Rural 3-phase, USA

I live in a rural area where 3-phase was installed a few months ago. I would like to go to 3-phase power in the shop, but I'm concered about reliability. During storms or heavy rains, one or more of the phases will drop. I know this by which homes loose power. (Easy to see at night!)

My question is; Can 3-phase equipment be damaged by loss of a phase? What happens to the motor if power (one of the phases) is lossed during operation?

Three phase motors can run fine on 2 legs. If more than 1 leg is lost the motor will just coast to a stop. 3 phase equipment (unless electronics are involved) is pretty much safe from phase failure on small motors. If you are concerned you could put a phase failure on your equipment to shut it down.

Before you switch, check the rates. We're switching an existing farm operation from three phase to single phase with a rotary converter.

If you have a three phase motor running, and one phase is dropped, the motor will "single-phase." This means that the motor will indeed continue to run, but will deliver only aprox. 2/3 its rated H.P. If your motor is connected to a control circuit which starts and stops the motor automatically, and the motor is required to start under heavy load, the motor may not start and run, because it cannot develop full starting torque due to the single phasing condition. The heavily-loaded motor will draw excessive current in trying to start, and -- if it is properly protected by a motor controller -- the controller will interupt power to the motor, preventing damage to the motor, and possibly a fire.

It's actually even worse that stated above if you lose one of the phase legs on a three phase motor. If running already, it will continue to run single phase on the remaining two legs. But if it stops and is then restarted, either automatically or manually, it will simply hum and the shaft will quiver and not rotate at all. In a few seconds, a fuse or circuit breaker will trip and hopefully save the motor windings from burnout.

You cannot start a three phase motor on single phase, unless you mechanically get the shaft rotating, or use a starting capacitor. Only three phase provides the rotating magnetic field, while single phase provides only a reciprocating magnetic field (hence the quivering of the shaft).

Thanks for all the responses. Down here in the South Texas farm land, we thought 3-phase was "Big City". I was hoping that going to 3-phase would also let me split lighting, small machines, air and heat on different phases. If one leg went out, we wouldn't be dead in the water.

"Dloc" mentioned a rate variance. Does 3-phase cost that much more? Now I'm wondering if getting another meter (transformer) on a different phase might be more cost effective. That would allow me to split draw between lines. (Maybe even fudge a 3-phase)

In any case, thanks again.

It is not a good idea to combine power systems. If you want true 3 phase you will have to get it from the power co. You can get vfd's that will run 3 phase motors great.
As for getting another service, most utilities charge quite alot for having 2 meters on the same property. They can do it ok but even so you would still likely have the same 2 phases, as the third is probably high leg. Good luck.

I agree with Chip's comments.

There are two types of three phase service available from power companies, delta and wye. Chip mentions the "high leg", which is delta-connected three phase. This is the type which most phase converters supply. You get true 240 volt three phase, but you only have the possibility of regular single phase 120 volt branch circuits with the delta connection from a single phase. The reason is that the voltage to neutral of the third or "high" leg is 208 volts, which you cannot use for ordinary household loads.

The wye connection supplied by the power company is at 208 volts three phase. There is no high leg, and the voltage to neutral for all three legs of the wye connection is 120 volts. So you can, with the wye connection, truly do load balancing between all three phases for household use. Many small businesses, schools, and churches are supplied this way.

One drawback of the 208 volt wye supply is that most three phase machine tools want 240 volts and virtually all single phase household machines such as dryers, ranges, and air conditioners also want 240 volts and not 208. Most of these devices would run on 208, but would draw more current and have slightly lower output and efficiency.

I agree with Chip. You probably just want to keep your plain vanilla single phase supply, and either build yourself a good rotary converter or go the VFD route. I built myself a great rotary, and I have piped its output to a separate three phase distribution panel, which I find works great.

Just a note that I used to live out in the middle of dairy farm country and had Delta 3 phase at the shop next door. I can't remember if I had a seperate meter for the house or not, but I'm thinking that I just had one meter for both. Never had any real problems with it...during a few lightning storms I recall loosing one phase but all my machines had magnetic starters so this was no problem. I can imagine with something like a Bridgeport with just a manual rotary switch it could be a problem, but if you think about it, how often are you running a Bridgeport when you're not in there with the machine to cut it off instantly if need be ?