power for pitman servos If a little is good...

[MrWild]

I'm putting together a control/drive box and wondering how many transformers I should use in series for it. I have several 13.5v 20A transformers and need to know if I should mount two or three. The servos are three 24v Pitman 14207 and possibly later a 48v 14203 for an A axis.

Each 14207 takes 7A continuous and 40A peak. They won't ever hit their peak though due to the AMC drives having a 30A peak (30A8). Does the power formula that is used for pwm stepper drives hold for pwm servo drives? That'd be 3 x 7A x .7 = 14.7A. But I'd also have peaks of 30A but doubtful all three drives would peak together.

With three transformers I'd have ~55v 20A. I'm assuming that the extra volts (power) would help the drives meet 30A need, or even bursts of two drives needing 20A each. With two transformers it comes out to ~36v 20A. Would just two work, or should I go with three?

Speaking of peaks, I have a 200v 7400mfd capacitor. Is that enough? The cap is huge.

Thanks for the help

[H.O]

Hi,
What's the rated speed of the motors and how much speed do you need? What's the armature resistance?

I'd say 2 transformer will be enough for the 24V rated motors. You start off with 36VDC, you'll drop at least 10% across the amplifier so you're down to ~32V. Figure an armature resistance of 1ohm, then you'll drop 7V across it at full current, now you're down to 25V actually "driving" the motor which is right on the mark for the 24V rated motor.

Also, I'd say a supply rated for 20A continous will be fine. A transformer can be quite heavily overloaded (>200%) for short periods so it should be able to handle peaks during acceleration etc.

The capacitor however seems a bit small. It depends a bit on how much ripple the amplifier can handle but a rule of thumb is ~1000uF per ampere so you'd need 20000uF. The voltage rating of the cap should be at least 20% higher than the rated voltage of the supply, a 50V cap would be a good fit.

/Henrik.