480V Motor at 240V

[jlamb]

What happens if you apply 240V to a 480V motor?

I have a coworker who is having trouble with a submersible pump. It is only pumping about half the volume-per-minute at half the head as it is supposed to be moving. The pump is wired at 240V 3-phase. I am wondering if the pump motor is actually tapped for 480V.

I know a 480V motor will spin if 240V is applied (after all, that is how reduced-voltage starters work, right?), but I don't know exactly what the failure mode is if the motor is wired at the wrong voltage (higher current? higher slip-rate? lower torque? or will it just burn up?)

Any insight would be appreciated!

(P.S. - my other thought is, maybe a phase is switched and the pump is running backwards)

 

[Besoeker]

I have a coworker who is having trouble with a submersible pump. It is only pumping about half the volume-per-minute at half the head as it is supposed to be moving. The pump is wired at 240V 3-phase. I am wondering if the pump motor is actually tapped for 480V.

I know a 480V motor will spin if 240V is applied (after all, that is how reduced-voltage starters work, right?), but I don't know exactly what the failure mode is if the motor is wired at the wrong voltage (higher current? higher slip-rate? lower torque? or will it just burn up?)

(P.S. - my other thought is, maybe a phase is switched and the pump is running backwards)

If it is running and pumping with only half volts applied and not tripping the overload protection it is most likely lightly loaded. Half head and half flow indicates that it is very lightly loaded.
It could be because the pump is running backwards - I don't know enough about pumps to know if it would pump at all if running in reverse.
I'm sure there are others here who might venture an opinion on that.

 

[GBShade684]

If the motor is rated 480Vac at the nameplate FLA, when you run 240Vac you will double the running amperage. The windings won't / aren't designed to handle the heat that will build up. Motor will burnup and / or catch on fire. How long it takes depends on the robustness of the motor. You may not trip the overload protection depending on the level of current the pump is ideally supposed to run on.

 

[texie]

If it is running and pumping with only half volts applied and not tripping the overload protection it is most likely lightly loaded. Half head and half flow indicates that it is very lightly loaded.
It could be because the pump is running backwards - I don't know enough about pumps to know if it would pump at all if running in reverse.
I'm sure there are others here who might venture an opinion on that.

Good points. It could be operating at half voltage and backwards which is allowing to run without overload.

 

[iceworm]

What happens if you apply 240V to a 480V motor?

I have a coworker who is having trouble with a submersible pump. It is only pumping about half the volume-per-minute at half the head as it is supposed to be moving. The pump is wired at 240V 3-phase. I am wondering if the pump motor is actually tapped for 480V.

I know a 480V motor will spin if 240V is applied (after all, that is how reduced-voltage starters work, right?), but I don't know exactly what the failure mode is if the motor is wired at the wrong voltage (higher current? higher slip-rate? lower torque? or will it just burn up?)

Any insight would be appreciated!

(P.S. - my other thought is, maybe a phase is switched and the pump is running backwards)

assumptions:
The motor is down with the pump - integral with the pump. Otherwise you would go look at the nameplate.
The pump is a centrifugal.

Qs;
Did it ever run right? Or is this the commissioning?
If it did run right as some time, what was done/changed?

Yes it will run at 50% V, but the available HP is only 50% and the slip will be a bit higher. And that does not match up well with the described flow/head. The required power for a centrifugal pump at half speed is only 1/8. But that depends on the pump efficiency. And the efficiency could well be in the toilet at half speed. So that is a resounding, "I don't know - more data please."

If the motor/pump was replaced or new, how certain are you the right one was installed? Have you looked at the purchase documents/paper that came with the pump?

If is is a new install, consider opening up the piping and removing the welder's socks out of the pump suction.

Since you can't look at the nameplate, try switching the motor rotation. Generally speaking - can't hurt anything. However, 50% flow is less than I would expect for reversed rotation

I'm curious to what you find. Let us know.

ice

 

[broadgage]

Any standard type of induction motor will run fine at reduced voltage, provided that it is not heavily loaded.
Any attempt to use the nameplate HP of the motor would result in a current about twice the design figure and rapid failure.
Provided however that the line current does not exceed the nameplate figure, and the load will function at the reduced HP it will be fine.

