Generator

[Guardiano]

Hi forum members,
I have a diesel engine running at 750 rpm but the aternator is only rated at 1500 rpm 50 Hz. The prime mover is rated at 1 MW and the alternator 1500 k VA. I understand that a multiplying gearbox will do the job but is there any other possible solution ? I understand that underfluxing the alternator will result in reduced voltage and frequency.
Thanks for any comments.
Guardiano

 

[FreddyNurk]

Buy an 8 pole alternator instead of using the 4 pole one you have?

 

[edison123]

For 1500 KVA alternator, minimum requirement of prime mover is 1.2 MW. So, no match there.

Speed of the alternator is what is mentioned in the nameplate. No getting around it.

I have not heard of diesels having step-up gearing. So, whose idea was it to get such a mismatched set?

Muthu

http://www.edison.co.in

 

[ScottyUK]

Theoretically you could rebuild the rotor as a four-pole design, but I'm not at all convinced you could achieve rated output.

 

[waross]

I understand that underfluxing the alternator will result in reduced voltage and frequency.

Under-fluxing will result in reduced voltage.
Under-speed will result in reduced frequency.
1MW at 750 RPM, How old is this engine?
In North America, standard agricultural PTO speeds are 540 RPM and 1000 RPM.
There are generators available with gear boxes to take the PTO speed up to 1800 RPM so that the generators may be driven by the farm tractor.
It should be no problem to find a suitable 1:2 gearbox. A gearbox may not succeed with a one bearing generator.
With 1 MW in and a rating of 1500 KVA your power factor rating will be 67%
While pricing suitable industrial 1:2 ratio gearboxes, it may be well to also investigate the availability and price of suitable 750 RPM generator ends. It may be cheaper to go with a used 750 RPM generator end.
Save the 1500 RPM generator end. It may make a usable synchronous condenser.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Guardiano

I went thru a similar process several years ago when a couple of customers wanted to go from high speed distillate engines to medium speed HFO engines.

One customer went the rewound rotor route, against our recommendation, results were very poor, vibration, high field currents and multiple diode failures, plus some unexpected torsional issues.

I have done three units with a speed increaser, changing out 3516's with 3606's, when it was all said and done with adding the gear box and it's cooling system, a new base that had to be very stiff, a new torsional coupling and required changes to the foundation, the costs and efforts way exceeded what the price of a reworked generator could have been bought for.

On one of the units we added a speed increaser to, we could not use a 2:1 due to critical speeds after the torsional analysis, the end user insisted using his old generator and we had to run the engine at a slightly slower speed with an odd gear box ratio to get 50 Hz. That unit didn't stay in service long.

I'm assuming your 1500 rpm tail ends are two bearing? if not you may have additional install issues.

Not sure what your application and expected service life target is, but at least based on what I've dealt within the past, getting a proper tail end for the unit would be the best solution.

MikeL.

 

[GroovyGuy]

Guardiano
Is this a prime power or standby application? If prime power, then efficiency must be a big concern. Even a 750kW unit will burn through a ton of diesel per day; a few % in efficiency loss will cost a bundle over the long haul.
A good gear-box must be close to the cost of a new alternator. Don't forget the efficency loss through the gear-box as well. The only outfit that I know who does this regularly is Jenbacher who, in North America, offer a line of natural-gas engines operating at 1500 rpm, together with a gear-box, and driving an 1800 rpm alternator. In Europe, the gear-box is not required.

waross
Actually low rpm engines are popular, in prime power applications, due to reduced maintenance and improved fuel economy. For standby applications, not so much. A 900 rpm engine can cost 2x that of a 1800 rpm engine, but some Owners are willing to pay this premium.

 

[iop95]

Diesel engine speed is 750rpm only? can't speed up to 1500rpm? Sure, torque / life will reduce but may be even a temporary solution.

 

[iceworm]

...is there any other possible solution ? ...

Solution to exactly what? Are you saying you want to turn a 4 pole alternator at 750 rpm and get 50Hz?

