Sizing standby generators for large reactive loads

[Jon456]

We have two large storm water pumps; one pump is fairly new, the other is probably >50 years old with no nameplate. The pump & motor specs are as follows:


Pump #1
Size: 18" x 10', 8,200 gpm
Motor: 40 HP, 3 Ph, 230 V, 100 A, 1.15 SF, 117 SFA, 84.3 PF, Code F


Pump #2 (best estimates)
Size: 16" x 8.5', ~5,000 gpm (est)
Motor: 30 HP (est), 3 Ph, 230 V, 60 A


Both motors are powered through soft-starters which ramp up the starting current.


These pumps keep our commercial property from flooding. One time, we experienced a blackout and had to scramble to rent a transportable 70kW generator to power the pumps during a heavy storm. After that, a decision was made to install a diesel-powered standby generator.


Our general manager had some early discussions with a used generator broker. Based on their communications, he had thought that a 40kW generator would meet our needs. At the time, I ran some locked-rotor current calculations on our pumps (562 LRA & 421 LRA) and advised that the 40kW generator he wanted to purchase was undersized based on the generator's spec of being able to "start a 20HP, 3-Ph, Code G electric motor" (316 LRA).


Some time later, a local contractor was purchasing two 60kW generators and we were given the opportunity to add a third 60kW unit onto that purchase at a discounted price. We purchased that generator (ratings & specifications attached as PDF files).


After installing the generator, we discovered that it would not start our 40HP pump #1. In fact, it sounds like it's straining somewhat when starting our ~30HP pump #2. We contacted the generator distributor thinking there's something wrong with the generator, but they simply stated that their 60kW generator was too small to start a 40HP motor.


I then contacted two major brand-name generator manufacturers to see what generator they'd recommend for starting our pump #1. Both companies ran calculations using the motor nameplate data and other info; one company recommended a 60kW generator, the other recommended 50kW generator.


I have since spoken to a gentleman at a generator service & testing company. He stated that the power rating of a generator is not enough to determine it's suitability to a load. For example, he told me that the power of the diesel engine driving the alternator can influence whether or not it can handle a large step-load. That makes sense. But how could a 50kW rated standby generator outperform our 60kW rated standby generator? And why does it seem that our 60kW generator is straining to even start our 30HP pump?


At this point, we're trying to determine if the generator we purchased is defective or simply incorrectly-sized for our application.

 

[zach_zachary1@yahoo.com]

When the motor is up to speed on pump 2 does the generator handle the load? Is there a setting on the soft start you can adjust? If you could slow down the starting speed you could limit the amount of inrush current.

 

[Jon456]

If I can get the #1 pump (40HP) started, then the generator can keep it running. (Although I don't know for how long because we don't have enough water to evacuate for long operating cycles.) The #2 pump (30HP) is able to start and run on generator.

I have tried lowering the max start-up current setting on the soft starter, but then the motor won't start at all, even on utility power.

 

[kwired]

I have since spoken to a gentleman at a generator service & testing company. He stated that the power rating of a generator is not enough to determine it's suitability to a load. For example, he told me that the power of the diesel engine driving the alternator can influence whether or not it can handle a large step-load. That makes sense. But how could a 50kW rated standby generator outperform our 60kW rated standby generator? And why does it seem that our 60kW generator is straining to even start our 30HP pump?

The ability of the prime mover is likely part of your problem. How much short term heavier then rated load it can deliver at rated speed is an issue. If speed drops too much then output voltage and frequency of your generator drops further complicating the genererator output ability.

Just having a heavier flywheel on the prime mover will make a difference in how well it rides through demand from such surges, but will decrease efficiency at lower load levels. Ideally you want to be able to maintain input speed before you can start to blame output abilities of the alternator for anything.

 

[GoldDigger]

The ability of the prime mover is likely part of your problem. How much short term heavier then rated load it can deliver at rated speed is an issue. If speed drops too much then output voltage and frequency of your generator drops further complicating the genererator output ability.

Just having a heavier flywheel on the prime mover will make a difference in how well it rides through demand from such surges, but will decrease efficiency at lower load levels. Ideally you want to be able to maintain input speed before you can start to blame output abilities of the alternator for anything.

