Can a gas engine be used to startup a larger electric motor? 2

[cdcosta]

This is outside my area of knowledge...

We have access to a large compressor and electric motor, which we want to use in an application. However, the motor is not "sized" for this particular application. The start time for motor coupled to the blower is greater than the manufacturers' recommended start time.

One idea is to use a "start-up" motor to get the blower/ motor assembly upto some intermediate speed, where it then is uncoupled (magnetic coupling) and the main blower motor is energized. This "start up " motor could be a gas engine or an electric motor.

Does this scenario seem plausible, or is it a far fetched idea?

 

[motorsdirect]

HI
HOW ABOUT TOTALLY OFFLOADING THE COMPRESSOR ,,ie.A DECOMPRESSI0N DEVICE, TO ALLOW THE MOTOR TO GET UP TO FULL SPEED WITH VIRTUALLY NO LOAD!! THEN IT WOULD BE DOWN TO WHETHER OR NOT THE MOTOR CAN DRIVE THE COMPRESSOR INSIDE ITS FULL LOAD CAPACITY.
JEFF

http://www.motors-direct.co.uk

 

[HCBFlash]

Sounds like a good idea to me! You'd probably get best results bringing it up to near full load speed for the elec motor tho.

Are you willing to accept possible tripping of the motors overloads or branch circuit pretection though? If you mechanically overload the system it might happen, and then a fairly complex startup sequence will be needed again.

I'll bet you won't have much trouble with it though.

remember: An opinion is only as good as the one who gives it!

 

[BJC]

Electrical Apparatus Magazine had an artical within the last year on stating a large crusher motro with a gas powered pony motor. The pony motor was a pickup engine (350 chevy) still on the frame. The cab, engine, transmission, rear axel etc were all still on the frame. to start it the operator went to the cab, started the truck ran the crusher motor up to speed and then started the electric motor. I think they had a hydraulic cylinder to raise and lower the truck.
Rube Goldberg but it worked well enough for them to put it in a magazine.

http://www.barks.com/eacurr.html

 

[jraef]

What you are asking about is referred to as a "Pony Motor" start. If you search for that term you will find lots of references and discussion of issues. In a nutshell, there are several main concerns:

Timimg the disconnection of the Pony Motor. You dont want it to become an additional load on your main motor.

Cost. You have to have an engine big enough to do the job, a transmission of some sort (because you will not be able to start it and accelerate it directly), a magnetic coupling to the electric motor or the load, a fuel source (and the associated safety issues that come with that), and a control system to do it all properly.

Electrical issues. You will still have inrush current and associated torque spike from the AC motor when you energize it, just not as much.

I'm with motorsdirect on this one however. Did you double check that it can start with the compressor (blower?) completely unloaded? If everything is open to atmosphere there should be just mass and a little friction to accelerate.

For that matter if you are going to buy a magnetic coupling anyway, put that on the electric motor and start the motor uncoupled!

"Venditori de oleum-vipera non vigere excordis populi"

 

[cdcosta]

BJC... Would you know which issue that article would be in. It sounds like an interesting read.
I've looked at the monthly summaries at Electrical Apparatus
website, but there don't refer to the article in question.

Thanks

 

[BJC]

cdcosta
I don't ave that issue any longer. I have tried searching their website and so far have not found it. I'll keep looking. You mignt try writing them and asking them.

There was one nuke plant that use an electric motor to drive a pneumantic tire to start primary coolant pumps. I rember looking at the drawing and wondering what they would do if it was flat. There was no instrumentation to meonitor pressure if I remember correctly.

 

[cdcosta]

Thanks BJC... I was able to find that issue (March 2004)

 

[mc5w]

A pony motor that is electric could also be left connected to the main motor as the friction losses of a deenergized pony motor would be rather small.

Your pony motor would need to be Design D which is high rotor resistance/ high starting torque/ high slip. The breakdown torque of a design D motor occurs at zero speed and is about 300% to 400% of full load torque. Since the pony motor will only be used for starting duty the poor efficiency of a desigh D motor would not be an issue.

You will also need a belt or coupling that can take the 300% to 400% starting torque of the design D pony motor. For a pony motor that is 1/4 the HP of the big motor a coupling that is rated for the big motor will work.

This is essentially a semimechanical version of a single would rotor motor. In both cases you start with high rotor resistance and run with low rotor resistance but the pony motor uses easier to obtain motors. A lot of design D motors are built for wye-delta starting so that you can avoid a heavy belt or coupling if you add in a wye delta starter.

You would still need an electric valve that decompresses the unit so that the pony motor would not be working against
back pressure.

Using all of the ideas mentioned so far you can have 5 step starting:

1. Open decompression/unloading valve.

2. Start design D pony motor on wye connection.

3. Transition pony motor to delta connection.

4. Energize bif motor.

5. Close decompression valve and turn off pony motor.

This would allow you to use a belt or coupling rated at 150% of the horsepower rating of the pony motor.

Mike Cole, mc5w@earthlink.net

 

[mc5w]

Oops, I meant big motor, not bif motor.

 

[RRaghunath]

The detailed procedure for starting the compressor using a pony motor and specifications of pony motor given by mc5w are interesting and think should definitely work. However,I thing the gas engine starting is also a workable option form what I have seen in gas turbine based power plants.

Gas turbine generators are run up using three methods.

- Pony motor starting
- Diesel engine
- Powering the GTG through Static frequency converter (SFC)from grid supplies.

While "diesel engine coupled to the shaft of GTG" method is used for smaller machines (such as Frame 5 machines of GE that is, about 20MW), the pony motor and SFC methods are used for all sizes.

Hope the above is of help.

 

[gsimson]

What is the rating of the motor? Many solutions are possible. If LV applications VVVF is one of the simplest & economical alternative.