# 3 phsae motor wired for wrong voltage



## WhatsBurning

Hello all,
I have a question as what really happens when a 3 phase motor is wired for the wrong voltage. Case in point I found a failed motor at work that had been wired for 480 but the actual voltage is 230 for that building. So what happens to the current, voltage wear and tear of the motor. In this case the motor was a 1 HP, 3400 rpm motor used for a magnetic pump.


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## dronai

Overloads will trip hopefully !


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## circuitman1

usually they want start, or on something like this they may go very slow, also the amp draw may double.:whistling2:


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## 347sparky

I looked at a milling machine with the same scenario. The guy said nobody in the shop wanted to use it because it didn't want to cut very well and seemed slow. It never tripped out the overloads and always ran. Did some checking and found it wired for 480 and fed with 240v. Told him I did not know if the motor would last and for him to try it, it's been at least 2 months now and no calls on it. Also found out when they bought the machine, one of the machinists wired it up.


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## 8V71

WhatsBurning said:


> Hello all,
> I have a question as what really happens when a 3 phase motor is wired for the wrong voltage. Case in point I found a failed motor at work that had been wired for 480 but the actual voltage is 230 for that building. So what happens to the current, voltage wear and tear of the motor. In this case the motor was a 1 HP, 3400 rpm motor used for a magnetic pump.


What has failed on the motor? Do the windings look or smell like they have been overheated? I would think there is a good chance that the motor has not been damaged by the undervoltage.


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## JRaef

WhatsBurning said:


> Hello all,
> I have a question as what really happens when a 3 phase motor is wired for the wrong voltage. Case in point I found a failed motor at work that had been wired for 480 but the actual voltage is 230 for that building. So what happens to the current, voltage wear and tear of the motor. In this case the motor was a 1 HP, 3400 rpm motor used for a magnetic pump.


Motor base speed would be the same, speed is based on frequency and number of poles.

Motor STARTING torque drops by the SQUARE of the voltage reduction. So if you have a Design B motor where LRT is 160% of FLT, applying 1/2 voltage will reduce it to 1/4th of that value, so 40% of FLT. If that's enough to accelerate the load, then fine. If not, the motor stalls.

Assuming the reduced starting torque was enough to accelerate the load, motor running torque would drop proportionately to the voltage reduction. That may cause a precipitous increase in slip if the load is designed for the full torque output of the motor, in which case the current will increase as the motor tries to return to normal slip speed. That current increase generally results in an overload condition, but it is entirely dependent on the load.

Accelerating a saw blade: easy peasy so it looks OK. KEEPING a saw blade running as load is applied requires torque. 

Accelerating a magnetic pump: not as easy, unless maybe the magnet can be decoupled to let the motor start. But once the motor is at speed and full load is applied, the same basic principles apply. Assuming it is a centrifugal pump then means that there is little flow at first, so that might be OK, but as flow increases, so does load on the pump. If someone compensated for the reduced capacity of the pump by restricting flow, it could conceivably do that forever, as long as the amps remain below the motor FLC rating.

Damage would take place only if the motor overload protection is missing or faulty.


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## micromind

An autotransformer type of starter intentionally applies a reduced voltage to a motor during starting. Typically they have taps at 50, 65, or 80% of full voltage. 

But this is only during starting. After a set time, full voltage is applied. 

A wye-delta starter is similar, but it uses the motor itself as the voltage reducer. 

Any motor will start and run on half voltage as long as the load it drives will allow it.


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## WhatsBurning

*Thanks for the info*

Thanks for the info. The motor was pretty old and most of the fan guard was rusted away. Not sure how long the motor had been wired wrong. The load is very low as its only pumping salt water a short distance. What was a little odd is when I tried to rewire the old motor for 230 the starter would not pull in. I removed the output wires from the bottom of the starter (nothing going to the motor) and the starter would again pull in. So I removed the motor and started looking for a spare motor. I found a motor but no replacement magnet as the old one was also falling apart. I passed on to the next shift. The installed and wired as told to wire as the old one was. So right back to square one. So I guess as long as the load stays low the motor will last a while anyhow. Any over current issues should be covered by the breaker and the overloads. (I hope)

Ok on another topic as I am new here, how come there is an ad added to my post? Why when I log in then enter this thread and try to respond I get a popup saying I need to log in? I just kept refreshing my screen and then it show me logged in in the upper right of the web page. Do I need to adjust any settings?
Thanks again


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## oldtimer

WhatsBurning said:


> Thanks for the info. The motor was pretty old and most of the fan guard was rusted away. Not sure how long the motor had been wired wrong. The load is very low as its only pumping salt water a short distance. What was a little odd is when I tried to rewire the old motor for 230 the starter would not pull in. I removed the output wires from the bottom of the starter (nothing going to the motor) and the starter would again pull in. So I removed the motor and started looking for a spare motor. I found a motor but no replacement magnet as the old one was also falling apart. I passed on to the next shift. The installed and wired as told to wire as the old one was. So right back to square one. So I guess as long as the load stays low the motor will last a while anyhow. Any over current issues should be covered by the breaker and the overloads. (I hope)
> 
> Ok on another topic as I am new here, how come there is an ad added to my post? Why when I log in then enter this thread and try to respond I get a popup saying I need to log in? I just kept refreshing my screen and then it show me logged in in the upper right of the web page. Do I need to adjust any settings?
> Thanks again


 
Just Nosy !

