Disadvantage/Problem with using a 1200 RPM, Wye wound imotor for an RPC idler motor

Bill Widmer · Nov 26, 2021

I have a US Electrical Motors Uniclad 10HP, Wye wound motor I was going to use as idler for a RPC, but looking at the tab see that it is 1200 RPM. Reading about these much mention is made of using either 1700 or 3400 RPM motors with 1700 RPM preferred. American Rotary has a 25% off sale but requires use of a Delta wound, 1700 RPM motor for to use with their panels. Wondering what the problem is with using a 1200 RPM motor rather than 1700 RPM? Have read a couple of posts stating the Delta wound provide better balance. Comfortable with wiring & rebuilds but not an electrical engineer. Thanks for any insights.

johansen · Nov 26, 2021

Electrically, delta and Y motors are the same for both use as a rotary phase converter, and as a motor when run on single phase.

However: thermally, they are not the same. The Y motor spreads the heat out over 2/3rds of the stator core when run on single phase.

The delta motor spreads 75% of the heat out across one third of the stator core, the other 25% across the other 2/3rds of the stator core. so they have to be derated more.

So anyhow, regarding a 6 pole motor as an idler I would go ahead and try it, but my estimate is that it will perform as an idler in a similar manner as a 7.5 hp 1700 or 3400 rpm motor.

The reason why is because 6 pole motors typically have 10-20% more leakage inductance due to some fundamental geometry problems.

However, this also depends on the length to width ratio of the coils. your motor only draws 27 amps at 10hp which is not bad at all. I have a 5hp motor of 91% efficiency and 83% power factor that draws 12.3 amps at 230volts. yours is only 10% higher, some of which is accounted for by the 5% difference between 220 and 230volts.

so basically your 6 pole motor assuming the same efficiency as mine, is probably running at around 75% power factor.

the higher power factor makes adjusting the voltage with capacitors easier, but the generated leg is not as stiff.

I have an 8 pole motor and it runs at a power factor of just 0.6, and would be somewhat useless as an idler.

Bill Widmer · Nov 26, 2021

Johansen, Thanks for your reply. I also have a Westinghouse 7.5 HP, wye wound 1745 RPM, 20 amp draw at 220V, 10 amp at 440V, either 1.25 or 1.20 service factor (hard to read). Since the 1200 RPM, 10 HP motor as an idler would perform like a 7.5 HP motor seems like/would I be better off/more efficient to the use this 7.5 HP 1745 RPM motor as an idler? Here is the plate.

CalG · Nov 26, 2021

Slower motors are more difficult to start.

I've got an 850 rpm motor that just about won't start on caps. Does fine on a VFD though.

johansen · Nov 27, 2021

This is really too close to call without measuring it.

Start the idler motor with start caps only. No run caps to push the generated leg higher.

Then take a small motor and measure its starting torque. Compare with the other idler.

Froneck · Nov 29, 2021

I have Delta wound 8 pole Idler, runs quiet! Starts Wye wound 8 pole motors like they are wired 3 phase! 6 pole start too especially the small HP ones. I do not us balance caps nor run caps! Only Capacitor used is to start! My Idler is connected to 3 phase wiring connected to 6 lathes, 3 mills 3 Jig Borers, HBM and smaller motor items like surface grinder and cut-off band saw. It would be impossible to determine the correct run cap or balance cap due to the number of different combinations of motors run at the same time. I have no problems, been running the system for 10 years at about 10 hours per day. Sometimes quite a bit longer! I find it the other way, faster motors seem harder to start!
I prefer the 8 pole motors, I check HGR and buy the 8 pole motors, I have 4 that are new never used! They are on my shelf just in case I need a motor.

rons · Nov 29, 2021

Get a 3 pole switch or contactor and connect a test RPC with the 1200rpm motor. You have nothing to lose except the "sound of silence". I bet that sucker is super quiet.

rons · Nov 29, 2021

Froneck said:
My Idler is connected to 3 phase wiring connected to 6 lathes, 3 mills 3 Jig Borers, HBM and smaller motor items like surface grinder and cut-off band saw. It would be impossible to determine the correct run cap or balance cap due to the number of different combinations of motors run at the same time. I have no problems, been running the system for 10 years at about 10 hours per day. Sometimes quite a bit longer!

