Motor- Alternator-Coupled To External Primemover

[IndiaElecEngg2010]

Hi All,

I Have a tractor which is coupled to a 32kVA alternator which is meant to supply three nos. 7.5HP motors used to extract water from ground for our fields.

When we accelerated the RPM of tractor to get desired water we found that all the motors burnt and the shaft connecting tractor and alternator was also broken, what necessary protections will be needed to protect the system (mechanical and electrical) in future.

 

[itsmoked]

You need the usual motor protection which will trip out your generator when an overload occurs.

You need a system to control the generator's output voltage to the correct rated voltage. ANd prohibit the motors from being driven if the voltage is too low.

You need a frequency monitor to make sure the generator is putting out the correct frequency power. Too low and the motors will burn up. Too high and the motors will burn up.

You should have some ground fault protection to keep from also burning up and breaking the humans that are using the system.

If there is even the remotest chance that the wires could be hooked up differently between usages you need to include phase detection to make sure the pumps don't get run backwards.

You should also have the system set up to start the motors individually with time delays between them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

First of all, you need a constant speed engine. Get your tractor engine fitted with a constant speed governor.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Motors are generally designed to operate on either 50 or 60 Hz. The generator speed is regulated by a governor to typically 3000, 1500 or 1000 rpm for a 50Hz set or 3600, 1800 or 1200 rpm for a 60Hz set, with the speed depending on how many stator poles the set has. It doesn't sound like you have been doing this, which is why you've broken things. If you speed the motor up then it will draw more power. I'm sure you can see where this is heading...


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Well, you can't just run the tractor engine as fast as possible hoping to pump more water, as your failed experiment has proved. So, the best protection is a better operator.

Typically, tractor driven gensets have indicators to show they are operating at the correct speed. The throttle is a level that is manually set. A retrofit to automatically control the throttle would be costly and complex. So, honestly, if you can't trust the operator then the second best way to protect the system would be to install a solid very hard to tamper with throttle position stop that does not allow another experiment.

On the electrical side, you should look at motor overload relays and a over-frequency relay to turn-off the generator power if someone tries that again. You may also have to install a breaker with a trip coil in it so it can turn off the output.

 

[rbulsara]

Never too late to learn to do the things in a professional way.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Unless you win the Darwin award! grin

Most tractors that I see have quite a good governor. It will hold a set speed quite well. Most tractors have a notch or mark on the throttle quadrant marked P.T.O. That is where the throttle should be set for the proper speed at the Power Take Off.
Check that the ratio of the reduction gear ahead of your alternator matches the PTO speed.

Bill
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"Why not the best?"
Jimmy Carter