480v Applied to 3ph Low voltage motor

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GoldDigger

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All the smoke will leave the motor and it will need to be replaced.

If the insulation on the motor windings could take the higher voltage, is there at least a chance that the motor's overload protection would shut it down before thermal damage occurred?
Or would the effects of magnetic saturation make even that unlikely?
In any case it would not overspeed. :)
Also, I imagine that a tightly selected OCPD might trip on the excess starting current....

Tapatalk!
 

iwire

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If the insulation on the motor windings could take the higher voltage, is there at least a chance that the motor's overload protection would shut it down before thermal damage occurred?

I imagine it would depend on the motors rating.

Is the motor 400, 277, 230, 208, 120 .... 12 volts? :)
 

Jraef

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What would be the conditions that would occur if this were the case?
If you want to get technical, 480V IS considered "low voltage".

Did you perhaps mean what you happen if you applied 480V to a motor that is designed for a lowER nameplate voltage rating, such as 230V or 200V? (And for the sake of argument, we'll assume the motor cannot be re-connected for the higher voltage)

Here's the chain of possible events, using the image below as a general guide (it does not even consider this much of a voltage increase however).
motorlife1.jpg

On energizing, the starting current and torque would be increased, but by the SQUARE of the voltage difference. So if for example the motor nameplate said 200V (for use on a 208V system), the difference is +280V, so 240% of rated. Therefore the current and torque would increase to 576% of normal. Since NORMAL starting current is already 600%, that means in theory, the current spike could be 3456% of normal! That would not happen though, because no properly applied breaker or fuse would allow it.

But let's say someone "fixes" that perceived problem by installing a breaker that is 4X+ the normal size and the motor does manage to start. The 576% current spike would create a corresponding TORQUE spike, and since normal starting torque is 160% of rated FLT, that means the spike would reach 921% of rated FLT! It would likely twist off a shaft or something in the process. If not, one or more of the rotor bars inside of the motor would likely separate.

Assume now however that your system mechanically manages to survive that onslaught. Now the motor is getting 2.4X it's rated voltage, so the motor windings are saturating without producing any additional useful work. The motor is heating up at an exponential rate because all that added flux is just making heat. The temperature increase would happen much faster than the standard motor thermal overload relay curve could react to protect it, so the winding insulation would cook itself in a matter of moments, signaled by, as mentioned, the release of the magic smoke and corresponding fireball...
mushroom-cloud-hb1.jpg
... followed shortly thereafter by TEOTWAWKI.
 

Jraef

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I know that at least one 230 volt motor connected to 480 that caught fire.

I didn't do it. I just had to repair the wiring going to it. That also caught fire.
That's usually what precedes questions like this in my experience.
  1. Fire happens,
  2. Investigation takes place,
  3. Determination that the wrong voltage was applied (or more often that the wrong connections were used),
  4. Someone sees the nameplate and says "But it says it is 230 OR 460V, which means it can handle either one. So why did it catch on fire just because we hooked it up wrong?"
I see it the other way more often actually, where they have a 230V supply but they connect the motor for 460V, then can't figure out why it stalls all the time.
 

K8MHZ

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Michigan. It's a beautiful peninsula, I've looked
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Electrician
That's usually what precedes questions like this in my experience.
  1. Fire happens,
  2. Investigation takes place,
  3. Determination that the wrong voltage was applied (or more often that the wrong connections were used),
  4. Someone sees the nameplate and says "But it says it is 230 OR 460V, which means it can handle either one. So why did it catch on fire just because we hooked it up wrong?"
I see it the other way more often actually, where they have a 230V supply but they connect the motor for 460V, then can't figure out why it stalls all the time.

This was more like

1) Shear quits working.

2) Investigation takes place and determined motor is not working.

3) Motor from nearly identical shear in the spare equipment room is located and installed (without checking the nameplate).

4) Operator tries to run parts and the shear still doesn't work so he keeps pushing the button. Operator smells acrid smoke and notices flames coming from the back of the shear. E stop is hit as operator runs toward the nearest exit. Operator notices other workers standing near the exit and they form a small crowd watching the flames die and the room filling up with smoke.

5) Fire goes out, smoke clears, workers finish their cigarettes and go to their stations with the exception of the shear operator who went back to his station and sorted out parts until the end of the day.

6) Friend of mine that works at the factory calls me that evening and asks if it's OK to give my number to his boss.

The rest is history. The place made safe type cabinets for use in banks. The owner used to joke that he was one of the few people in the world that could hold up a bank before it ever opened. I could write a couple chapters of a book about the stuff I ended up doing over the years at that place.
 
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