150KVA diesel generator to start 75HP pump motor? 1

[suntop]

We have a 75 HP motor(400v,50Hz,FLA=105A)operated through a solid state starter(soft starter),usually working on the utility grid powr supply.
We tried to connect it to a standby diesel generator of 150 KVA rating,the generator failed to start the motor.
The strange thing with this motor that we measured the starting current of this motor on different runs we got different readings of the starting current (they are ranging between 287Amp. to 450Amp.).
According to the soft starter manufacturer the starting current using this type of starters shouldn't go above 300Amp.during the starting period.
What could be the problem?
Is this size of diesel generator appropriate to start a motor of 75Hp?

By the way during this week we connected 3 different sites to standby diesel generators(each genset is 150KVA rating) having the same size of motor pumps and larger up to 90HP one step starting and they all workig very well using either soft starters or autotransformer.
I appreciate your suggestion about what could be done whether with the genset,the starter and the motor itself.

thank you.

 

[busbar]

You might contact the generator manufacturer for the unit’s maximum recommended motor-starting size.

It the genset is applied within its ratings for motor starting, then it may have a voltage-regulator/excitation problem.

 

[suntop]

Thank you busbar for your quick response,according to the manufacturer the max. sarting capacity is 160% of the rated generators capacity(i.e 344 Amp.) for 6 sec.
Do you think this is suffecient to start up the motor through the soft starter?

 

[shepperd]

test

 

[busbar]

I’ll side with sheppard on that. It will depend on the starter settings and mechanical-load characteristics.

 

[jbartos]

Suggestion: If the motor has all winding ends in the motor terminal box, try to start the motor over a manual start-delta starter. This will narrow down causes, if the motor starts properly with reasonable inrush currents.

 

[Marke]

Hello suntop

A genset has two major components, the engine or prime mover, and the alternator. The engine supplies power (KW) and the alternator supplies current (KVA)
If the KW loading is too high, the engine will slow. If the current is too high, the voltage will drop.

With your application, was the engine changing speed? If it was then it was being overloaded. This could be because of the connected load, or because there is a problem with the governor which is restricting the output power.

Often with gen sets, there can be a problem of alternator stability becuase of the harmonics produced by the starter. You mentioned that the current went high, and I suspect that you have a problem with the output voltage regulation. There are two major types of AVR in common usage. One is a peak reading AVR and measures the voltage on one phase only and is resposive to the crest voltage on that phase. This AVR can be sensitive to harmonics and waveform distortion. The alternative is a three phase averaging AVR and this is generally relatively immune to waveform distortion and harmonics.
Some soft starters do not behave well on a high impedance supply (such as a gen set) and when coupled with a harmonic sensitive AVR, chaos can occur.
I would expect that the rating descriped should be very adequate for the job, and suggest that you verify the operation of the AVR on this machine.
Best regards

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion to the original posting marked ///\\\:
Is this size of diesel generator appropriate to start a motor of 75Hp?
///The size of 75HP appears to be within the range the 150kVA generator. Very high LRA of the motor might cause problems.\\\

 

[suntop]

Dear all
Thanks very much for your input.
As an updated information,yesterday we made an assessment and got measurements from the field with motor's stater specialist and he confirmed that such problem is due to a distorted and non uniform starting wave from the soft starterand this was not noticed as the motors were run during the past time on the utility grid power supply.

He says that replacing the existing soft starter with a 4 taps autotrans. starter will solve the problem totally and he assured that the genset will be working fine with this load.
Please advise whether to go or no go with this solution?

Thanks

 

[Marke]

Hello Suntop

Yes, I would agree, see earlier post, that the waveform distortion could be causing the problem, but the real problem is that the AVR is too sensitive to this distortion. I suspect that the AVR is not performing correctly as the loading is not excessive. You may find under and overshoot problems with the auto transformer starter. I believe, from experience, that provided your AVR is performing correctly, and the starter is designed to operate from high impedance supplies, that you should not have a problem with a soft starter on that gnerator.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ScottyUK]

Suntop,

It sounds like your supplier is trying to make the shortcomings of the AVR on the generator into a motor-starter problem. A generator of 150kW should not have any problem with a 75HP drive. Unfortunately there are a lot of generator suppliers who are using cheap AVRs which can't handle modern non-linear loads such as your soft starter.

