Motor Power Supply (-) voltage question

[beekeep]

Does the negative side of the motor power supply have to be the same as my control circuit ground? i.e. I have a 60 volt power supply and the negative side is 30 volts below ground and the positive side is 30 volts above ground. My DC control circuit negative is the same as chassis ground. What I am worried about is that I will burn up the controller card if these grounds are connected internally.

TIA
Greg Ferris

[Al_The_Man]

You do not mention what type of system you are using and drives etc, you really have to know the exact nature of the present power supply connections, your 60v supply may be completely isolated at present but you are most likely getting the reading you do by using a typical high impedance meter.
For the 60vdc common to be connected to Earth ground, the transformer ac secondary has to be isolated from ground, if using a linear supply.
Switching supplies are generally isolated anyway.
Al.

[beekeep]

I'll try to be a little more specific. I have six motors total. Three need to run at 78 volts DC and three run at 60 volts DC. For the power supply I have two step down transformers wired with 120 VAC to the 220 volt winding. This gives me about 55 VAC on the secondaries. The secondaries are each fed to their own variac to adjust the AC voltage down a little more before going to the full wave rectifier bridge. On the DC side of the rectifier bridge I have 7800 mfd capacitors rated at 200 volts.

Motors are bipolar Keling KL34H180-60-4A and KL34H2120-42-4A
Controllers are 6202 Gecko drives
Transformers are 2000 watts
Variacs are 1KVA

thanks
Greg Ferris

[Al_The_Man]

I assume these are normally 120vac secondary transformers?
It also seems that not only are you using the transformers at half voltage, if the variacs are 120v input type, these are also then being operated at half voltage?
Not very efficient.
Unless they are potted well, I would have looked at taking turns off the transformer secondaries.
Variacs are auto-transformers, which run direct off the main supply can pose a safety problem but at least you have them isolated by virtue of the transformers,
If the above is true and they are 120v, why not run the transformers at 240v and the variacs on the 120v secondary?
If you have two separate DC supplies then you should be able to connect the DC-ve of each together.
You should also have the choice of earth grounding it, if you so wish.
Al.