A smart convertor would be able to handle that.

In your wiring there is probably a average balance position that could be made.

Froneck · Dec 1, 2021

It might be possible to determine balance and run Capacitors for a motor running at idle and full load. But when multiple motors are connected to a 3 phase line the only way to provide run and balance caps is at each motor! Too much work! I run my shop without any especially 8 pole motors that start as if connected to 3 phase. I have played with ideas to make a good RPC and have made a few that work great for friends. Have a few 4 trace o'scopes and other electronic test equipment and best I ever did was to operate a RPC with a truck fly wheel and a pony motor connected all the time via a one way bearing. However a number of simple ones that used 8 pole motors and Delta wound if I can find one.
I've also helped a few friend connect a RPC that failed to work when they tried connecting balance and run caps!
My advise to a newbie is to forget the balance and run caps until they are more familiar with what they are doing! 8 pole motes are better and quieter than 4 pole which are quieter than 2 pole! I've also found 2 pole hard to start! I keep watching HGR for even slower motors, few were available at price more than I wanted to pay for a used motor!

JST · Dec 1, 2021

Or you can use a buck/boost transformer to do the balancing. That is far less load sensitive.

The idler naturally produces a lower back EMF (on the generated leg) than the incoming voltage. That's part of how it works. All the schemes are better and worse ways to make it equal to incoming.

Froneck · Dec 1, 2021

Yes but being lazy like I am doing nothing for run and balance works plus it's better way for a newbie to get his machinery running especially if he knows very little about electricity and 3 phase.

JST · Dec 1, 2021

Froneck said:
Yes but being lazy like I am doing nothing for run and balance works plus it's better way for a newbie to get his machinery running especially if he knows very little about electricity and 3 phase.

Agree on that. The bigger the idler, the better that works.

jim rozen · Dec 1, 2021

JST said:
Agree on that. The bigger the idler, the better that works.

Bigger that is, with respect to the load motor. My SWAG is the 1200 rpm motor will work just fine as-is.

Froneck · Dec 2, 2021

I agree the 1200RPM motor will work with only start Cap. Just like I use. Mine is quite quiet, so quiet I forgot to shut it down a few times so I wired the control circuit of the contactors to the lights. System will not start unless lights are on and will turn off when lights are turned off. Will always require manual start but only when lights are on.

jim rozen · Dec 2, 2021

Froneck said:
I agree the 1200RPM motor will work with only start Cap. Just like I use. Mine is quite quiet, so quiet I forgot to shut it down a few times so I wired the control circuit of the contactors to the lights. System will not start unless lights are on and will turn off when lights are turned off. Will always require manual start but only when lights are on.

Start capacitor, or pony-start.

Froneck · Dec 2, 2021

I use a start capacitor. Two contactors, one to connect start cap to motor that does not lock on and another that locks-on proving single phase to the motor and connecting 3rd wire to my 3 phase shop wiring. All my machinery is wired so that the manufactured leg goes only to the motor and not to any other wiring such as transformers or contactor coils. It's used only for items needing 3 phase.
My start cap will start the RPC motor in 2 to 3 seconds and will be removed when the start button is released by dropping out the non locked contactor. Of course the off button will drop out the locked contactor as well as lost of power from lights. Also using a locking contactor any power-line loss will also shut the system down preventing the motor from receiving single phase power when power-line power is restored. It was done prior to light line connection and should be done if coupling to shop lights is not used.

Bill Widmer · Dec 2, 2021

Thanks for all your replies. Will work on getting 800-1000 uf of starting caps and accessories to give the 1200 rpm motor a try. It is also the better motor in terms of condition compared to the 1750 rmp. I do know they both ran 10-12 years ago as they were both tested on 3 phase power at that time prior to storage. I do remember the 1200 rpm motor was pretty quiet. Already have a nice panel box.

rons · Dec 3, 2021

If you have the space for oil filled caps then they will not give you problems in the future.

johansen · Dec 4, 2021

rons said:
If you have the space for oil filled caps then they will not give you problems in the future.

They wont blow up but the motor will instead when they are left in circuit.

On the other hand the motor overload should trip. But i suspect a lot of folks run without one.

Disadvantage/Problem with using a 1200 RPM, Wye wound imotor for an RPC idler motor

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