I have a 3- phase dual voltage motor 230/460v with 9 leads available for connection to either low or high voltage. The source voltage is 480V. This motor is matched to a jockey pump. I know if wired correctly there will be no problems. But am not sure if wired incorrectly what problems will or can occur. So what would happen if the motor leads were wired for a low voltage connection (230V) and fed with 480V
Three Phase Dual Voltage Motor
- Thread starter arnolds
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crossman
Senior Member
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- Southeast Texas
I guess the first question has to be "why not just wire it for 480 and be done with it?" Question 2 would be "why are you asking this? Did someone do it be accident?"
Anyway, good question. My educated guess:
Under start-up, the motor will draw excessive current. I say this because we are essentially doing the opposite of reduced-voltage-starting which lowers the starting current. Putting over-voltage during starting should make the current much higher. So the start-up surge will be pretty high.
Now, once the motor is running:
If not loaded, the motor will probably do just fine, assuming the winding insulation is good for 480 which it most likely is... will probably pull more current than if connected to a proper LV source, but still would be fairly low. Of course, where would we be running the motor with no load?
Now, if loaded, it is going to act like a higher HP motor and is going to pull excessive current, maybe enough to burn up the windings. Now, if this motor is properly protected by fuses and overloads, one or the other would probably shut the motor down.
Looking forward to more commentary from the rest of the brainiacs...:smile:
Anyway, good question. My educated guess:
Under start-up, the motor will draw excessive current. I say this because we are essentially doing the opposite of reduced-voltage-starting which lowers the starting current. Putting over-voltage during starting should make the current much higher. So the start-up surge will be pretty high.
Now, once the motor is running:
If not loaded, the motor will probably do just fine, assuming the winding insulation is good for 480 which it most likely is... will probably pull more current than if connected to a proper LV source, but still would be fairly low. Of course, where would we be running the motor with no load?
Now, if loaded, it is going to act like a higher HP motor and is going to pull excessive current, maybe enough to burn up the windings. Now, if this motor is properly protected by fuses and overloads, one or the other would probably shut the motor down.
Looking forward to more commentary from the rest of the brainiacs...:smile:
Three Phase Dual Voltage Motor
I agree crossman, the codo assoc. installed a brand new jockey pump and let their maintenance man wire it. I doubt it was an accident. I got a call from the property manager telling me that the motor was smoking and found it wired that way. When I wired it correctly the motor worked fine but the pump was leaking water because the seals were damaged. Guess who got blamed? Do you know if incorrect wiring the motor will damage the pump seals
I agree crossman, the codo assoc. installed a brand new jockey pump and let their maintenance man wire it. I doubt it was an accident. I got a call from the property manager telling me that the motor was smoking and found it wired that way. When I wired it correctly the motor worked fine but the pump was leaking water because the seals were damaged. Guess who got blamed? Do you know if incorrect wiring the motor will damage the pump seals
crossman
Senior Member
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- Southeast Texas
My first thought is that i don't think the improper wiring would have damaged the seals. The motor speed in this situation is mostly dependent on the frequency, not the voltage. It would not have turned much faster at 480 then it would have with 240.
Now, I suppose the start-up "jolt" to the equipment would have been greater. Another reason for using low-voltage starting on a large motor is to lessen the strain on the equipment being started. In this instance of incorrect wiring, the pump would have jumped from zero speed to full speed very quickly as compared to 240... still, I don't see that it would be enough to damage the seals.
If the motor was smoking, then there may be some reduced insulation life, in other words, the motor may not last as long as it normally should.
Now, I suppose the start-up "jolt" to the equipment would have been greater. Another reason for using low-voltage starting on a large motor is to lessen the strain on the equipment being started. In this instance of incorrect wiring, the pump would have jumped from zero speed to full speed very quickly as compared to 240... still, I don't see that it would be enough to damage the seals.
If the motor was smoking, then there may be some reduced insulation life, in other words, the motor may not last as long as it normally should.
crossman
Senior Member
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- Southeast Texas
Reread your second post. It is my opinion that neither the improper or proper wiring connection would have damaged the seals.
ptonsparky
Tom
- Occupation
- EC - retired
Wiring the motor for the wrong voltage creates a lot smoke. Smoke means heat. I could certainly see why you could be blamed for the rubber seals failing. Especialy on a new installation.
Insualtion value of the windings has nothing to do with the amount of heat produced by a motor that has been misswired.
Insualtion value of the windings has nothing to do with the amount of heat produced by a motor that has been misswired.
gar
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- EE
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All inductors (coils in transformers, coils in motors, and plain inductors) with a magnetic material core (steel, iron, etc.) exhibit core magnetic saturation as the magnetic flux density increases. The maximum flux is proportional to voltage and inversely to frequency.
To minimize size for a given power level the magnetic circuits are designed to operate at a flux level starting into the saturation area. As you go more into saturation the magnetizing current rises very rapidly. If you apply double voltage to one of these devices the current becomes extremely high and the coil will severely overheat. Internally things get very hot, that could include the seals. If on long enough, maybe a few minutes or less, the insulation will fail and the coil will short and burnout or catch fire, whatever. Obviously internal or external overload or overtemperature devices should open the circuit.
If you have a coil of a fixed number of turns and winding construction and you double the voltage then voltage between adjacent turns will double. If an individual winding that is rated for 240 V has 480 V to it, then the insulation stress level is much higher.
A lot of thin insulating materials have a voltage breakdown of about 1000 V/0.001" of insulation. However, for sustained application of the voltage a more realistic value is 200 to 300 V for these materials. This breakdown voltage degrades as the temperature rises.
Here is a simple question. If a motor was rated to operate at 240 V why would you logically expect it to run without overheating at 480 V? There is a reason for ratings.