:thumbsup:
With a heavy enough load, and cooperation from the OCPD and the voltage regulator, it is possible to stall the prime mover. That is not the fault of the alternator unless you would prefer to have it fail in a different way when confronted with that load.

 

[kwired]

:thumbsup:
With a heavy enough load, and cooperation from the OCPD and the voltage regulator, it is possible to stall the prime mover. That is not the fault of the alternator unless you would prefer to have it fail in a different way when confronted with that load.

Had a farmer during a power outage about 10-12 years ago that needed grain out of a bin for livestock feeding. This bin was not on same source as his main power for the rest of the farm and had no transfer switch on that service. He decided to just connect a cord from the bin auger to his tractor PTO powered generator - keep in mind the tractor PTO output was well above the generator rating.

He happened to mis-wire the motor when connecting the cord and had a line to line short in his connections. That generator on an oversized prime mover had enough load that a universal joint in the connecting shaft just snapped from the stress.

 

[Jon456]

Just having a heavier flywheel on the prime mover will make a difference in how well it rides through demand from such surges, but will decrease efficiency at lower load levels.

So beefing up the flywheel is how manufacturers are able to get a particular genset to withstand large step loads?

I understand that it takes more power to overcome the rotational inertia while starting up a heavier flywheel. But once at speed, I'm not sure how efficiency is decreased under lower alternator loads.

Ideally you want to be able to maintain input speed before you can start to blame output abilities of the alternator for anything.

It's quite apparent that our generator engine is straining as the pump is first brought online; I can hear the engine lug. Still, I'm having a difficult time seeing how a 50kW genset, even one with a heavier flywheel and more powerful engine, can dramatically out-perform a 60kW genset.

 

[GoldDigger]

In principle it is simple. In one unit the engine is specified more conservatively, with power reserve but running at less than full engine output at full rated electrical load. This increases the engine life and also provides more reserve for short time overloads.
The other generator is running the engine closer to its limit to save on initial cost.

 

[gadfly56]

So beefing up the flywheel is how manufacturers are able to get a particular genset to withstand large step loads?

I understand that it takes more power to overcome the rotational inertia while starting up a heavier flywheel. But once at speed, I'm not sure how efficiency is decreased under lower alternator loads.


It's quite apparent that our generator engine is straining as the pump is first brought online; I can hear the engine lug. Still, I'm having a difficult time seeing how a 50kW genset, even one with a heavier flywheel and more powerful engine, can dramatically out-perform a 60kW genset.

With a heavier flywheel, the generator will have a harder time following fluctuating demand levels; it will be overpowered when loads drop and underpowered when loads increase. With a high steady load, the flywheel helps the generator through the "burps" but makes it less agile in load following.

 

[GoldDigger]

With a heavier flywheel, the generator will have a harder time following fluctuating demand levels; it will be overpowered when loads drop and underpowered when loads increase. With a high steady load, the flywheel helps the generator through the "burps" but makes it less agile in load following.

I disagree except for the special case of inverter type generators.
Unlike a car engine with an automatic transmission with a torque converter, the engine on a generator does not increase its RPM to deliver more power nor does it decrease it's RPM when servicing a smaller load.
It has to turn at a constant RPM to deliver 60Hz.
Now a mechanical governor may rely on a small change from the programmed speed to cause the throttle setting to change, but the same flywheel effect that slows down that throttle change keeps the generator delivering full power while it is adjusting.
Your statement does not make sense to me. Can you explain in more detail?

 

[gadfly56]

I disagree except for the special case of inverter type generators.
Unlike a car engine with an automatic transmission with a torque converter, the engine on a generator does not increase its RPM to deliver more power nor does it decrease it's RPM when servicing a smaller load.
It has to turn at a constant RPM to deliver 60Hz.
Now a mechanical governor may rely on a small change from the programmed speed to cause the throttle setting to change, but the same flywheel effect that slows down that throttle change keeps the generator delivering full power while it is adjusting.
Your statement does not make sense to me. Can you explain in more detail?

The flywheels purpose is to store energy to help maintain the constant RPM. On an increasing load the engine/flywheel will tend to slowdown as the stored energy attempts to keep pace with demand and there may be a lag as the engine throttles up and accelerates the flywheel to bring it to the correct speed. If the engine is at high throttle under steady high load it may briefly overspeed the flywheel if there is a sudden drop in the load.