The motor will run a little slower due to increased slip, but nothing like half speed.

A centrifugal water pump run backwards will still pump water, but less effectively.

 

[charlie b]

It is only pumping about half the volume-per-minute at half the head as it is supposed to be moving.

I agree with Iceworm: these two symptoms do not match. Perhaps the person reporting the symptoms is not being precise in his or her measurements or reported numbers. I might suggest following up on the reported symptoms.

The first question I would have asked has already been asked: Is this a new installation, or a recent repair, and has the system ever worked properly?

My first inclination would be to look for a mechanical cause: clogged strainer, low volume of water in the sump, downstream valve not fully open, bad bearings, etc.

The pump is wired at 240V 3-phase.

I don?t see a lot of installations that have that voltage available. Is the motor being supplied with 208V three phase? That could account for some volume and pressure issues.

 

[Besoeker]

Any attempt to use the nameplate HP of the motor would result in a current about twice the design figure and rapid failure.

Probably a lot worse than twice.
[/QUOTE]

 

[SShaffer]

Should run but with 1/4 the power.

Should run but with 1/4 the power.

If the motor is rated 480Vac at the nameplate FLA, when you run 240Vac you will double the running amperage. The windings won't / aren't designed to handle the heat that will build up. Motor will burnup and / or catch on fire. How long it takes depends on the robustness of the motor. You may not trip the overload protection depending on the level of current the pump is ideally supposed to run on.

Lets look at ohms law on this one. The inductance (resistance) of the motor hasn't changed by hooking a different voltage to it so the current draw is exactly 1/2 of the 480v current draw. I=V/R so that won't burn up the motor. The power input for the motor is however 1/4 of the rating. (P= 1.73*(V^2)/R) (240/480)^4 (the actual output will be less and 1/4 due to the mechanical losses of the motor being a greater fraction of the output total).

The Reverse will quickly burn up a motor (set for 240v run on 480v) (4x the power consumption).

 

[mike_kilroy]

I was going to look this up to refute the 1/2 power comment but did not make time too; I too thought power out would go down by V^2 term.

But in addition to 1/4 the power, said another way - 1/4 the torque, means if they DID try to pull full or even 1/2 NP torque, it would likely send the motor over the breakdown knee and into the very high current draw area; so if more than 1/4 torque is asked for the motor will burn up from very high current also!

 

[Besoeker]

Lets look at ohms law on this one. The inductance (resistance) of the motor hasn't changed by hooking a different voltage to it so the current draw is exactly 1/2 of the 480v current draw. I=V/R so that won't burn up the motor. The power input for the motor is however 1/4 of the rating. (P= 1.73*(V^2)/R) (240/480)^4 (the actual output will be less and 1/4 due to the mechanical losses of the motor being a greater fraction of the output total).

It isn't quite as straightforward as Ohm's law.
The available torque is approximately proportional to the square of the applied voltage. Half volts, quarter torque. The actual torque produced will be such as to balance the torque assuming the motor can produce that. Otherwise it will stall or crawl neither of which is good news for it.

A bit of a non-technical explanation.
An induction motor can be modelled much like a transformer. The stator is the primary and the rotor is the secondary.
Torque is the product of the flux produced by the stator and the current in the rotor.
Half the applied voltage means half the stator flux and, for the same slip, half the induced voltage in the rotor to drive the rotor current. A half times a half.....

Here are characteristics for a motor driving a centrifugal compressor:

The torque is given for 100% volts and 80% volts. It was a reduced voltage (ATX) starter. In the event, the stater was set on the 90% tap. It was a trade off between run up time and limitations on available supply current.

The Reverse will quickly burn up a motor (set for 240v run on 480v) (4x the power consumption).

It isn't a linear effect.
If you double the applied volts, the stator will go into magnetic saturation much like a transformer would. The current would be huge.