Problem In Starting Motor Generator Set

[AB2005]

Hello

 

We have a rotary cutting machine in our department which has two cutters. Each cutter has a DC motor 85KW. The drive of cutter #1 is powered through main distribution transformer but cutter #2 has a motor generator set (self exciting) from Finland which spec is as following;

 

Motor

110KW, 400V, 196A, 1470rpm,

Generator

300Kva, 415V, 50Hz.

Generator has also a flywheel which weight is about 500KG.

 

There is an information typed on the control panel of gen-set which always confuses to me as

 

“Do not switch off as motor takes 30 minutes to stop & start again”

 

I have no proper documents which help me for understanding the meanings of above mentioned sentence. Actually, this machine is 30years old and owners purchased this machine from Ireland 10years ago. We have only a control diagram of this unit. I have asked to the engineer which had involved in the erecting of that machine and he answered that it means once the gen-set power is off, it shouldn’t start again until it stop.

I have checked the all control circuit as well as the inside Self-exciting circuit of generator but found nothing which resist it to start again if power flickering is occurred.

 

Have any body any idea about above mentioned information?

[AB2005]

Any time when power flickering problem is occurred, we have to wait 25-30 minutes for stopping the gen-set then re-start it. This is a production loss which forced me to find an alternative solution.

I have made an experiment, switch off the gen-set and start after few seconds for checking that what happen but no problem faced. I fear about possible problem which may occur by doing so.

[mariomaggi]

Dear AB2005,

your generator 289 kW @ 400 VAC seems to be enough strong for that load, also in case of repetitive startings.

 

You don't mention a thyristor drive between generator and DC motor.

You don't mention the flywheel maximum speed ad diameter (to calculate inertia) and normal working speed for 110 kW motor.

Anyway, probably they should write:

 

"Do not switch off FOR A LONG TIME as motor takes 30 minutes to stop & start again"

 

I would recommend to command thyristors only when the generator is restarted, to avoid bad regenerative voltage peaks from spinning motor to generator.

 

Regards

Mario

 

 

[marke]

Hello AB2005

 

I assume that the motor in the input side is a standard AC induction motor and has standard starting equipment.

 

If the flywheel effect is very high, so that the effective inertia is high, then there will be a long run time and a relatively long start time.

 

If the motor and starting equipment are standard, there should be no problem restarting while the motor is still rotating. i.e. you should not need to wait for a complete stop before restarting, however, during start, there is considerable energy dissipated in the rotor of the motor. For this reason, the number of starts per hour will be very limited and perhaps the requirement is to limit the starts per hour and to provide a long cool down period before a start.

 

Best regards,

[AB2005]

Hello Marke & Mariomaggi,

 

Thanks for replying.

(1) The motor is a standard Asynchronous motor with a star delta starter.

(2) The gen-set takes approximately 35sec to run at star for taking full speed then change to delta.

When we start the gen-set, it takes approximate 1250A which slowly drops with increasing of speed.

(3) I can’t measure the exact diameter of flywheel as it is covered. It is approx 700mm.

(4) Generator supplies power to a 4 quadrant thyrister drive. The drive not only rotates the heavy duty cylinders of knife but also apply a brake to stop the motor at exact position.

(5) The working speed of gen-set is 1470. However, when knife motor rotates from steady state position to matching machine speed, it takes a large amount of current which is caused the speed variation of gen-set at 1455-1470.

 

If the motor and starting equipment are standard, there should be no problem restarting while the motor is still rotating. i.e. you should not need to wait for a complete stop before restarting, however, during start, there is considerable energy dissipated in the rotor of the motor. For this reason, the number of starts per hour will be very limited and perhaps the requirement is to limit the starts per hour and to provide a long cool down period before a start.

Yes, your point is valid. We don’t take this factor into our account. I think the meaning of “Caution” typed on the panel is may be that the gen-set is limited only 2starts per hour.