I've started a 15 HP (480 V, 3 phase) motor while running a similar 25 HP motor, and my Onan 37 kVA generator with Ford 6 cylinder LPG engine running at 1800 rpm only bogged for a short while, around a second or so. Even though the motors are never started under a mechanical load (the air compressors being driven by the motors are unloaded, though of course there's still the mass of the compressor flywheel, etc to get up to speed), the total current being drawn during the 2nd motor start is certainly higher than the rating of the generator. I would like to continue this practice, even at accepting some loss of longevity of the equipment, since I only use the equipment about 4 hours/day 5 to 10 days a year. I can't afford a VFD or soft-starter or other pricey stuff.
Does this practice damage the generator (or the motors), either in the long run or sooner? (Or does the generator or motor suddenly fry one day during a motor start?) If so, what exactly gets damaged and how? Is there a way to over time monitor the condition of whatever is going bad? (What do people use for measuring insulation integrity, if this is what goes?) Or protect it? There is a 50 amp breaker on the generator that didn't trip during this one test.
I assume I should start the larger motor first.
The "service rating" on the generator is "standby," whatever that means.
I've tried to talk to the outfits that sell the generators on the questions above but they just give me approximate rules of thumb for sizing generators. I'm not sure that factory service engineers (if I could even locate them and get them to talk to me) would be willing to answer my questions, since they might be perceived as condoning my overloading of their product. Any suggestions as to other sources of knowledge would be appreciated.
If the total running load on the generator does not exceed 80% of it's rating you are unlikely to do it any harm. A 37 KVA set probably has a surge capacity of 45 KVA. A standby service rated generator is designed to be used exactly as you are using it. The other service rating is prime which is were the set is designed to run continuously for very long periods on the order of weeks or months.
Detailed generator ratings are based on several things. The amount of true power available from the prime mover limits the amount of true power available out of the generator. Conservation of energy says you can't get more out than the engine driving it can put into it. In your example, you mention how it 'bogged' for a short while. That simply is the extra load slowing down the engine and the engine governor trying to restore the speed to 1800.
The maximum field current on the rotor is limited by the ability to keep cool and not overheat the insulation. Sometimes, the field current is limited by the voltage regulator or other components. The maximum field current available limits the amount of lagging reactive load that can be supplied.
The maximum armature current on the stator is limited by the ability of it to keep cool and not overheat. This limits the total kVA (regardless of pf) that can be carried. Your starting the second motor is probably exceeding this rating, at least for the few seconds it takes to start the motor. More on this in a moment.
Another limit is sometimes mentioned that is the worst *leading* power factor. It is rare that this limit gets in the way of things unless you're transmitting the power over some distance.
Now, about your motor. Starting a large motor draws a lot of current. If the voltage from the generator drops appreciably when the motor starts, then it will take longer to accelerate, and draw even more current. In the worst cases, it may not get up to full speed and burn out. But you mention not starting under any load. This is a good way to limit the time it takes to accelerate and limit the surge current. And you mention that it comes up in 'around a second or so'.
Drawing too much current through a motor can damage it just like drawing too much current through a generator. The windings heat up from I^2R losses and the cooling isn't enough to keep the temperature below the point of damage. But motors are *not* designed to draw the high starting current for a long time. They *can* draw the current for a short time because of the large mass of copper/iron in them. So while the I^2R losses during starting are much higher than the cooling ability, if it is only for a short time, then the excess heat can only raise the large mass of copper/iron a limited amount and the insulation isn't damaged. *Large* motors are limited to only a certain number of 'starts' in a given period of time. This is to allow that large amount of heat generated during starting to dissipate/diffuse away from the windings and insulation.
Now, think about the kVA rating on your generator. That is the steady-state rating. With that amount of current, the I^2R losses will warm up the windings until the cooling/heating reach equilibrium. In a well designed unit, this will happen at a temperature much lower than what would cause damage to the insulation. But just like a motor, the windings *can* handle short bursts of higher current (and higher I^2R heat buildup). But you have to be careful to limit the duration and how often they happen.
So, with no guarantee or liability attached (expressed or implied ;-), I would say you can continue to do this, *IF* you limit how often *AND let the generator windings cool sufficiently between successive starts. AND IF the normal load of these motors is within the rating of your generator. AND given that you state that the motor accelerates to normal speed in just a second or two.
*Large* motors are allowed something on the order of no more than two 'starts' in a 3-hour period, or one 're-start' after its been running at normal temperature. I would suggest using that as a guide. Start the big motor, then start the small one. Don't repeat the operation more often than once every three hours (or longer). If you need to start more often, you'll need a bigger unit.
Too frequent starting and you'll let the 'magic smoke' out of your generator ;-)
Thanks for all the info. Do you know if there's a way to tell if I'm starting the motors too often for generator insulation longevity? (I assume that the 15 and 25 HP motors are much cheaper to replace than the generator.) Do people put thermocouples into the generator windings to keep track of temperature? If so, can this be retrofitted?
For large commercial units, that is *exactly* what is done. RTD's are often embedded directly between the windings of two coils as they are placed into the slots. Several are placed around the unit in various slots. During initial testing, all of them are monitored while the unit is loaded and the highest 3 - 6 of them are noted and connected to external alarm circuits, while the others are de-termed and left as spares.
Retro-fitting would be a problem though. To get an accurate temperature reading, you would need to 'jam' it into the end of the slot between the upper and lower coil without damaging the turns. You could easily scrap the enamel coating and short the winding, rendering your generator useless (rewinding small units isn't always cost-effective).
You could try putting a sensor on the outside of the iron half-way between the end turns, then run the unit for a prolonged period under full load and record the temperatures. Not on the casing, but on the laminated iron that holds the windings. This is often separate from the outside housing. This would tell you how warm the iron gets when the unit is under full-load and dissipating rated heat. (if you can accurately determine full load ;-)
But the temperature of the iron will lag behind the winding temperature by quite a bit. If significantly overloaded (or too many surges from starting motor), by the time the iron temperature sensor shows a problem, the inner winding temperature will already be toast. That is why large units have the RTD stuck right in the slots.
Of course if you take the full load temperature rise and set your own limit of 1/2 that or something, you can afford yourself some protection.
But this is *all* very speculative. If you do frequent starts of the heavy motor (say, 15 in an hour), you might still let the 'magic smoke' out even with the outer iron temperature rise as low as 1/2 full load. Or not. We're really extrapolating pretty far afield here. You might be safer just limiting the number of starts in an hour to one or two than to trust such an instrument.
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