 

[GoldDigger]

The flywheels purpose is to store energy to help maintain the constant RPM. On an increasing load the engine/flywheel will tend to slowdown as the stored energy attempts to keep pace with demand and there may be a lag as the engine throttles up and accelerates the flywheel to bring it to the correct speed. If the engine is at high throttle under steady high load it may briefly overspeed the flywheel if there is a sudden drop in the load.

I understand that. I just do not see how a heavier flywheel makes it any worse. You would have the same underspeed and overspeed as the throttle responds with a lighter flywheel, but the dip or peak would be greater.
The flywheel does not slow the engines response to the throttle, at most it slows the throttle control input slightly.

Now if you have a speed control which is trying to maintain phasing to an external source, as when synchronizing two generators, I can see that there might be an effect.

 

[mpoulton]

After installing the generator, we discovered that it would not start our 40HP pump #1. In fact, it sounds like it's straining somewhat when starting our ~30HP pump #2.

Someone else asked if the 40HP pump would start if you already had the 30HP unit up to speed. Does it? Sequencing the pumps may be an easy solution. Once a motor is running at full speed, it will assist in starting other motors connected to the same line by acting as a flywheel/generator. As the supply voltage dips from the high load of starting the second motor, the other running motor briefly converts some of its rotational energy into electrical power flowing back onto the line. It's similar to how a phase converter works. Try it. There's a good chance the big pump will start if the smaller one is already running.

Also, when you say the generator won't start the 40HP pump, exactly what is the failure? Does a breaker trip? Does the generator stall? Does the motor starter's overload trip? If the failure is anything other than actual stalling of the generator's engine, then your setup is not giving it's all to start that motor and you could improve it's chances of success by adjusting breaker ratings and overload settings so that the limiting factor is only the generator itself.

Finally, you can almost certainly fix this whole problem by using a VFD to run the larger motor. Your existing "soft" starter apparently can't get enough torque out of the motor at low RPM to get it spinning without also requiring more peak current than your system can deliver. This is a limitation of non-electronic starters since they cannot change the supply frequency to match the rotational speed of the motor. A VFD fixes this problem and lets you start the motor as softly as you desire without any significant current spike. A 40HP 240V VFD seems to start at about $1800 for a basic unit, which is not cheap but is probably cheaper than replacing any of the other equipment here.

 

[Barbqranch]

I wouldn't want to depend on the 30 hp unit being up and running before being able to start the 40 hp. Doing so would sacrifice a lot of the redundancy of having 2 pumps.

 

[zach_zachary1@yahoo.com]

[QUOTE
Finally, you can almost certainly fix this whole problem by using a VFD to run the larger motor. Your existing "soft" starter apparently can't get enough torque out of the motor at low RPM to get it spinning without also requiring more peak current than your system can deliver. This is a limitation of non-electronic starters since they cannot change the supply frequency to match the rotational speed of the motor. A VFD fixes this problem and lets you start the motor as softly as you desire without any significant current spike. A 40HP 240V VFD seems to start at about $1800 for a basic unit, which is not cheap but is probably cheaper than replacing any of the other equipment here.[/QUOTE]


Great advice. I was about to recommend a VFD also. Quick question. Since the motor is already on a softstart is it safe to assume that it is inverter duty rated?

 

[Ingenieur]

If I can get the #1 pump (40HP) started, then the generator can keep it running. (Although I don't know for how long because we don't have enough water to evacuate for long operating cycles.) The #2 pump (30HP) is able to start and run on generator.

I have tried lowering the max start-up current setting on the soft starter, but then the motor won't start at all, even on utility power.

the alternator looks sufficient at 225 kva starting
the engine is marginal but should do it if the starter limits starting i to 300-400% fla range

the fact that it won't start on utility tells you something else is going on

did you mechanically inspect the pump?
spin freely?
no binding/rubbing
amp it while running? All 3 phases
megger the windings

 

[kwired]

So beefing up the flywheel is how manufacturers are able to get a particular genset to withstand large step loads?

I understand that it takes more power to overcome the rotational inertia while starting up a heavier flywheel. But once at speed, I'm not sure how efficiency is decreased under lower alternator loads.


