DC Servo power supply help.

[drumsticksplinter]

Hello,

I need to design a power supply for a machine that the control system has died on. The machine is quite old (1989 era...). There are 2 brushed DC servo's that drive 2 axes, these are both 65v, 4.5A, 200w each. I'm replacing the old servo cards and power supply so that I can drive them with more modern control. The old power supply is a big transformer with 75v and 150v taps on the secondary, its 630VA in power. However, I can't seem to figure out how this is then rectified and smoothed to somewhere near to the 65vdc for the motors? I have a feeling that the card with the big transistor is responsible (see attached images). The second card in the attachments is 1 of the 2 servo cards, I have no idea how it works though...

Would there be a way to use the exisiting transformer to build a simple linear power supply? My new servo drives will accept upto 80vdc, but will need the input to be smoothed by the power supply. I already have a 7400uf capacitor that I was hoping would come in handy from another project....

Many thanks in advance,

Adam

 

[Minder]

1/ Your DC supply does not need to be the same rating of the motors, when using a servo etc controller, the recommended supply voltage is motor plate voltage +10% minimum.
The power semi's appear to be SCR's so likely the original was a full wave, 4 quadrant SCR bridge type controller which was common back then.
2/ You could look at removing a few turns off the secondary to come a little closer to the required voltage.
M.

 

[drumsticksplinter]

Thanks for the advice! So I've been looking a bit more into building this power supply, I can get a reasonably priced 54v 700w toroidal transformer, which will give me around 76.3v once rectified.... What sort of smoothing capacitance should I be looking for? I found a spreadsheet that works out a capacitor value to a target voltage ripple, but I'm not exactly sure what is considered acceptable. A 5% voltage ripple (3.8v) requires around 20,000uF, is that realistic? Or could I possibly get away with a 10% ripple? I've read on another servo drive manufacturers data sheet that 1000uF per motor amp should be used and their drive powers up to 20A, which allows a 10% ripple according to the spreadsheet formulas.

 

[Minder]

10% is ample allowance, especially that servo's are not operating at max current for a great deal of the time, also if the supply is for more than one servo, it is very rare that all are operating at the rated current at any one time.
M.

 

[drumsticksplinter]

Thank you for your helpful replies, I have ordered my parts to build up this power supply, hopefully it will work fine.

 

[sundy]

I don't know anything about your type of machine, but if you don't get your PS working you can get a open frame linear PS pretty cheaply.