Generator starting of autotransformer start 1500 HP motor

[GirlEE]

We're unable to start a 1500hp, 4000 volt motor operating a centrifugal pump under generator power. Pump starts against closed valve that slowly opens in 3 minutes. But we don't get there. Gen manuf.-set CB trips on undervoltage. Generator manufacturer assures us that the gen-set is sized correctly, 1750 KW. Very little other loads. Starter is autotransformer type with taps at 65%. Starter has power factor correction capacitors. Generator voltage before adding load is 4160. New installation, generator manufacture will be out to take a look next week.

Any ideas?

 

[dpc]

My first idea, unfortunately, is that your generator is too small. It's going to be really tough to start a 1500 hp motor on a 1750 kW generator, even with autoxfmr start.

You can try the 50% tap - since this is a centrifugal pump it can probably accelerate on the lower torque. But the generator is really its own reduced voltage starter in this case - have you tried measuring the actual voltage on either side of the starter during starting?

Does the generator have a voltage boost or voltage support option on the voltage regulator?

You also need to question the setting of the generator undervoltage relay. What it is set at, and why? Undervoltage doesn't really bother the generator too much - if your other loads can tolerate the voltage dip, you may be able to lengthen the time delay on the UV relay.

 

[jraef]

Couldn't agree more. The basic gross rule-of-thumb is that you need the genset kW to be AT LEAST 1.5x the HP rating (twice the kW of the motor) when using a RV starter on a centrifugal pump, more if trying X-Line. I'd venture to say that at 3 minutes your autotransformer has long been out of the circuit anyway. They are typically rated for no more than 30 seconds.

 

[LionelHutz]

Also agree the generator is likely too small. It will likely require adjustment of the under voltage relay so it does not trip the breaker during the start.

 

[dpc]

Replacing the starter with a VFD would likely solve the problem, but will be expensive, obviously.

 

[waross]

Rather than a VFD, I would be looking at ways to reduce the excitation during starting so as to let the generator act as a "Poor mans" VFD.
Lose the undervoltage trip.
Connect the motor to the generator.
Start the generator and ramp the voltage up with the speed. There are a number of ways to do this, both with and without the AVR.
You may use one excitation system for starting and then switch to the AVR.
You may use one or more CTs with resistors across them to develop a load dependent voltage that may be used to fool the AVR into reducing the excitation during starting.
respectfully

 

[itsmoked]

Wow waross... Gnarly!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

I would first try what dpc said, set at lowest tap and bypass the UV trip. If this motor is the only load, even a 30-35% voltage dip should not matter for a moment (as long as the contactors/relays, if invovled do not drop out, hopefully they have separate source of back up power). It should recover pretty fast.

How big is the alternator? The alternator at least should be sized for twice the HP rating.

waross's approach would be like last resort, its far too facny for a new installation. At that point adding a generator in parallel would be more attractive or replace the unit with a bigger one.

 

[rbulsara]

The kW requirement during starting is very low ( the p.f. is about 0.2). In the worst case just replacing the alternator with bigger one would be required.

 

[dpc]

waross,

There must be an great story behind that technique.....

Did it happen in Alaska?

 

[rbulsara]

It may sound conuterintutive but sometimes trying to start a motor on higher tap may help. The voltage dip may be bigger but it also recovers faster and motor speeds up quicker.

 

[Marke]

Hello GirlEE

There are two major components to consider when starting a motor on a gen set.
The engine produces KW and the alternator produces Amps or KVA.

If the engine is too small, the frequency will drop, and if the Alternator is too small, the volts will drop.

Both the engine and the alternator, have an overload capacity and the ability of each to start a motor, is dependant on the overload capacities.

At this stage, it sounds as though you are having a problem with voltage drop, rather than frequency droop, so lets concentrate on this in the short term.