It's quite apparent that our generator engine is straining as the pump is first brought online; I can hear the engine lug. Still, I'm having a difficult time seeing how a 50kW genset, even one with a heavier flywheel and more powerful engine, can dramatically out-perform a 60kW genset.

Why do you think you have steady voltage and frequency from the utility? voltagte may drop from line losses or transformer impedance but frequency changes are nearly non detectable because the have such massive rotors on generation equipment that starting your 40 hp motor has nearly no noticeable effect on the prime mover.

Because of laws of inertia the inertia of that heavier flywheel will also limit over voltage and frequency on the other extreme when load is suddenly removed. Yes there may be increase, but not as severe as with a smaller inertia on the generator rotor.

 

[Jon456]

Someone else asked if the 40HP pump would start if you already had the 30HP unit up to speed. Does it? Sequencing the pumps may be an easy solution. Once a motor is running at full speed, it will assist in starting other motors connected to the same line by acting as a flywheel/generator. As the supply voltage dips from the high load of starting the second motor, the other running motor briefly converts some of its rotational energy into electrical power flowing back onto the line. It's similar to how a phase converter works. Try it. There's a good chance the big pump will start if the smaller one is already running.

When I first read the earlier question by zach_zachary1, I thought he made a typo and was asking if pump #1 was able to run on generator after getting started. I can try this test and report back. There are two problems however:

1. As Barbqranch mentioned, we'd lose the ability to start our pump #1 if pump #2 fails.
2. Our pump controller is fairly 'stupid" and unable to sequence the starts. Without replacing the controller, in the event we'd need to operate the pumps on generator, a person would have to be on station for the duration to manually start & stop the pumps.

Also, when you say the generator won't start the 40HP pump, exactly what is the failure? Does a breaker trip? Does the generator stall? Does the motor starter's overload trip? If the failure is anything other than actual stalling of the generator's engine, then your setup is not giving it's all to start that motor and you could improve it's chances of success by adjusting breaker ratings and overload settings so that the limiting factor is only the generator itself.

Okay, this is where it starts getting detailed. The motor requires 230V/3-phase and that's what passes through the input/output bus of the soft starter (Siemens Sirius 3RW4055-2BB34). But the soft starter's control supply voltage (for its internal electronics) requires 115V/1-phase at less than 0.5A.

When I first tried running pump #1 on the generator, the motor would begin to turn briefly, then the Sirius would go into failure (indicated by a red LED on its front panel). As soon as that happened, no more power to the motor, the motor would shut down, the Sirius would automatically come out of failure, the Sirius would again try to start the motor, and the process would repeat.

I discovered that the reason the Sirius would go into failure mode was because the single-phase branch supplying the Sirius's control circuits would jump to ~130V (as the generator is trying to start the motor load). Apparently, as the genset struggled to keep up with the load placed upon it by the motor, the voltage would surge and the Sirius would shut down to protect its circuitry. I tested this by connecting utility power directly to the Sirius control input, while feeding generator power to the input bus to run the motor. When I did this, I was able to start and run the motor on generator power several times.

Next step was to try a power conditioner to keep the control voltage stable while on generator power. I initially tried a ferroresonant transformer, but it was too frequency sensitive: it worked on utility power, but with the genset producing 120V @ ~62Hz, the ferroresonant transformer was outputting 130V so the Sirius would go into fault immediately upon power-up, even without even starting the pump. I wanted to avoid a double-conversion online UPS (expensive, large, wastes electricity even in standby, heat-producing, and uses batteries with limited life), so I decided to try a Tripp-Lite conditioner. This is a consumer-grade servo-tap-switching transformer, so not the ideal device for our industrial application, but at less than $100 it was good for testing and proving if the system would work.

Running power to the Sirius control input through the Tripp-Lite conditioner, I started the pump on generator power and it appeared to work: the Sirius never went into fault condition! However, the pump would not start either. The motor would begin ramping up, but then the generator would completely shut-down with an over-voltage error showing on the genset LCD control panel. I tried starting 4 times and only once was the genset able to stay online and start/run the pump.

So this I don't understand. Because when I ran utility power to the Sirius control input, the generator was able to start and run the pump several times without dropping offline. But with the generator power running through the voltage conditioner to the Sirius control input, 3-out-of-4 times the generator was unable to start the pump.

Any ideas?

