Quick electric motor question (stemmed from another discussion on another forum with no real electricians on it so far...)

Say you're running an electric 220V single-phase AC motor. Now, ordinarily, I understand that the breaker should be a single 220V double-pole breaker, but for the purposes of this discussion, lets say the hot leads are connected to two single pole 15A breakers.

Now, lets say that, for some reason, one of the 15A breakers trips, taking out one leg of the 220V feed. What happens to the motor?

Does it run at half-speed sending the juice back through the neutral instead of using the other hot?
Does it try and run at full speed, drawing more current and potentially tripping the other circuit?
Does it just stop running all together?

Just curious... I would never recommend wiring it that way, but heard a tale of someone with a generator that was wired that way... (a cheapie Harbor Freight special) Just got me thinking about what would happen to a 220V motor...

Thanks!

-Joe

...The motor, if in proper working order will not run until it has 220v...The failure of the 15 amp breaker is not a surprise as I'm sure it doesn't meet up with the min. amperage breaker that is required ...

...Most motors and appliances use a max. amperage initially at start up ...

If you vary the voltage in an AC motor it will not change the speed. In order to change the speed in an AC motor you need to vary the frequency of the line voltage or the number of pole pairs in the motor. In other words an AC motor designed to run at 3600 rpm will run at 3600 rpm whether its getting 120VAC 60Hz, 220VAC 60Hz or 440VAC 60Hz. If you want that same motor to run at half speed (1800 rpm) you would need to either change the line frequency to 30Hz or double the number of pole pairs in the motor.

However, changing the incoming voltage will change the amperage draw of the motor. So if its rated at 220VAC 1000W (~2.6A), cutting the voltage in half to 120VAC the current would jump to ~4.6A and doubling the voltage to 440VAC would drop the current to ~1.3 amps.

On the other hand, a few motors were designed to be "convertible," in that their original windings were designed to work on multiple voltages by altering the wiring at the junction box terminals on the motor.

the info could be found on a label inside the motor's junction box...

You guys are all missing the question...

The motor is already running on 220V.
One leg drops out.

What does the motor do?

...It will not start on 110v...

...It will not start on 110v...


I know that. That's not what I'm asking.

The motor is already running on 220V. One hot lead of the 220V feed drops out while the motor is running. What does the motor do?

Should stay running, however the current draw will increase. So it will either trip the breaker or start running warm.

...You will have a brown out effect...A short condition if you may...Hopefully it will trip the reset (If it has one) before it fries the motor...

...I've seen this several times with pool pump motors and landscapers thinking they are electricians...:rolleyes:

...The end result is usually a replacement motor being bought...:dunno:

I know that. That's not what I'm asking.

The motor is already running on 220V. One hot lead of the 220V feed drops out while the motor is running. What does the motor do?

The motor will just stop, open circuit, there is no nutural on a 220 circuit.

Should stay running, however the current draw will increase. So it will either trip the breaker or start running warm.

It can't run, Open Circuit.

Go to a 220v outlet with your volt meeter, put one probe on one leg of the 220v and the other probe on NOTHING. What voltage do you read? 220v,110v no the correct answer is 0.

I may be getting things confused with 3-phase power. I've had several motors and equipment in the past running 440VAC 3-phase that would lose a phase and keep on trucking.

I may be getting things confused with 3-phase power. I've had several motors and equipment in the past running 440VAC 3-phase that would lose a phase and keep on trucking.

Thats why they make phase protectors. Without it the motor will fry.

The motor will just stop, open circuit, there is no nutural on a 220 circuit.

It can't run, Open Circuit.

Go to a 220v outlet with your volt meeter, put one probe on one leg of the 220v and the other probe on NOTHING. What voltage do you read? 220v,110v no the correct answer is 0.

That's what I was thinking, but forgot about there being no neutral on a 220V circuit...

So, correct me if I'm wrong, but if the generator in question tripped the breaker on just one leg of the 220V feed, there should be no potential damage to the breaker, motor, or generator, correct?

That's what I was thinking, but forgot about there being no neutral on a 220V circuit...

So, correct me if I'm wrong, but if the generator in question tripped the breaker on just one leg of the 220V feed, there should be no potential damage to the breaker, motor, or generator, correct?

You are correct, although it still is a hot circuit. If you touch anything grounded and the 1/2 tripped circuit you will get lit up.

Would a tool like this help you out? http://cgi.ebay.com/Amprobe-PRM-3-Phase-Sequencer-Motor-rotation-Tester_W0QQitemZ170295169074QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item170295169074&_trksid=p3911.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318

Each breaker has 110 to 120 volts (depending on the line voltage) and an amp rating for its size (15, 20 etc). If you shut off one breaker, the motor will stop because it will be an incomplete circuit. The breakers should be side by side in the panel and both of the switches attached so if one breaker goes off the other does too. If it doesn't you could get fried. It is in the electrical code that they must be joined. If they aren't and you have a fire then most likely you don't have any home insurance even if this wasn't the cause of the fire. Insurance companies look for easy ways out of paying. For 220- 240 volts you need to be on both buss bars (one breaker on each). Some electrical panels will let you put a double breaker on the same buss bar. If this is the case, then you won't be able to operate that motor. Be careful and don't hold onto a cat's tail to check for power, unless you don't want to see the cat around anymore.