3 Phase Motor

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MOD85

Member
Hello Everybody

The past friday i was talking with a co-worker and we where arguing about 3 phase motor.

So he says that if a configure the connections of a motor for work at 220VAC, and by making an arrange of connection, switch the configuration for working with 440VAC, and still supplying the 220VAC the motor RPMs will drop down...

what do you thing? i can be done??

Thanks in advance.
 

haskindm

Senior Member
Location
Maryland
Huh? The RPM of an AC motor is controlled by the frequency of the AC circuit - thus the need for "variable frequency drives". I don't know how you could run a 220 volt 3-phase motor from a 440 volt 3 phase supply.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
I got the impression they want to use a 440 volt connection on the motor and supply 208 volts (220V??) to it. Will it run?

I think it will run very slowly and not sure what problems it may cause.
 

zog

Senior Member
Location
Charlotte, NC
Hello Everybody

The past friday i was talking with a co-worker and we where arguing about 3 phase motor.

So he says that if a configure the connections of a motor for work at 220VAC, and by making an arrange of connection, switch the configuration for working with 440VAC, and still supplying the 220VAC the motor RPMs will drop down...

what do you thing? i can be done??

Thanks in advance.

Speed = (Hz *120)/ # of poles

Only way to change speed is either change frequency or the # of poles.
 

Cold Fusion

Senior Member
Location
way north
I'm out of my area, but here is my understanding of the physics:

Torque is porportional to V^2. So if the voltage is dropped to half, the available torque drops to 25%. The torque-speed curve shape doesn't change much. So the 40hp, 460V motor you started with is now a 10hp motor at 230V. It should make rated speed at 10hp output.

Just curious, where are you guys at? An old mining camp maybe? I haven't seen any 440V systems in maybe 45 years. All of the 440V U frame motors I've dealt with were eventually rewound to 460V - long ago.

cf
 

david luchini

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Location
Connecticut
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Under no load, the synchronous speed would not change, that is a 440V 1800RPM motor would still spin at 1800RPM if only 220V were applied to it.

Under load, however, it is a different story. Slip increases with a decrease in voltage, so reducing voltage will cause the rotor speed to decrease. But decrease in speed also reduces maximum torque. At 50% of rated voltage, I don't know if there would even be enough starting torque to start the motor under load.
 

david luchini

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Location
Connecticut
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David that would not apply to a dual wound motor is that a correct assumption?

Yes, this appeared to be a question of a dual voltage single winding motor. But the same would apply to either winding of a dual winding motor. As you decrease the voltage, the slip would increase.
 

dicklaxt

Senior Member
Okay are you then saying a dual voltage dual winding is not intended to meet the same torque specs,,,,,,,if that were the case why have a dual wound dual voltage motor,,,,,,,,,,,,confused once again but thats normal for me :grin:

dick
 

Cold Fusion

Senior Member
Location
way north
Okay are you then saying a dual voltage dual winding is not intended to meet the same torque specs,,,,,,,if that were the case why have a dual wound dual voltage motor,,,,,,,,,,,,confused once again but thats normal for me :grin:

dick
Dick -
I'm not sure what you are asking - I suspect you understand the motor model as well as I do.

The motor has 6 coils. Connect each pair in series, the motor is rated for 460V. Connect each pair in parallel, the motor is rated for 230V. The volts/turn is the same, core flux is the same, FLA through each coil is the same. Torque, speed, hp are the same.

Now take the 460V (series) connection and connect to 230V. Look at the equations for a standard induction motor model. Available torque is porportional to V^2. So at 230V, the applied voltage is .5xVn. So the available torque is 25% of that available at 460V. Will the slip increase? Depends on the load. Once the voltage is dropped to .5X the rated motor hp dropped to .25x.

cf

Whoops had to fix my math
 
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dicklaxt

Senior Member
What I'm saying is, why have the ability of being able to run the motor on 230 volts if it is not going to buy you anything or function adequately as it would at 460 volts:confused:

dick
 

Cold Fusion

Senior Member
Location
way north
What I'm saying is, why have the ability of being able to run the motor on 230 volts if it is not going to buy you anything or function adequately as it would at 460 volts:confused:

dick
It will function fine when wired for 230V and connected to 240V system - and there are still some 240V D systems out there that need motors.

cf
 

dicklaxt

Senior Member
Right on,,,, that was never the question(I thought) the OP was trying to rewire from 460 to 230 and keep it in the same service.If that wasn't the case then all bets are off.

dick
 

Cold Fusion

Senior Member
Location
way north
Right on,,,, that was never the question(I thought) the OP was trying to rewire from 460 to 230 and keep it in the same service.If that wasn't the case then all bets are off.

dick
Well close. He wants to leave the motor internally connected for 440V and connect it to a 220V supply. No reason why - I suspect just casual conversation between him and his buddy.