If so, you already seem to have a handle on that.

... I understand that underfluxing the alternator will result in reduced voltage and frequency.

Speaking with tongue in cheek - or perhaps typing with fingers in mouth)
However, if you don't mind 200V and 25HZ. I work on one type that used a steam turbine as a variable speed source to turn a two pole alternator driving a 2pole motor. Variable speed drive - 1955 technology. (truth)

Or, just plan on using 25 hz. All the motors will turn half speed and put out 1/4 nameplate power.

How about xfm from 200V tO 400V (still 25 Hz of course. Power an inverter to get 400V, 50Hz. Likely none of the rectifier magnetics will have the right V/Hz ratio - likely will need a custom rectifier section.

Okay, back to reality:
What is it exactly you are trying to do?

ice



Harmless flakes working together can unleash an avalanche of destruction

 

[ScottyUK]

iop95,

Do you see the 100Hz output from the generator being a problem?

 

[waross]

I understand that underfluxing the alternator will result in reduced voltage and frequency.

May I comment on this?
Underfluxing will result in reduced voltage. It will not affect frequency.
However, if the speed is reduced, the voltage will be reduced. However the AVR will sense the reduction of voltage and try to increase the field strength to maintain the set point voltage. Full voltage at reduced frequency is not good for motors. They may easily saturate and quickly burn out. Reduced frequency and full voltage will drive the V:Hz ratio too high.
Another issue is burnout of the AVR. When I was young, most AVRs DID NOT have UFRO. An AVR could withstand a normal start, but extended operation (a few minutes) at low speed would destroy the AVR. A combination of high current trying to maintain the setpoint voltage and internal transformers saturating due to V:Hz issues.
Most sets had either a switch to turn the AVR off or some automatic relay to switch the AVR off at slow speeds.
Then came the Under Frequency Roll Off feature in small AVRs. This feature is almost universal now.
This is described as reducing the voltage proportional to reduced speed.
This does not imply reduced flux or excitation. Rather the excitation and the flux are kept relatively constant (Subject to load swings) rather than being increased to compensate for lower voltage.
As the set slows and the frequency drops, with constant flux or excitation, the voltage will drop proportional to the drop in speed or frequency. The flux strength is the same but the rate that the flux is sweeping the stator windings is slowed resulting in proportionally lower voltage.
When the speed drops the frequency and the voltage both drop in the same proportion. The UFRO feature of the AVR holds the flux relatively constant rather than trying to increase the flux or allowing it to decrease.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop95]

@ScottyUK
Sorry, why 100Hz? Alternator is rated at this speed: 1500rpm - 50Hz.

 

[ScottyUK]

iop95,

I mis-read your intentions - my apologies.

 

[waross]

A generator will carry the same current at any frequency, subject to cooling issues. Based on experience with smaller sets, at no load up to about 50% load, the generator will be capable of developing full voltage. This is subject to the UFRO of the AVR. The field strength and the excitation current will be about double at 50% frequency. Somewhere past about 50% load, something in the excitation circuit may be expected to saturate. It may be the main rotating field, it may be the exciter, but we can expect the field strength to be limited.
Once the field strength reaches the limit, further increases in loading and the attendant voltage drop will not be able to be compensated by increasing the field strength.
At less than about 50% load the generator may be able to develop full voltage however at full load we can not expect much more than 50% voltage.
Motors supplied with 50% voltage will deliver about 50% power IF THEY CAN START. The starting torque will be about 25%. However if a motor can come up to speed the current depends on the slip frequency rather than the applied frequency and the motor should develop full torque when loaded.
I suspect that this engine will sit in storage until a suitable 750 RPM generator becomes available.
Likewise I suspect that the generator will sit in storage until a suitable 1500 RPM prime mover becomes available.
Back to the Original Post:

I understand that a multiplying gearbox will do the job but is there any other possible solution ?

The long answer is YES BUT at great expense and/or reduced capacity and/or reduced capability......


Bill
--------------------
"Why not the best?"
Jimmy Carter