Realising an Automatic Switch for Test Circuit

[papachulo]

In our testing laboratory we perform electrical tests on Circuit Breakers and other devices. We take the required electricity from a Generator that is powered by a Motor.

The test circuit includes the load (electrical device to be tested) and other needed resistances and reactances with the generator as the power source.

The issue we have is that the motor is used to power up the generator and keep it running at a constant speed during our testing procedure. Every instant of a test usually last about 300ms and we have about a series of 10 such tests within a test procedure. There is short time between every instance of a test within a test series.

So far we have only a manual control process, that first a worker turns on the motor and when the generator is up to speed he signals the engineer that the machine is ready, so the engineer can proceed with tests. When the engineer wants to turn off the generator he has to signal the worker and he turns off the motor.

As you can see a lot of energy is lost just keeping the generator running even when no energy is being take out.

What I am trying to do is automate this process and develop a sort-of button that the engineer can press when he wants to start the test.This should result is energy taken out of the generator for the test exactly when the generator is up to speed, then circuit turned off (motor circuit and test circuit). For the next instant of the test the motor-circuit turned on once again and when the generator is upto speed energy again take out... and so on till the end of the procedure.

This should help us reduce our energy bill since most of the generator's life is spent running idle.

My question: How can I realise just a switch that turns on/off the motor and turns on/off the test circuit when the generator is ready (up to speed)

NOTE: I am assuming the energy taken to speed up the generator from a bit below the nominal speed is not much greater than the energy needed to keep the energy running at a constant speed.

 

[jraef]

How is the Prime Mover motor being started now? If it is a Magnetic Motor Starter, it's just a matter of adding a simple remote start/stop circuit and maybe a timer tied to the "test" button that shuts it down 1 second after the test. If it is being started by a manual device, you will need to add a magnetic motor starter to the circuit.

But you must first also evaluate the length of time in between tests in a series. Your assumption that it takes the same energy to re-accelerate as it does to idle is false, it totally depends on the idle time and the rate of speed decay (friction in the load for the most part). kW consumption in an idling motor is only a small fraction of the loaded kW. If it's only a few minutes or less between tests and it takes 5 or 10 minutes to spin down, I would leave it idling.

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[papachulo]

The motor is connected to the main grid and it drives the generator. The time period between each instance of a test (within a test series) is pretty short... around 30seconds perhaps. It takes the motor about 7minutes to accelerate the generator from 0 to 3000 RPM.

The worker has to manually turn on the motor switch to get the generator up and running.

Having a switch between the Motor input and generator output that switches on/off the motor circuit when needed by the generator is something I imagine. I believe a solution involves not only a simple load-sensing switch.

 

[davidbeach]

You'll waste more energy than leaving it running. You'll also get more life out of the equipment if you just leave it running.

Measure the power into the motor (not the current) and you'll see that the power is very low while idling. The current may not change a whole lot, but the power factor, and therefore the power, does change considerably.

 

[papachulo]

How will we waste more energy ?

The motor runs around 3 hours every day and the total time of all the tests summed up is around 15 minutes spread out over that interval.

I have been looking at the power and current curves at the motor circuit and they both look the same i.e have the same shape.

 

[LionelHutz]

This sounds suspiciously like some posts that were here a few months ago. That system was described as a wound rotor motor with resistors for starting, running at 3000rpm and used for testing circuit breakers. I my suspicion is correct, then the same recommendations posted at that time still apply. Here was one of a few threads concerning that system.

http://www.eng-tips.com/viewthread.cfm?qid=299896&page=1

Overall, what you are asking for is a simple control system that any capable electrical engineer could design. We can't help you design a system because we don't have the details of what you presently have. Also, this is eng-tips, not eng-spoon-fed-solutions so we tend to give some tips and ideas here, not a complete solution.

 

[IRstuff]

If there is no generator electrical load, then the generator mechanical load must reduce, hence the motor mechanical load is reduced. When the generator is presented with an electrical load, its mechanical load is increased, and so is the motor's; that's just conservation of energy.

TTFN

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