There will be an overload in current during start and the minimum current required is a function of the load torque and the motor starting efficiency. You are already starting against a closed valve, so that minimizes the starting torque requirement.
Using an auto transformer starter can reduce the start current provided that it is set up correctly. In many cases that I see, the motor effectively goes DOL because the transformer steps to full voltage at part speed.
I expect that you are going to need at least the 65% tap to get the pump to full speed, and in many cases, the 80% tap is required to get the motor from 60% speed to full speed. i.e. on the 65% tap, the motor runs out of torque at around 60% speed. This is very much a function of the motor starting characteristics and the load torque.

The first question that I would ask is: Is the trip occuring at the same time as the transformer starter switches to full voltage? Try extending the changeover time ans see if the generator runs for longer.

If the generator is tripping while the starter is on the 65% tap, then you have an excitation problem which is limiting the overload characteristics of the alternator. - This may not be a fault, rather a weakness in the system.
If the alternator is self excited, then the chances are that it will have a short term overload capacity of around 120 - 130%. This is very common on lower cost generator sets. If the alternator has external excitation from a PM generator, then it should be able to supply a short term overload in the order of 300% current.
For this type of application, I always specify a three phase averaging AVR and a PM Generator excitation system. These can usually be added if required.

Self excited systems will commonly require an alternator rated at 250 - 300% of the motor rating to start, PMG excited systems commonly require alternators rated at 120 - 150% to start a pump.

see

http://www.lmphotonics.com/genset.htm

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

It is also possible that the setpoints on AVR and/or the governor controls are not set at optimum level. Specially the setting for v/hz response during step loads.

 

[rbulsara]

For what it is worth refer to: faq237-766

 

[itsmoked]

I would also caution that your various starting attempts be far apart or you may end up cooking the motor the generator or both! You won't find specs for either of your machines relating to failed starts, almost starts, partial starts, etc. So you need to look at both your machine's starting duty cycles (starts/hr) and then add 30-50% as I suspect things get pretty warm during your aborted starting attempts.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

as I suspect things get pretty warm during your aborted starting attempts

As you will not from various other threads related to this topic in this forum, what gets "warm" the most during these aborted starts is the rotor and it can take a long time for that heat to make it out to the housing of the motor. Never rely on the housing temperature of motor to decide that it is cool enough to try to start it again. As long as the prime mover continues to run the generator should be getting plenty of cooling air, but watch that motor rotor.

 

[waross]

I see the undervoltage trip as a hinderance.
A VFD was suggested. Rather than using a VFD to ramp up the frequency and voltage, it would be very simple and cheap to use the generator to do the same thing.
With further thought, after disabling the undervoltage trip, if the motor is connected directly to the generator and the engine then started, the underfrequency roll-off feature of the Automatic Voltage Regulator will do a pretty good job of ramping the voltage up as the frequency increases. It should bring the motor up to speed efficiently and with a lot of the advantages of a VFD start.
If this technique is used, I would go direct online and not use the autotransformer starter.
Use a UPS to energise the main contactor in the starter.
A variation of this technique is used to energise Generator Step-up Units. Why not motors?
respectfully

 

[rbulsara]

waross:

Your approach has merit. The only difference in a step up tranformer and starting a motor is the starting torque requirement and some real power. Real power is not a problem in this case, the reactive power and the torque requirement is. At very low voltage motor may just stall or draw excessive current.

I believe by defeating UV protection during the start should help a great deal. I would also try at higher voltage than lower, if necessary. All other cautions about motor heating up mentined above are also valid.

 

[dpc]

Ramping up the motor with generator is a good idea as long the the other loads mentioned in the OP can be shut off while this is done or can tolerate the low voltage & freq.

There may be a fair amount of protection that would have to be circumvented. I agree that that UV relay is probably not necessary unless they plan on operating in parallel with utility at some point.

 

[itsmoked]

Other suggestions:

Got a +500HP motor lying around? Hook it up in a manner where you start it with the generator. Once it's running start the large motor. It will return energy to the system in parallel with the generator. It may be enough to do the trick. You could disconnect it once it has returned all the energy it's going to, based on the current direction to it. For more stored energy add a flywheel to it.

Another alternative would be a pony motor to get the 1500 up to some speed that the generator can then support a start of.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com