Start Motor

When building a large (30hp) converter would there be any advantage to using a start motor. It would seem that if the armature were spinning at half speed when power is applied there would be less starting load.

Comments??

Thanks, Bob W

Main advantage would be if your power company charges using a "demand factor", which means you pay overall higher rates on ALL the power you consume based on your peak demand for a given month -- even if that peak occupied only half a second.

The demand factor can eat you alive if you have a large shop and start a 30 hp idler by throwing it across the line when demand rates apply. A starting motor will help enormously. I'm not sure how many rpms you need to get to; 50% of nameplate may not be enough. Anybody know?

Another option is to use a 'compensator' or reduced current starter. One method is to put a large resistor in series with the motor during starting. Forrest Addy wrote me he once built a 75 hp rotary converter to run a sawmill, and used stove heating elements wired together to make a gigantic starting resistor. The beauty of this method is that YOU are in control of the size of the resistor, so you can cause the motor to start as slowly as you like, and cut down on the peak demand. You cut the resistor in and out during starting with contactors, just as with starting capacitors.

You can also use a buck autotransformer to cut the starting voltage during starting. If you can find a good used 2:1 dry type general purpose transformer, you can wire it as an autotransformer to cut the voltage 50% during starting.

Finally, a word about being a nice neighbor. Even if your power company doesn't come knocking on your door about deep line sags from throwing your 30 hp motor across the line, your neighbors may find their electronically-controlled ceiling fans change speed or turn on/off, and their garage door openers open or close without warning. To say nothing of the annoying light dimming which can affect all houses connected to your transformer.

I know someone here in Indy who built a 30 hp rotary using start caps and a large commercial resistor. He told me if he ever builds another one, he'll use a starting motor with a centrifugal clutch. And we don't have demand rates here.

I'm still wiring all my pieces together (actually working on the panel layout with all the pieces I have).

What's the likelikood that starting my 15HP idler will cause voltage sags sufficient to affect the neighbors? Is it worth removing the shaft to reduce the innertia the motor has to overcome during startup?

Rob, a 15 hp idler should cause no problems to your neighbors. In your own house, you'll notice light dimming, similar to when a big outdoor A/C starts up, for about half a second. As the portion of the shaft protruding is but a small fraction of the total armature mass, there's no benefit to cutting it off. Some do it for safety reasons, however, but of course you could never use it for anything else if you did that.

My concern is mostly with line sag when starting more than cost. I was using 50% speed so you could use a 3600 start motor belted 2 to 1 and not go over the rated speed when the 1800 converter is runing.

Bob,

I misunderstood you; I thought you meant that the converter motor would only be spinning at 50% rpm when you would apply the single phase to it.

If you want to avoid line sags, a start motor is the way to go.

A start motor system which is integrated with the converter's potential relay, and which employs a solenoid (or other appropriate means) to engage and disengage the start motor, and a slave relay to start and stop the start motor, would be ideal.

Peter,

That would be nice -- it would offer the convenience of using standard start cap circuit. The only motor start rotary I have ever seen was for a 30 hp idler; it had a 3 hp single phase capacitor start/capacitor run start motor with pulleys, a big belt, and a huge spring-loaded (12 inch spring, about an inch in diameter) lever which moved the start motor in and out. It had a very nice guard over the belt for safety, which also prevented the belt from coming off the pulleys while disengaged. It squealed a lot for a couple of seconds, but it did work very well.

Do you think a solenoid setup as you describe could do the work of this lever?

Re: Solenoid starting mechanism ...

I think it could be made to work, if the mechanical advantage of the linkage and the solenoid throw length relationships could be worked out.

For a commercial quality unit, I would prefer the (near) absolute certainty of an overriding clutch.

Incredibly enough, *very* large IPCs (AKA, RPCs) have been employed for many years for supplying high-powered FM radio transmitters, which, of necessity, are often sited in very isolated locations, such as mountain tops, where only single-phase power is available, and with capacities of 20 kW, and higher.

In a commercial broadcast station, just about the only equipment which is likely to require three-phase power is the transmitter itself, and usually those with an output power of 5 kW (and, for very old transmitters, with an input power of perhaps 15 kW), and above.

One AM transmitter I am familiar with has an input power requirement of 86 kW, and a 125 to 150 HP "idler" motor (AKA, "pilot motor", in the patent literature) would likely be required.

The mind boggles at the thought of constructing such an IPC.

[This message has been edited by peterh5322 (edited 03-27-2003).]