Star motor wiring with brake

[gary10]

I have a 208VAC 3ph motor with a internal brake. The motor schematic shows a star (wye) layout but with the common point also going to ground (not the motor case). This is a 400Hz application if that makes any difference.

My concern is that the 28VDC brake coil shares this ground and the 28VDC supply I will use to release the brake doesn't share a ground with the 208VAC motor supply.

I have been unable to find any info as to the purpose of hooking the center of the star to ground. I'm assuming there is a reason for this sort of layout. It is possible to break this connection to ground. Any info will be helpful.

 

[Skogsgurra]

Hi,

Not an expert on 400 Hz systems. But I suspect that such systems have lots of little single-phase loads (I can think of resolvers, synchros, little motors and other equipment). Some systems have a problem with phase-neutral loads if they are not built for that. Solid state frequency changers are among those that can have a problem with phase-phase loading. That may be why your motor is connected four wires instead of three.

The brake. If you have a 28 V DC supply that is only used for the brake, I see no problem connecting it to the 400 Hz neutral. You are using ground and neutral in a somewhat imprecise manner. If the star point isn't connected to motor frame, I would say it isn't grounded.

There may be local codes saying that it is a nono to connect the two - but I would do it without any hesitation at all. (OK, I would do some measurements first...)

If you want to do it really, really well; install a small transformer and rectifier at the motor and use that to operate the brake. BTW: Do you want to operate the brake and motor separately? If not, you can connect that transformer directly to motor voltage.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

I concur with skogs.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gary10]

Thanks for the quick reply!

I should have said a 4 wire hookup and left out the "grounding" term. The brake will always be operated with the motor, so the transformer trick would be a nice touch.

I would still like to know (forgetting about the brake circuit) the difference between a 3 wire and a 4 wire hookup in a motor like this.

Some of us mechanical guys have trouble understanding 3 phase power, especially when delta and star get into the mix.

 

[waross]

Hi gary10;
Can you give us a little information on the source of the 400 cycle power?
As skogsgurra stated, the reason for the fourth wire may have nothing to do with the motor, but may be required by the power source.
It may be that the motor will run well on one type of power source without the fourth (neutral) wire, but cause problems with a different type of power source if the neutral wire is not used.
I am trying to restate skogs' information in different words.
A little more information about the 400hz. system will be a big help to us. We are doing a lot of guessing here. (But educated guesses!)
respectfully

 

[gary10]

Hi waross,

The 400Hz power will come from a 35KW generator. The motor in question (new, surplus) was originally part of a piece of equipment which was also powered by a 400Hz generator. As this motor is only going to be used for a demonstration (new, dedicated motors will be used for for subsequent equipment) and the motor manufacturer isn't being fully cooperative (I did get a schematic). I am trying to get through the demo without frying something. Unfortunately, I haven't found anyone connected with the domonstration (my custiomer) that is too versed in these matters. They usually rely on their vendor for help. In this case, I'm not much help!

Thanks!

 

[powerjunx]

gary10,

Contact the vendor or the manufacturer, exactly they know better than anyone else.
If you tried that they aren't cooperative, i think there's a way to settle down any differences. Vendor or any other sales person loves to promote good relation. In your case i wonder, why?


for me " i hate to guess, but i love to think."

As to waross; "educated guess"!

 

[waross]

Now we know that it's a generator and not an inverter, we can stop guessing.
I would go with skogs' suggestions.
respectfully