I have a customer that bought a 17kw generac single phase 240/120v generator/transfer switch to power freezers in a commercial food wareshouse. His service is 240/120v delta 3-phase. Most of his freezers are resi chest style. However, he does have one walk-in freezer. It is 240v 3ph, 7.5hp. His idea is to put a phase inverter up and power it off of that. Then he can use this generator. The generator is non returnable. Is this even possible? Will the 17kw gen have enough power to get the 7.5 motor started. I say no. My calculations say that 7.5hp * 746w = 5595watts. Now it takes roughly 5 times running watts to get a motor started. So 5595w *5 = 27975 watts. Am I correct in thinking it will take a 30kw gen to get this motor going?
Generator sizing for motor starting inrush currents
- Thread starter aglick87
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rcwilson
Senior Member
- Location
- Redmond, WA
Your reasoning is close, but there is an outside chance that the generator could do it. A rule of thumb is size the generator for 300% of the motor rating. 7.5 HP = 7.5 kVA, 7.5 kVA x 3 = 22.5 kVA, just a little over 19 kW (18%). Well, not so little.
The 5-6 times inrush current on a motor is not all watts since the power factor during starting is low - maybe 0.2 or 0.3. The generator engine only has to supply enough kW to accelerate the motor. However, the generator has to still have enough voltage capability to supply the vars by holding up the voltage during the inrush. Generac might be able to tell you if it will work, but they will probably rather sell a bigger unit.
The phase inverter might help if it is a rotary style. It will act like a fly wheel and deliver energy to the big freezer during starting. That might make up for the converter losses. I am not that versed on phase converters, there may be some other issues.
If the client is willing to risk burning up his freezer motor from excessive stalling, you could give it a try. Bail out if it doesn't get up to speed in a couple of seconds. If possible, raise the voltage as high as the generator allows (250V?) prior to starting. Don't have other loads on until after the big unit is running.
(The manual switching seems to defeat the whole idea of an automatic transfer switch and backup generator).
The 5-6 times inrush current on a motor is not all watts since the power factor during starting is low - maybe 0.2 or 0.3. The generator engine only has to supply enough kW to accelerate the motor. However, the generator has to still have enough voltage capability to supply the vars by holding up the voltage during the inrush. Generac might be able to tell you if it will work, but they will probably rather sell a bigger unit.
The phase inverter might help if it is a rotary style. It will act like a fly wheel and deliver energy to the big freezer during starting. That might make up for the converter losses. I am not that versed on phase converters, there may be some other issues.
If the client is willing to risk burning up his freezer motor from excessive stalling, you could give it a try. Bail out if it doesn't get up to speed in a couple of seconds. If possible, raise the voltage as high as the generator allows (250V?) prior to starting. Don't have other loads on until after the big unit is running.
(The manual switching seems to defeat the whole idea of an automatic transfer switch and backup generator).
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Maybe yes, maybe no.
If he uses a VFD as the way to convert from 1 phase to 3 phase, one added bonus is that he can probably set it up to that the generator will NOT see any significant inrush. But I say probably because he will have to check that out with the refer compressor manufacturer. Some do not like to be ramped to speed slowly because they use the refrigerant as lubricant and if that's the case, the inverter may be impossible to implement.
Even if that works, I would also implement a control scheme that monitors power and after a power failure that causes the generator to start, always starts the largest load BEFORE it applies power to anything else. By the time you get done doing it right it may have been cheaper to have used a 3 phase generator. If that's a done deal though, his idea has some validity.
Just as a side note if you're not aware; to power up a 7.5HP 3 phase motor from a 1 phase source, you have to use a 15HP VFD. Then the NEC says that the circuit feeding the VFD must be sized for 125% of the VFD input current rating, not the motor behind it. So being that most 15HP 230V VFDs are going to be rated for around 46A, that means a 60A circuit to feed it. Your generator will not have to actually supply that much current, it's just the component sizing that needs to be considered.
If he uses a VFD as the way to convert from 1 phase to 3 phase, one added bonus is that he can probably set it up to that the generator will NOT see any significant inrush. But I say probably because he will have to check that out with the refer compressor manufacturer. Some do not like to be ramped to speed slowly because they use the refrigerant as lubricant and if that's the case, the inverter may be impossible to implement.
Even if that works, I would also implement a control scheme that monitors power and after a power failure that causes the generator to start, always starts the largest load BEFORE it applies power to anything else. By the time you get done doing it right it may have been cheaper to have used a 3 phase generator. If that's a done deal though, his idea has some validity.
Just as a side note if you're not aware; to power up a 7.5HP 3 phase motor from a 1 phase source, you have to use a 15HP VFD. Then the NEC says that the circuit feeding the VFD must be sized for 125% of the VFD input current rating, not the motor behind it. So being that most 15HP 230V VFDs are going to be rated for around 46A, that means a 60A circuit to feed it. Your generator will not have to actually supply that much current, it's just the component sizing that needs to be considered.
kwired
Electron manager
- Location
- NE Nebraska
17 kW is common enough generator you shouldn't have too much trouble selling it. Sell it and get what you really need. Will be less trouble and may even cost less in the end.
I thought it would be close. The only downside to a rotary phase converter or VFD is that it will have to be operating on single phase even when the 3-phase utility power is up. So there will be some efficiency losses with the phase conversion. Not to mention another device in the circuit that needs maintainance. The VFD would be ideal. But as mentioned, the slow start-up may affect compressor lubrication. The price increase from a residential 17kw generator to a liquid cooled 30kw 3-phase unit is almost $6,000. A 15 hp VFD would run $2000 plus installation. About the same for a rotary converter install. I will forward this conversation to my customer so he can review these options. If there are any other suggestions, I am open to them. Thanks
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
It is 17kW @ what pf? BTW: this unit also has no short time overload rating.
If it is rated at 0.8pf(typical), then it is a 21KVA machine, and should be able to produce around 12 KVAR at rated KW output. The motor when it starts may pull around 7kVAR. It may work. Can you hook it up temp and try it, before you go looking at other alternatives.
If it is rated at 0.8pf(typical), then it is a 21KVA machine, and should be able to produce around 12 KVAR at rated KW output. The motor when it starts may pull around 7kVAR. It may work. Can you hook it up temp and try it, before you go looking at other alternatives.
The alternatives are to get a 3-phase genset or to get a unit that produces the third phase and try this single phase generator. I will have to talk with the customer and explain to him that there are no guarantees that his current equipment will do the job. The bad part is, I have to get a $2000 phase inverter to even try it. Then if it doesn't start it, we have not only a generator, but a phase inverter that is useless. Why don't customers talk with their contractors before going out and buying equipment? Especially non returnable equipment. Makes no sense, but it continues to happen day after day.
weressl
Esteemed Member
The regulator may not 'like' the front end of the ASD.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Good point, that may very well be an issue. Most newer commercial/industrial gensets now come with filtering on the AVR circuit that takes into account the effects of non-linear loads such as VFDs and SMPS front ends on electronics, but residential grade gensets may not.
Amen to that...aglick87 said:
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