Generator for 75 H.P. motor
Generator for 75 H.P. motor
(OP)
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
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





RE: Generator for 75 H.P. motor
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!).
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
RE: Generator for 75 H.P. motor
Dave
RE: Generator for 75 H.P. motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator for 75 H.P. motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
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
RE: Generator for 75 H.P. motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Generator for 75 H.P. motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Generator for 75 H.P. motor
Best Regards
RE: Generator for 75 H.P. motor
Some have Permanent Magnet Generators fitted and some don't.
My observations;
The PMG fitted sets have less voltage dip and less flickering of the lights when starting a large A/C when the size of the A/C is well within the capability of the set.
Starting larger motors, where the set is marginally sized will often drag down the frequency of the set. Once the frequency drops more that 3 to 5 Hz. on a modern Automatic Voltage Regulator, the Under Frequency Roll Off feature becomes active. The UFRO starts to drop the output voltage in proportion to the drop in frequency. Even though the PMG is supplying adequate voltage to the AVR to maintain the output voltage, the UFRO feature of the AVR is dropping the voltage to shed load and give the engine a chance to get back to speed. Most AVRs now supplied on new sets have UFRO. As well as shedding momentary overloads, AVRs with UFRO do not burn out if the generator is operated at reduced RPMs. Any one who has been around older diesel gen-sets will remember the warnings to turn off the AVR before stopping the set. Most of you will also remember changing AVRs when the operator forgot to turn off the AVR when he ran the set at low RMs for some reason.
I have found PMGs to be more useful starting smaller motors that big motors. (Relative to the size of the set.)
Once the motor starting load slows the generator more than 3 to 5 Hz. the PMG becomes less helpful.
If a generator is not powerful enough to start a motor with a standard AVR it will not be able to start it with a PMG.
That said, I spec. PMGs whenever The customer is willing to pay for them.
respectfully
RE: Generator for 75 H.P. motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com