Power Saving during Generator operation

[jimmynora]

We have a slip-ring asynchronous (induction) Motor than runs a generator needed for electrical tests. We are trying to improve the energy efficiency of the set-up.

The generator spends most of the time running idly and the motor has to keep running during this process to maintain the speed. Any suggestions how we can save/reduce this idle-running power of the Motor-Generator set-up ?

 

[russ_watters]

Use a UPS and shut off the generator when the load is low, a la a hybrid car.

 

[jimmynora]

The load is the device we are testing... and the duration of a test is usually 200ms. A series of such tests are conducted during a time interval for example 2 hours in which the motor-generator is most running idly.

The UPS idea means we should be storing the energy in batteries ? That means changing the circuit at the generator end switch on/off the battery charging right ?

We turn off the motor if there are long intervals (more than 7minutes) between two series of tests and the generator is allowed to run down to a low speed and then fired back up for the resumption of tests.

My question relates to storing/saving/reducing energy during idle running .

 

[david90]

1. You can try to mechanically disconnect your AC motor from your generator when idling by using some sort of clutch system. This will reduce load on your AC motor.

2. You can use a VFD to reduce speed to your AC motor during idling and then ramp it back up for testing. You can do this automatically thru programming with most VFDs.

 

[jimmynora]

I am already trying to implement your second suggestion in a different way. We can not reduce the speed of the motor during normal operation because then the interval between the test series would be much greater due to the time needed to ramp up the generator speed plus more energy would be drawn by the motor side of the circuit to provide the additional torque.

A flywheel is also a suggestion I am looking at but I am sceptical if the added load on the generator will cause a huge power spike at the motor end to drive the additional load.

 

[NascentOxygen]

With no load on the generator, then the motor is developing only sufficient power as is necessary to overcome windage, bearing friction, and electrical losses. A flywheel isn't going to reduce these. It will just contribute a bit more loss of its own.

The only way a fly wheel could help, is that it would/may allow you to replace the motor with a lower hp motor. If you used a smaller/lower power motor without a fly wheel, then the sudden application of your test load to the generator may cause a significant and undesirable drop in speed of the generator and consequent drop in output.

 

[david90]

You are using a VFD right? If so, you might want to look into reducing the "carrier frequency" of your inverter output to your AC motor. Reducing the carrier frequency will increase motor efficiency but it will also increase the motor noise.

Why do you have to use an ac motor coupled to a generator to generate power? Why not just use power straight out from an outlet? Unless the generator is the unit that you're testing...

 

[kneeslider]

I assume you want to run the generator at some constant speed during the test interval (with load or idle). Tell me more about the setup. Does the motor run much slower than the synchronous speed during idling? I.e, do you turn up the rotor winding resistances during idling? If so, then your biggest losses are resistive in those resistances.

If you are already using a VFD then you shouldn't need to use the rotor resistances at all, just short the rotor windings- you are using the motor as a regular induction motor. This is the most efficient you can get with VFD and your current motor generator setup.

Alternatively, you may want to use some sort of PWM circuit with the rotor windings to control the rotor winding currents. This will probably be cheaper and simpler than a VFD setup. The efficiency should be same or better than VFD.

If your costs permit, the most energy efficient setup would be to replace the motor with a modern permanent magnet motor and variable speed drive.