Question about single phase motors

[ltlvt]

Does wiring a single-phase motor in 220-volt mode make it any more efficient than 110 volts. In other words, use less $$$ to operate? If not, what are the advantages of 220 vs 110. Thanks in advance.

 

[sdelivery]

Yes and no. Volts times amps equals watts so in theory, no but in practicality
The motor temperature is slightly less it is slightly stronger feeling and faster acting , the bigger benefits are less strain on your electric system, requires smaller conductors and raceway, lower rated circuit protection (circuit breaker)
All comments belong to the perpetrator of this response, your actual results may very.

 

[rabler]

Basically what @sdelivery said,
The advantage of 220 over 110 is you get away with lighter wire in the walls to deliver the same amount of power.

Where it gets a bit interesting is looking at startup current. Motors can easily peak at 2-3 times rated current or more, very briefly, during startup. This can and often does exceed the rated breaker current. It occurs briefly enough that wire heating is not a fire hazard, so this is acceptable behavior. In the old days, this was why you had slo-blow fuses.

So, lets say you have a 1.5 HP motor that draws 12 amps rated full load on 120v. It may peak at 35 amps when first started. (Just pulling some reasonable ballpark numbers) on your 20 amps circuit. When drawing 35 amps, that is enough to drop the line voltage to the motor some, which means the motor struggles more to start. (And maybe more flickering of your lights). Now, if you wire that motor to 220, you are probably still running it on a 20amp circuit. But it will draw 6 amps full load and 17 amps startup. No problems with that 220v 20amp circuit delivering that startup current, motor sees much less voltage drop, better startup.

Ironically, it is possible that a motor startup load on 110 can cause your lights to brighten rather than go dim. Can you guess why?

 

[DavidR8]

Ironically, it is possible that a motor startup load on 110 can cause your lights to brighten rather than go dim. Can you guess why?

Hmm backfeeding voltage?

 

[rabler]

Hmm backfeeding voltage?

While we think of neutral as always being zero volts, the reality is that if you run current through your neutral wire, there is some voltage drop on it. On large startup currents, that peak startup current causes voltage drop on the hot wire, but voltage RISE on the neutral wire. Well, it's AC, so the neutral voltage moves toward one of the two hot legs. That reduces the voltage on the loaded hot wire, but increases the voltage on the "lightly" (pardon the pun) other leg.

If your shop is being fed with a 220v subpanel, and the lights are on one120v legs when the motor is on the other 120v leg, then the voltage on the lights increases because of that offset in the neutral voltage. When I was in college a fellow student had a house with a garage/shop where we'd both work on woodworking projects. This was common when starting the table saw.

 

[DavidR8]

Thanks, I did not know that.

 

[Provincial]

There are times when upping the voltage is necessary.

I had a RF30 mill drill with a single phase 120/240V motor wired for 120V. The motor burned up, so I replaced it with a motor rated the same 2 HP. The new motor tripped the 20 Amp circuit breaker on start-up if the belts were set for higher spindle speeds. It wouldn't trip the breaker if set for lower spindle speeds. The old motor was probably only 1.5 HP, and labelled for 2 HP as a marketing ploy.

Rewiring to 240V fixed the problem.

 

[markba633csi]

My brothers both did roofing work. They had a compressor that would start just fine on a skinny extension cord but would trip breakers if you used a proper heavy duty cord.
I had to explain to them that the skinny cord was acting like ballast resistor and kept the current low enough to start the motor without tripping

 

[K30]

Where it gets a bit interesting is looking at startup current. Motors can easily peak at 2-3 times rated current or more, very briefly, during startup. This can and often does exceed the rated breaker current. It occurs briefly enough that wire heating is not a fire hazard, so this is acceptable behavior.

Thumb rule we always use is about 5x running current. You haven't had fun until you start an 8000 hp motor and watch it peg the gauge to 1600 amps for a solid 30 seconds on 13,800v...

 

[rabler]

Thumb rule we always use is about 5x running current. You haven't had fun until you start an 8000 hp motor and watch it peg the gauge to 1600 amps for a solid 30 seconds on 13,800v...

My 20hp RPC idler peaks 210a on startup. That’s big enough for me. That amount of current/voltage for 8000hp just frightens me.

We have several coal power plants nearby. One of them was down for significant time (months to year) when a generator was brought online out of phase. It tore out of its mounts and plowed into another unit.