Generator for 75 H.P. motor 2

[Lazerdog]

Hey everybody

I've got a customer who needs a generator to run a 75 H.P. electric motor at prime. The electric motor is rated 440 V, which I assume means he needs a 480 V L-L. I'm looking for a product to purchase that would reduce the size of generator that he'd need. What would you guys suggest? A soft-start or a VFD?

Can anyone link something to point me in the right direction?

Thanks

 

[dpc]

http://www.cumminspower.com/powergenerationproducts/systemtools/home.jhtml?SMSESSION=NO

Cummins Power has generator sizing software you can probably get for free. Or call local Cat distributor. They do this for a living.

Basically, you'll need an absolute bare minimum of 75 kW if you have a soft-start. A 100 kW would be much safer.

If the load is a centrifugal pump, you might be able to start it across-the-line with a 125 kW, but 150 kW would be better.

Also, just because you have a 75 hp motor doesn't mean the driven equipment is going to use 75 hp. Could be less (could be more!).

 

[jraef]

In my experience 75kW may be a little small to use a soft starter, the general rule-of-thumb that has never failed me for stand-alone soft starter applications is:
Minimum generator prime kW rating = 1.5 x largest motor HP (or 2 x largest motor kW).

So a 113kW prime rating would be ideal (75 x 1.5 = 112.5), which will typically be sold as a 125kW standby rated genset. Keep in mind that if other significant loads must be running before the largest motor can be started, it gets more complicated.

A 1:1 ratio as dpc mentioned above would be adequate for a VFD, but the VFD will cost a lot more than a soft starter if buying strictly for starting and probably more than going up one size in the genset as well. If reduced flow has other benefits for you however, by all means use a VFD. That may be worth looking into even if you had not thought about it previously.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Marke]

I agree with jraef, 75KW will be too small.
During start, you need to have an engine rated high enough to provide the total starting KW draw which includes all the losses.
The genset does have an over load capacity, but that is usually insufficient to start the machine on it's own, plus a 75KW motor needs more than 75KW to run. It has a shaft power of 75KW and an electrical input of around 82KW plus there will be up to 5% cable losses giving an electrical load of around 88KW.
If you have a cable loss of 5% and a 400% start current, then you cable loss during start will be 4 x 4 x 5% = 80% or 65KW. Then you have the slip losses and stator losses to add on as well as the shaft power.

Have a look at

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

were there is a little bit of a guideline.

If the Alternator is fitted with pmg excitation, you can get a good overload of the alternator If it is not, then the overload is commonly limited to around 30%.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

The smallest generator that I have seen succesfully start a motor of that size was twice the motor size. This was a dedicated generator-motor installation. The generator slowed down and the Under-Frequency-Roll-Off feature of the Automatic Voltage Regulator dropped the voltage. It started the motor quite well but I wouldn't have wanted to see any other load sharing the power.
I agree with jraef's minimum sizing.
For shared loads I have had good luck using 300% of the largest motor plus 100% of all other loads liable to be online together. I have done quite a few residential installations with a lot of large A/Cs. with this sizing and not had problems. I have seen a few generators sized smaller by others that do have problems starting big A/Cs.
respectfully

 

[itsmoked]

I would point out that most soft starters are really lousy for generators that are marginal in that they often monitor and complain about frequency sag and then trip off before fully starting the load motor. They often have this problem when specifically used as a "generator stretcher".

So if you use one, be aware of this possible tripping point and find one that you can defeat this so called "feature" or has a very wide frequency allowance.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Marke]

Hi Keith

So do modern contactors. If you really want to marginally size a genset, just Direct On Line it via a switch and let the alternator drop in voltage provide a reduced voltage start, and the sag in frequency of the engine pick up th motor at part speed. - the inertia of the genset can get transferred to the load and they will meet at part speed and then ramp on up, but if you have any electromechanicals, they will vibrate, letgo or weldup.

Bottom line, design the system correctly and it will work with a soft starter, or a larger unit for a full voltage starter. Two points to consider, use a three pase averageing AVR and a PMG excitation system when you are controlling motors, and many of the problems disappear.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

Also review faq237-766

 

[dpc]

Yes, I also agree that 75 kW would be small for a soft-start. I was thinking VFD but typed "soft-start". Thanks for correcting me.

Dave

 

[itsmoked]

Thanks Mark, good point about the contactors.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Not all soft starters have trouble with generator supplies, but I do agree that some will. The old phase-locked-loop Saftronics firing circuit and all of their offspring have that problem. The old Vectrol firing circuit used by Motortronics and all of their offspring does NOT have that problem, it self-adjusts the firing circuit to match the drifting frequency.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sacem1]

Most of you guys in this thread are electrical, I'm mechanical so I look at things from a different point of view, if the 75HP motor is permanently going to be hooked up to this gen-set and the idea is to downsize it as much as possible then it is not going to be used for anything else, could you connect your load directly to the motor via a clutch (which usually is supplied with most stationary power engines) and forget the generator, controllers, starters AND motor. Just use the thermal to mechanical power transmission instead of thermal-mechanical-electrical(gen)-electrical(mot)-mechanical power use.

