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Motor Control - RC Networks

Posted By: Potseal

Motor Control - RC Networks - 12/09/14 05:11 AM

I have no experience with this item and have not seen it used in a motor controller until today. I was looking at a "blown up" motor controller in the shop today that was used for a large fan motor - 3 phase @ 600V. I don't have a great deal of experience with motor control so I had no clue what I was looking at when I seen a little black plastic block with two leads coming off of it - one from the L2 termination of the O/L's and the other lead went to a terminal strip which then went...somewhere. I never had time to cut the Ty-raps and trace it out. Trying to figure out what the device was wasn't easy since the information on the block was partially charred from the arcing that occurred during the breakdown. Finally I managed to track it down via Google using a couple readable values and the letters "o-c-u-b-e" . It's an Electrocube RC Network. This is the information I found online:

The seasoned journeyman in the shop at the time when I was studying the failed controller didn't know what it was either. He has many more years of experience than I do and is well respected in the shop. So my questions are - how common are RC Networks when it comes to motor control? Is this style of RC Network old tech or still used? Thanks.
Posted By: Potseal

Re: Motor Control - RC Networks - 12/09/14 05:15 AM

Here's a picture of a similar looking RC Network that I found online:

[Linked Image from]
Posted By: gfretwell

Re: Motor Control - RC Networks - 12/09/14 06:25 AM

They are just there to reduce burning of contacts or EMF spikes blowing solid state switches.
You do have a problem on 3 phase since there is really no "zero crossing". The switch, mechanical or solid state, is opening under a load every time. At IBM we ended up sizing SSRs many times higher than the load would imply, like 45a SSRs on 3/4hp 208 motors.
If the RC was gone, the SSR was on borrowed time. The SSRs failed shorted more often than not. That usually cooked off a motor breaker.
They were tweaking the spec all the time, usually going with higher voltage ratings on the capacitor.
Posted By: Potseal

Re: Motor Control - RC Networks - 12/09/14 06:28 AM

You may have to walk me through this but the RC Network in this case was on the secondary side or low voltage side of the controller - does that make a difference? Transformer was 600V primary and 100V secondary. Does it serve a more obvious purpose or better use in that instance?
Posted By: JoeTestingEngr

Re: Motor Control - RC Networks - 12/09/14 06:48 AM

We usually just call those snubbers. They are very common with inductive loads. When contacts break, the cap charges through the resistor and the current in the inductor takes longer to approach zero. That means that you don't get as large a counter emf from the collapsing field.
Posted By: Potseal

Re: Motor Control - RC Networks - 12/09/14 07:10 AM

Thank-you for the replies.

Are "snubbers" being used like the one pictured above in modern motor control or are they now integral to other equipment? I really don't know how old the equipment I was looking at is but I'm guessing early 90's.

I'm trying to gauge if this equipment is somewhat outdated or still being put into use in the manner in which I found it. Thanks.
Posted By: JoeTestingEngr

Re: Motor Control - RC Networks - 12/09/14 07:39 AM

Snubbers are routinely used in many, many, (Did I say many?), types of equipment. Light dimmers use them to reduce RF emissions. Same with phase control and PWM battery chargers, UPS, inverters. A majority of the equipment that I've worked on used discrete R's and C's to make the snubber network. There's more flexibility and higher power and voltage withstand with discretes.
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