer with a secondary output
of 48 - 55 V at 10 A or so. This will give roughly 66 to 76
volts DC at the filter capacitor. For 3 Gecko drives, something
around 100,000 uF would be a good start. It is possible to put
2 identical 40 V capacitors in series with balancing resistors
in parallel with each. For such a supply, a 400 Ohm, 5 or 10 W
resistor in parallel to each capacitor would balance the voltage
between the two caps, and also provide a discharge path when the
power supply is turned off. You need to double the value of each
cap when in series, so to get 100,000 uF at 80 V, use 2 200,000 uF
40 V caps. Since the voltage balance is not perfect, stay down at
70 V or find 50 V caps.

Jon