Finally, you can almost certainly fix this whole problem by using a VFD to run the larger motor. Your existing "soft" starter apparently can't get enough torque out of the motor at low RPM to get it spinning without also requiring more peak current than your system can deliver. This is a limitation of non-electronic starters since they cannot change the supply frequency to match the rotational speed of the motor. A VFD fixes this problem and lets you start the motor as softly as you desire without any significant current spike. A 40HP 240V VFD seems to start at about $1800 for a basic unit, which is not cheap but is probably cheaper than replacing any of the other equipment here.

Interesting idea. I had actually considered installing a VFD, although for a different reason: I wanted the ability to run the pump at less-than-maximum speed to reduce its output, thus extending pump "On-time" and minimizing short-cycling of the pump. However, when I spoke to a pump company about it, he said that VFD's are finicky and create problems. Although he agreed with what I was trying to accomplish, he felt that a VFD was more trouble than it's worth. But if a VFD will allow easier starting and will allow us to reduce pump flow output, that may be worth the downsides.

Thoughts?

 

[Jon456]

did you mechanically inspect the pump?
spin freely?
no binding/rubbing
amp it while running? All 3 phases
megger the windings

Yes, the pump appears to be operating within normal parameters. I am able to turn it by hand, and if I give it a good hard 180 deg spin by hand, it will continue to rotate about 90 deg before overcome by friction. This is similar to the amount of rotational resistance it had after the pump was factory overhauled in 2010.

Yes, I've put an ammeter on all three phases while running. While running, two of the legs typically pull about 92A, while the third pulls about 104A. I can't explain the current imbalance, and this may be part of the problem affecting our generator.

The windings haven't been meggered recently, but the motor was meggered by another electrician back in 2010 and he said the windings were good. I do have a megger so I could re-test. My megger can test at 50V, 100V, 250V, 500V, 1000V. Clearly the insulation must be able to withstand at least 250V. Should I test at 500V or 1000V?

 

[mpoulton]

So this I don't understand. Because when I ran utility power to the Sirius control input, the generator was able to start and run the pump several times without dropping offline. But with the generator power running through the voltage conditioner to the Sirius control input, 3-out-of-4 times the generator was unable to start the pump.

Any ideas?

It's hard to say what the difference between those two conditions is that's causing the problem, but two things stand out to me: First, a tap-switching conditioner is not a great fit for coping with brief transient power quality problems and it can't do anything about the sagging frequency from the generator which might mess with the starter's operation. A UPS would be much better even if it's not double-conversion. As soon as the generator's output gets a little bit wonky, the UPS will switch to battery and the starter should run smoothly. The changeover happens at least as fast as the tap-switching on your conditioner.

Second, the generator's controller settings seem to be contributing to the problem here. It needs to not be shutting itself down due to motor starting transients. Either the regulator needs to be adjusted so it doesn't produce a voltage surge when a sudden load is applied, or the controller needs to be adjusted to tolerate a higher voltage surge without shutting down the unit.

You have proven absolutely that the generator is capable of starting the motor. The problem is strictly a controls issue. That generator can be made to run these pumps.

Interesting idea. I had actually considered installing a VFD, although for a different reason: I wanted the ability to run the pump at less-than-maximum speed to reduce its output, thus extending pump "On-time" and minimizing short-cycling of the pump. However, when I spoke to a pump company about it, he said that VFD's are finicky and create problems. Although he agreed with what I was trying to accomplish, he felt that a VFD was more trouble than it's worth. But if a VFD will allow easier starting and will allow us to reduce pump flow output, that may be worth the downsides.

Thoughts?

VFDs are finicky and can create problems, but so can electronic soft-starters... Like the problems your existing one seems to be causing you. A VFD isn't much worse. In this case it would probably fix more problems than it causes. There's no guarantee that whatever specific VFD you choose will play nicely with the generator's poor power quality, but they tend to be pretty tolerant of bad power sources. Many are rated for 50-60Hz input (great for a generator since the frequency is unstable) and wide line voltage ranges. Some will even run with a missing phase! The variable speed would be nice and you could even rig a float switch (or multiple float switches at different levels) to lower the pump speed as the reservoir level drops and raise the speed when the water is higher. The 30HP pump with no VFD provides some assurance that you can still move water even if the electronics fail.