Offline generator losses

[ccdubs]

Hi all,

The following is some manufacturer data for the losses of a synchronous generator at different loads. What I want to know is what power do I need to rate a pony motor that will drive this generator to synchronous speed?

Obviously its continuos rating needs to cover the friction and windage components. I assume as the genny is off line it will also have to supply the exciter load and therefore the rotor copper losses. Does the motor need to supply the iron losses? I guess it will as even off line there will still be hysterisis losses.

Using the following information I would choose a motor rating of greater than 7722W. Please ignore the fact that the motor is accelerating a considerable inertial load as I will be using a VSD.

I am assuming that 0% load in following table is when the generator is online but with no generating torque applied.

Load 0% 25% 50% 75% 100%

Fr+Wind 2450 2450 2450 2450 2450
Iron 4930 4930 4930 4930 4930
Str Cu 0 664 2860 7205 14723
Stray 0 444 1776 3995 7102
Rtr Cu 311 1035 1869 2910 4255
Exciter 31 103 187 291 426
Total Watts 7722 9626 14072 21781 33886

 

[rbulsara]

VSD or not, the motor still need to develope the torque to rotate the machine and bring it to near sych speed. Motor HP is still related to the Torque times the speed and torque required is related to the inertia and acceleration time.

Search the topic for motor sizing here.

 

[RAMConsult]

You didn't state the rating of the generator or the type of excitation system but in general there are two different operating conditions that need to be examined; i.e., accelerating and steady state.

During the acceleration period everything should be de-energised, therefore the only losses will be bearing drag, fan losses and windage- the only magnetic loss will be due to any residual magnetism left in the rotor and/or stator. All of these would be lumped into the 2450 watts, any motor over 5hp could overcome these losses. The determining question is how fast do you want to get to synchronous speed- starting a load with that much inertia from a dead stop will require a lot more torque than if you give it a push to get it going.

Once you are at synchronous speed you would energize the excitation system with minimum field current, increase it to achieve rated voltage, check the phase rotation and match the line voltage then synchronize. At this point you could disengage the pony motor because the line will keep your alternator at line frequency, in effect it is now a synchronous condensor. Sensitive line metering will show you the no-load iron and copper losses which will vary with the excitation level.

 

[ccdubs]

Thank you both for your help.

The excitation system is self exciting, i.e., the field current is generated by a small PM generator on the rotor. I have no control of when the excitation will start.

From the posts the only question that remains is: when at sync speed and NOT ONLINE, does the pony motor need to supply the energy for the iron losses? It must as that is the only source of input energy.

I think I have answered my own questions. What confused me was the 0% load column of the table. Is this offline or online? As there are 0 Cu losses I assume this is an offline rating.

I have an 11 kW motor available which I know will drive the generator to sync speed (it has been tested DOL and takes about 10sec to bring upto speed). I now want to purchase a VSD and am hoping to find a 8 kW VSD to save some money.

 

[aolalde]

You are right. The only source of energy at that time is your pony motor. When the excitation turns on, that motor should provide the generator iron losses, exciter power and rotor copper field losses (plus all mechanical losses at full speed, which include the main motor or engine if it rotates by the pony motor).

The generator manufacturer data at zero load ( no load) could be offline or online as far as no current is drive in or out the generator.

 

[jbartos]

Suggestion: Probably, the manufacturer might have dealt with this type of application. It may have a solution. The pony motor size may be fine-tuned by computer simulations.