The auto-transformer starter will work, however it is a an expensive item (in Europe at least) and will probably be less reliable due to the increased number of moving parts than a well-specified soft starter from a reputable manufacturer.

The real problem is the generator AVR, and IMHO it is here that you should concentrate your effort.

 

[Shortstub]

Are you sure the motor is not starting the pump under load?

 

[VicP]

Suntop,

What is the frequency rating of the Soft Start? Most of the newer starters will operate over a fairly wide frequency range, typically 45-65 Hz. However some of the older units often had a frequency specification of 60 Hz + or - 2 Hz. If the frequency went out of this range, the firing angle would not be consistent between the 3 phases, which could lead to your problems. The generator frequency may be dropping when supplying the motor starting current.

 

[suntop]

Thanks to all of you trying to help me identify the problem.
The supplier is still resisting the idea of having a problem with the AVR.Could any of you provide me with the main specs. for the appropriate AVR that is supposed to be used in our case.
The genset is made by Cat.and it has an electronic governor ,anything can be done to help me getting the appropriate specs for the AVR.

Thanks very much

 

[Marke]

Hello Suntop

I have had very good experience with cat generators over the years in NZ. Here, they always use three phase averaging AVRs and there are nvere any problems. If they use a single phase, peak reading AVR then there can be problems, especially if the current is near the maximum rating of the alternator.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[shahidiqbal]

Shahid iqbal electrical
Hello every body

You must be check motor's rotor if rotor is disballance creat problem like this .Maybe bearings are lose on rotor shaft 150kva is enough for 75hp

 

[dadfap]

suntop

Caterpillar in OZ are well aware of the problems associated with SCR type loads on the AVRs.

I used to run VSDs on Cat gensets and the supplier always ensured that the correct AVR was installed.

Unfortunately, that was too many sleeps ago for me to remember the AVR model, but CAT are WELL aware of these issues and will be able to resolve it for you.

all the best

dadfap

 

[jbartos]

Suggestion: The AVR is probably designed for the voltage Total Harmonic Distortion (THD) equal or less than 5%. The voltage waveform might be analyzed to obtain THD. If higher than 5%, a suitable harmonic content mitigation is suggested. The AVR data sheet might have some information pertaining to the harmonic content.
Visit

http://www.basler.com/downloads/1606-991_Rev_F.pdf

and check or follow step 4, as applicable

 

[Shortstub]

What is the supplier's responsibility. Have they complied with the Order Specifications? Were they aware of the starting requirement of the 75 Hp motor at the time of order?

 

[MShaat]

Hi everybody,
Refering to suntop's initial post, let us note the following paragragh
{{{The strange thing with this motor that we measured the starting current of this motor on different runs we got different readings of the starting current (they are ranging between 287Amp. to 450Amp.)}}}.


Now, we have to remember that motors are constant power loads: ie. if the applied voltage decrease the current will increase to satisfy the motor power requirments.

The change in starting current is , most probably, due to momentarily decrease in voltage.
if this decrease in voltage meets the alternator Tech. data sheets, that specifies the voltage drop of value xVolt for yy% starting current, then you have to select bigger genset ( or only bigger alternator), if not

there is aproblem some where:the voltage induced in alternator winding E~ Flux x Speed.

If speed is noticed to derease below 47.5Hz (for 50Hz application) or 58Hz(for 60Hz application), check if this satisfy the diesel engine load acceptance on the engine Tech data sheets,if not, it is a mechanical problem.

If the speed is found to be within limits, but the voltage decrease, check the cuases of excitation problem: rotating diode(s) open circuit, low quality AVR that causes high transient time reponce or affected by harmonic distorted signals , Rotor problems.

hoping this will help