Two Speed Two Winding Motor Speed Switch

[TM519]

We are installing a two speed two winding 3 phase motor and are looking into switches that can automatically switch from the low speed to high speed (in this case 450rpm to 720rpm winding) upon acceleration.

Just looking to learn more here, How exactly does a switch like this work? Are there contacts inside that open/close based on programming from the control system? Also, is there anything in particular we should be looking for when specing our this new switch from the vendor? We don’t have a lot of expertise in this space, so the plan is to tell the vendor what we want and see what they come back with, but it would be nice to know what we are looking at when it does come back, especially if there is anything that we should be looking for that we wouldn’t know to look for.

the two speed motor is 6000HP/1500HP, 13,200 volts. We are using it to start a fan, plan is to start the fan on the 1500hp, 16 pole winding and then we need to switch over to the 6000hp, 10 pole winding.

Really appreciate any and all input, just trying to learn all I can so any input is appreciated.

 

[drcampbell]

One thing the control system will need to do is assure a certain minimum dead time after switching off the low-speed winding and before switching on the high-speed winding. Bad things will happen if the two are ever energized at the same time, even for a millisecond.

 

[kwired]

My experiences are with maybe 2 hp or less motors, those had a fairly simple drum switch or two contactors with mechanical and/or electrical interlocks so you can't run both at same time (basically about same thing as reversing starter as the base build of the controller)

Size of motor you have - seems likely you have soft starting method of some sort to deal with as well, and controller that is just a little bigger and a little more complex than would be on 2 hp. VFD would be great controller but then you sort of have no need for two speed two winding motor - but there could still be a good reason to have such motor even if controlled by VFD I imagine.

 

[iceworm]

Disclaimer: Only experience with two-winding motors is a 480V, 400Hp, 3600rpm/90Hp, 1800rpm - several motors, all the same application.

The concept is pretty simple - two medium voltage contactors. As dr said, electrical interlock to require one to be opened before the second can be closed.

At 13.2KV, 6000hp, the current is less than 400A, so, likely vacuum contactors, available from several outfits, Schneider, SEL, Rockwell. The application will also need one, possibly two protective relays, GE Multilin, Basler, SEL. And, a 13.2KV feeder, in the 800A range is also required. If this is not existing, that will be an additional slug of MV switchgear/transformer.

The controls can be as simple as ON/OFF push buttons and a High /Low switch. The operator watches a current meter and switches from Low to High when the current drops. One can add an RPM switch or a current switch - or even a PLC monitoring the rpm/current.

We don’t have a lot of expertise in this space, so the plan is to tell the vendor what we want and see what they come back with, but it would be nice to know what we are looking at when it does come back, especially if there is anything that we should be looking for that we wouldn’t know to look for.

That is a pretty good idea, however the first thing you need is a Detailed Design Document - the more detailed, more specific, the better. This doesn't tell the vendor what to supply, rather tells the vendor what the equipment has to do. This requires an owner's SME that understands the process.

Now might be the time to consider hiring a consultant (engineer familiar with large motor installation). The vendor is concerned with selling equipment at the highest profit. The equipment has to meet spec, but may not necessarily be the best for your application. An engineer paid for by you is very much watching for your interest. The engineer's cost will be pretty small compared to any process installation involving a 6000HP motor.

is there anything in particular we should be looking for when specing our this new switch from the vendor?

Yes. The vendor offering meets each one of the items listed in the detailed design document.

 

[LarryFine]

Simply put, it works kinda like a transfer switch.

 

[Jraef]

At 15kV, the gear itself will be built to order for this. Before you start specifying, you need a thorough understanding of your existing system, the application profile of the equipment, the expertise level of your maintenance crew, where the gear will be located, etc. etc. etc. So I totally agree with Iceworm, this is likely something deserving of being done by an experienced EE as a consultant to YOU, not as a vendor designed system.

As just one example; a vendor might propose a system that uses vacuum circuit breakers (VCBs) instead of vacuum contactor based motor starters, because they “kill two birds with one stone”. If this motor gets started a few times per year and runs for long periods, that may be totally valid. But if it turns on once per day, you will wear it the VCBs in short order; they are not designed for that.

The part about the starter system automatically changing speeds is a actually a minor detail that can be added to the big picture after solving the bigger problems. Generally it’s done via timing or a speed sensor on the work shaft, but again, it depends upon what you are trying to accomplish with the two speed motor.