Are you an electrician ?

Simple question , only 2 possible answers YES or NO !


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## wdestar

micromind said:


> An autotransformer type of starter intentionally applies a reduced voltage to a motor during starting. Typically they have taps at 50, 65, or 80% of full voltage.
> 
> But this is only during starting. After a set time, full voltage is applied.
> 
> A wye-delta starter is similar, but it uses the motor itself as the voltage reducer.
> 
> Any motor will start and run on half voltage as long as the load it drives will allow it.


Hmm... Are you sure about that? My understanding of the induction motor is a relationship between attraction and repulsion. The rotor is designed for a certain magnetic attraction and inductance. If the voltage is reduced, so is the induction - thus, repulsion.


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## wdestar

wdestar said:


> Hmm... Are you sure about that? My understanding of the induction motor is a relationship between attraction and repulsion. The rotor is designed for a certain magnetic attraction and inductance. If the voltage is reduced, so is the induction - thus, repulsion.


It seems to me, an undervoltage would lead to loss of speed, regardless of frequency. Maybe I'm wrong.


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## McClary’s Electrical

wdestar said:


> It seems to me, an undervoltage would lead to loss of speed, regardless of frequency. Maybe I'm wrong.


Yep. Youre wrong.


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## wdestar

mcclary's electrical said:


> Yep. Youre wrong.


I thought so. Please explain my mistake.


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## wdestar

Clown after clown.


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## wdestar

wdestar said:


> I thought so. Please explain my mistake.


You're the one who brought up the point. Please explain where I was wrong.


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## wdestar

mcclary's electrical said:


> Yep. Youre wrong.


I have no idea why this guy hates me...? Very strange person.


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## wdestar

wdestar said:


> I thought so. Please explain my mistake.


You're the one who brought up the point. Please explain where I was wrong.


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## micromind

Here's my take, based on tons of experience in the real world plus considerable tinkering and experimentation over the years.

If a basic induction motor is supplied with 1/2 of the nameplate voltage, it will produce about 1/4 of its nameplate HP while drawing roughly the maximum amps the windings can safely handle. 

Its slip (the difference between synchronus speed and actual speed) will be quite a bit more than nameplate. 

For example, if a 1 HP 3ø 4 pole 460 volt 1.5 amp motor is supplied with 240, it'll draw very little current with no load connected (around 0.5 or so), and it'll turn about 1799 RPM. If it is loaded to about 1/4HP, the current will be close to 1.5 amps (if not a bit over) and it'll turn about 1650 RPM. At full voltage and 1HP, it'd turn at around 1750 RPM. 

I don't know for sure about the long-term effects of half voltage, but I strongly suspect that if the full voltage current is not exceeded, it'd run for a very long time.


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## wdestar

micromind said:


> Its slip (the difference between synchronus speed and actual speed) will be quite a bit more than nameplate.


If the slip is increased, wouldn't the speed be as well?


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## five.five-six

wdestar said:


> If the slip is increased, wouldn't the speed be as well?


It would depend upon the mechanical load.


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## wdestar

five.five-six said:


> It would depend upon the mechanical load.


I agree with that. Without a load, there is no slip at all.


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## bkmichael65

wdestar said:


> If the slip is increased, wouldn't the speed be as well?


All motors operate with some slip even at no load. As load increases, slip increases which creates more torque, but speed remains the same as it determined by frequency and number of poles.


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## micromind

wdestar said:


> If the slip is increased, wouldn't the speed be as well?


Increased slip = decreased RPM.

Synchronus RMP is constant; it is based on HZ and # of poles.


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## wdestar

micromind said:


> Increased slip = decreased RPM.
> 
> Synchronus RMP is constant; it is based on HZ and # of poles.


Right. Re-read my post and I believe you're absolutely correct. I knew what I was thinking but the post didn't come across as accurate. My mistake.


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## wdestar

*Maybe a good experiment.*

I'll have my students wire a 230/460 motor for 460 and apply 230 and see what happens. Will have current and tach measurements and hopefully a load - without a load, I doubt we'll see what we anticipate.

Oh - better throw some temperature sensor in there as well.


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## micromind

wdestar said:


> Right. Re-read my post and I believe you're absolutely correct. I knew what I was thinking but the post didn't come across as accurate. My mistake.


Lol.....

Been there many times.....

Lol.

Seeing the results of lab testing would be cool!


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## wdestar

micromind said:


> Lol.....
> 
> Been there many times.....
> 
> Lol.
> 
> Seeing the results of lab testing would be cool!


Going to try hard to make this happen. Sorry for the aggravation.


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## micromind

wdestar said:


> Going to try hard to make this happen. Sorry for the aggravation.


No aggravation at all......

I love to experiment with stuff like this. I've learned more from this type of stuff than anything else.


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