rhenders
Member
- Location
- Alabama, USA
I have a trailer with a 30 kW, 3 Phase (wye), 208 V diesel generator. The generator's primary purpose is to supply power to a Variable Frequency Drive (VFD). The VFD is controlled by software to start/stop/speed control a 20 HP pump motor (also 208 V, 3 Phase). All works as designed with no problems. As you can see, the generator is considerably oversized for the pump application. That was done on purpose so that we could increase the pump size if needed (this is a prototype system).
However, as is often the case, requirements change. We now find ourselves needing to power some 120 V, single phase circuits in addition to the VFD/pump. The short and easy method is just to tap off of one of the generator phases and power the equipment since 208/(square root of 3) = 120V. The generator manufacturer concurs that this is acceptable, but mentions that we shouldn't "draw too much current" from just one leg/phase. He didn't define what "too much current" means in real numbers, but I understand that to the extent possible, we need to keep the 120V load balanced so as not to run the generator in a highly unbalanced load situation.
The 120V loads are a 2.5 HP compressor (usually needs its own 20A circuit), some interior lighting (8 x 100W incandesent lights), a computer rack (up to 700W), and a 15A convenience outlet circuit (who knows - coffee pot, lap top, ...). Back of the envelope estimating means I might put the compressor on one 20A circuit, the computer rack on a 15A, and the convenience outlet on a separate 15A circuit. Each circuit could be powered from one of the 3 phases to help balance the loading, but this is still pretty lopsided since the compressor dominates. Adding the max circuit currents (breaker sizes) for worst case design means that I would need 50 Amps (20A + 15A + 15A) of 120 V power which is 6 kW neglecting power factor and other losses. So, by my estimate, the 6KW 120V load is only about 20% of the total generator capacity, but with the compressor, it's pretty unbalanced.
I want to do this right and I'm concerned that the relatively large 20A compressor load is a problem because it will be cycling on/off throughout the day. It will be a major portion (40%) of the 120 V load. Is it acceptable to just run this equipment from the three phases of the generator as I've described (assuming appropriate individual circuit breakers/protection), or should I find some other means of converting the 208/3 phase to 120V/single phase? Am I making this too hard? Other ideas/thoughts?
Thanks.
rhenders
However, as is often the case, requirements change. We now find ourselves needing to power some 120 V, single phase circuits in addition to the VFD/pump. The short and easy method is just to tap off of one of the generator phases and power the equipment since 208/(square root of 3) = 120V. The generator manufacturer concurs that this is acceptable, but mentions that we shouldn't "draw too much current" from just one leg/phase. He didn't define what "too much current" means in real numbers, but I understand that to the extent possible, we need to keep the 120V load balanced so as not to run the generator in a highly unbalanced load situation.
The 120V loads are a 2.5 HP compressor (usually needs its own 20A circuit), some interior lighting (8 x 100W incandesent lights), a computer rack (up to 700W), and a 15A convenience outlet circuit (who knows - coffee pot, lap top, ...). Back of the envelope estimating means I might put the compressor on one 20A circuit, the computer rack on a 15A, and the convenience outlet on a separate 15A circuit. Each circuit could be powered from one of the 3 phases to help balance the loading, but this is still pretty lopsided since the compressor dominates. Adding the max circuit currents (breaker sizes) for worst case design means that I would need 50 Amps (20A + 15A + 15A) of 120 V power which is 6 kW neglecting power factor and other losses. So, by my estimate, the 6KW 120V load is only about 20% of the total generator capacity, but with the compressor, it's pretty unbalanced.
I want to do this right and I'm concerned that the relatively large 20A compressor load is a problem because it will be cycling on/off throughout the day. It will be a major portion (40%) of the 120 V load. Is it acceptable to just run this equipment from the three phases of the generator as I've described (assuming appropriate individual circuit breakers/protection), or should I find some other means of converting the 208/3 phase to 120V/single phase? Am I making this too hard? Other ideas/thoughts?
Thanks.
rhenders