Useful?

No

cf
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Right on,,,, that was never the question(I thought) the OP was trying to rewire from 460 to 230 and keep it in the same service.If that wasn't the case then all bets are off.

Dick, you are correct. The OP was trying to take a dual "voltage" motor, configure it for the higher voltage, but run it on the lower voltage. In that case I don't believe the motor would have enough torque at 50% voltage to turn the load.

This is different than taking running the dual voltage motor at its proper configuration. If it is connected at 230V with the LO configuration, or connected at 460V with the HI configuration, then the motor should provide the same torque in both cases.

It should be noted that a dual voltage motor is different than a dual "winding" motor. A dual winding motor is usually used in a two speed application where the desired speeds are not at a 2:1 ratio. There are two separate windings to run the motor at two different speeds. Only one winding would be energized at a time.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Hello Everybody

The past friday i was talking with a co-worker and we where arguing about 3 phase motor.

So he says that if a configure the connections of a motor for work at 220VAC, and by making an arrange of connection, switch the configuration for working with 440VAC, and still supplying the 220VAC the motor RPMs will drop down...

what do you thing? i can be done??

Thanks in advance.
So to recap, based only on the highlighted text and foregoing all the side tracks...

The motor speed will not technically change. Speed in an AC motor is a function of the applied frequency and the number of poles in the motor winding configuration. Applying a lower voltage does nothing to change either of those.

BUT, when you reduce the voltage, you reduce the motor output torque capability by the square of the voltage reduction. So at 50% voltage, the torque drops to .50 x .50 = .25, so 25% of the available torque. And because motor HP is a function of speed and torque, and you have not technically changed the speed, you have reduced the motor HP to 1/4 of what it was. For example, if you had a 10HP motor and you applied 1/2 voltage to it, it effectively becomes a 2-1/2HP motor, but the speed is the same.

Now, if your load has not changed, and it took more than 2-1/2HP to move the load, then the motor will technically slow down. But as mentioned by others, it will stall completely and overload. That is the basics of what you need to know.



Now to the not-so-common occurrence.
When reducing the voltage AND THE LOAD but only to the point of not stalling, you will increase the slip, which causes an increase in current to probably around 300% of "normal". So if, from that 10HP example, you only needed maybe 2.6HP to move the load, it may do it. Because the voltage is reduced, the maximum current draw is equally reduced as well so that 300% is skewed by the voltage reduction; it is 300% of 25%, so it will really only draw about 75% of FLA and likely do so forever. But this is a very dangerous game. The 300% current draw comes from what is called the Breakdown Torque capability of the motor, defined as the torque produced before the motor slip increases beyond maximum. If you fall behind that curve, even a little bit, the effective increase in current rises rapidly. If you can perceive a speed droop then, that means you are ALREADY behind that curve! If, for example, you drop even 10% more speed than that peak, your current increases to 112.5% FLA (450% of 25%) and the motor will overload.

Bottom line, don't do it unless your load is SIGNIFICANTLY LESS than 25% of what it was going to be with the correct voltage.
 

btharmy

Senior Member
Location
Indiana
I just re-wired a vacum pump for a customer that re-located to a new building. It was wired 480v. so I wired it for 208v. for the new facility. He nearly jumped out of his shorts when I hit the starter. He thought something was wrong because it came up to speed in just a couple of seconds. Then he yelled at me when I leaned down with my hand on the motor, telling me I would burn my hand. I had to ask and he admitted their previous facility was also 208v. He had wired the motor wrong two years ago (for 480v.) and thought it was normal for it to take 30 seconds to get up to speed. And for it to run cool to the touch was new to him as well. I am suprised it lasted this long if it was actually running as hot as he says.
 
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