Maybe it could work unless the power is used in an equipment that is not able to accept the motor directly due to noise, exhaust, heat + that it creates and you want it clean with a electrical motor, in that case can you fit a centrifugal clutch between electric motor and machine it will allow the electric motor to start without load and then apply the load.

I know I will get busted in here but just thinking

Regards

SACEM1

 

[itsmoked]

Not a bad idea sacem1. Usually some electrical tripping point like the system also needs electrons (in bulk) to control auxiliary stuff too wrecks that idea. But you make a good point. Perhaps the control stuff could be supplied by an inverter running from the engine's alternator.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

gotta give you a star for that Keith.
I had a customer trying to find a generator. He had a diesel engine and an ice machine with a belt drive compressor. My advice was exactly as you suggested. I'm expecting to get a call from him someday to help him start it up, but he has a lot of work and this is a side project so it won't be soon.
Respectfully

 

[jraef]

sacem1 (and itsmoked to a certain extent),
It depends on the application. It's usually all about torque response. Engines are not as good at supplying torque for step load changes as electric motors are, so a directly coupled load may slow down too much to make the engine practical.

Think of this analogy. You are cruising along at night in your car with a damaged standard transmission that has no lower gears and you get to a steep rise in the road that you did not see in time to anticipate by pressing the accelerator a mile in advance. Even though you press the accelerator now, the engine slows down considerably before the additional fuel can provide more torque from the engine, but now you are so far behind your torque-speed curve that you have insufficient horsepower to maintain speed and ultimately stall the vehicle. Normally you could have shifted down in gears to get into a better power band with the engine HP you have, but in this case you can't. With an electric motor, extra torque is available (to an extent) to keep a load at a constant speed while only passing a small percentage change to the engine driving the generator, well within its normal torque speed curve range.

Depending on the application then, to properly compare a direct drive engine to an electric driven system you must also include a transmission and torque converter system and then your cost goes extremely high compared to generating and running electric (clutches are cheaper, but add a wear part to the equation).

Certainly if the application involves a variable torque load that never has a significant step load change, such as an irrigation pump drawing water from a shallow well or canal, then a direct coupled engine could work. Notice I said variable torque load also, because if it is a high inertia load the direct drive may not allow the engine to accelerate without a clutch / torque converter of some sort either. The OP however never stated the nature of his load, so we can assume that his reasons for wanting an electric motor are already accounted for.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sacem1]

Thank God I had my helmet on so the blows where softened by it.

I have used the idea here just for the example you mentioned pumping water from a shallow pond to another (shrimp breeding ponds) and we used direct conection for the pumps.

Regards

SACEM1

 

[waross]

Hi jraef
Those are good points. I am not contradicting you but I would like to suggest a few factors in the field that affect your suggestions.
If a generator is used the torque load will be passed almost directly to the motor. However in order to get the motor started in the first place the engine and generator had to be a minimum of 200% of the load.
The governor on a generator responds very well to load changes and holds the speed very close to the set point. The same governor may be used with a direct drive.
The point is, a 75 hp. electric motor is probably connected to a generator driven by an engine of 150 hp or more.
There are savings to be made in the cost of an electric motor and an alternator as well as in using a smaller sized engine.
The actual size of engine used for direct drive should be selected with consideration of the points you have raised.
respectfully

 

[jraef]

No contradiction waross, your points are valid. I have lots of experience in replacing diesel engine drives with electric motors and gensets. Here in California they are clamping down on environmental permits to operate engine driven equipment by limiting everyone to 1000 hours/year. Most quarry rock crusher applications that were engine driven have been replaced by electric motors, but because of the unreliability of power delivery in California of late they must have diesel backup generators to remain productive. I can tell you from that experience that the electric motor necessary for most of these high inertia applications is smaller than the required diesel, and everyone is amazed at the improved performance. Mind you, the losses through the generator are greater than when directly coupled, so your estimation of a 150HP engine for a 75HP motor is right in line with my experiences, but you get rid of the torque converter headaches because that governor is a much much simpler system, and the genset is no longer prime anyway. Again, low inertia variable torque loads like pumps, no argument. Then again, I have yet to see a deep-well submersible diesel engine!

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

I have seen lettuce fast freezing units, concrete pumpers, and Nat.Gas to liquid methane converters all run directly from an IC engine. So some odd loads can manage it. But I totally agree with the your points jraef.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[hogdawg]

I noticed you guys give a lot of thought for worst case scenarios. I agree with that but I would think along the lines (motor starting and sizing generators) with the technology that I see daily, i.e., permanent magnet generators. We have a lot of customers that are not aware of the motor starting capabilites because of the current boost (I know, the old TAB excitation support systems). Newage and people like that offer them as an option. This option will prevent the field from collapsing unlike a regular brushless generator. Check it out!
Best Regards