To see the effect of saturation connect a small transformer, 120 V primary, to a Variac. Use a current probe and oscilloscope to view the input current with no load on the secondary. Sync the scope on the AC line witha a sweep rate of 2 to 5 milliseconds per divison. This provides 20 to 50 MS for the full width of the screen. One full 60 Hz cycle is 16.7 MS. From about 100 to 140 V gradually increase the voltage and observe the rapid growth in peaking of the current near the voltage zero crossing.
.
All inductors (coils in transformers, coils in motors, and plain inductors) with a magnetic material core (steel, iron, etc.) exhibit core magnetic saturation as the magnetic flux density increases. The maximum flux is proportional to voltage and inversely to frequency.
To minimize size for a given power level the magnetic circuits are designed to operate at a flux level starting into the saturation area. As you go more into saturation the magnetizing current rises very rapidly. If you apply double voltage to one of these devices the current becomes extremely high and the coil will severely overheat. Internally things get very hot, that could include the seals. If on long enough, maybe a few minutes or less, the insulation will fail and the coil will short and burnout or catch fire, whatever. Obviously internal or external overload or overtemperature devices should open the circuit.
If you have a coil of a fixed number of turns and winding construction and you double the voltage then voltage between adjacent turns will double. If an individual winding that is rated for 240 V has 480 V to it, then the insulation stress level is much higher.
A lot of thin insulating materials have a voltage breakdown of about 1000 V/0.001" of insulation. However, for sustained application of the voltage a more realistic value is 200 to 300 V for these materials. This breakdown voltage degrades as the temperature rises.
Here is a simple question. If a motor was rated to operate at 240 V why would you logically expect it to run without overheating at 480 V? There is a reason for ratings.
To see the effect of saturation connect a small transformer, 120 V primary, to a Variac. Use a current probe and oscilloscope to view the input current with no load on the secondary. Sync the scope on the AC line witha a sweep rate of 2 to 5 milliseconds per divison. This provides 20 to 50 MS for the full width of the screen. One full 60 Hz cycle is 16.7 MS. From about 100 to 140 V gradually increase the voltage and observe the rapid growth in peaking of the current near the voltage zero crossing.
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crossman
Senior Member
- Location
- Southeast Texas
Some good points above. I didn't think about the shaft getting hot enough to transfer heat to the pump seals. Sounds reasonable to me.
This scenario brings up a whole 'nother question: What about the old maxim of "if you raise the voltage, you lower the current?"
For example, if we wire a motor for 480v and run it at 480v, but then we lower the voltage to 450v, does the current go up or down?
This scenario brings up a whole 'nother question: What about the old maxim of "if you raise the voltage, you lower the current?"
For example, if we wire a motor for 480v and run it at 480v, but then we lower the voltage to 450v, does the current go up or down?
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
crossman:
Magnetizing and load current are two quite different components.
Ignoring magnetizing current, and assuming frictional and I squared R losses are constant, then input power equals output power. Thus, for a constant power output when input voltage is lowered the input current must rise.
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Magnetizing and load current are two quite different components.
Ignoring magnetizing current, and assuming frictional and I squared R losses are constant, then input power equals output power. Thus, for a constant power output when input voltage is lowered the input current must rise.
.
crossman
Senior Member
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- Southeast Texas
Yes, but are there any real-world loads where the output horsepower would stay constant if the voltage is lowered?
Or does lowering the voltage increase the slip = the motor slows down somewhat and is thereby producing less horsepower = less current?
Or does lowering the voltage increase the slip = the motor slows down somewhat and is thereby producing less horsepower = less current?
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
080417-0957 EST USA
crossman:
So as to not get into questions of how much slip occurs, this is dependent upon rotor resistance, and how the load power requirement varies with speed, consider a synchronous motor. Now clearly until you break sync the speed is constant.
With an induction motor it is clearly dependent upon the motor characteristics inter-related with its load.
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crossman:
So as to not get into questions of how much slip occurs, this is dependent upon rotor resistance, and how the load power requirement varies with speed, consider a synchronous motor. Now clearly until you break sync the speed is constant.
With an induction motor it is clearly dependent upon the motor characteristics inter-related with its load.
.
LarryFine
Master Electrician Electric Contractor Richmond VA
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A slight increase in slip causes a large increase in current (if available) in an attempt to maintain the torque.crossman said:
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
The point about raising the voltage and lowering the current is an approximation that you get when you assume that the motor is 100% efficient...another thing that doesn't actually happen in the real world 
Because motors are often in the 90%+ efficiency range, it is actually a pretty good approximation around the nominal operating point.
Since real motors are not 100% efficient, then the voltage versus current characteristics will not follow an ideal constant power curve.
For any given drive frequency and torque, the motor will have an ideal operating voltage at which it is most efficient. So on top of the real world 'non-constant load' breaking our approximation, you have real world 'non-constant efficiency' breaking the approximation.
A real world induction motor at its nominal operating voltage and _no load_ probably draws somewhere between 30% and 60% of its nominal full load current. This is almost all reactive current (no real power delivered), and is drawn to create the motor magnetic field.
The relationship between the magnetizing current flow and the applied voltage is very non-linear, and strongly depends upon the saturation of the iron. With 480V applied to a 240V motor, I would expect so much magnetizing current to flow that I am very surprised that no OCPD or overload popped.
Is it possible that the motor was in fact wired for 480V, but with a coil reversed, or with the rotation reversed?
-Jon
Because motors are often in the 90%+ efficiency range, it is actually a pretty good approximation around the nominal operating point.
Since real motors are not 100% efficient, then the voltage versus current characteristics will not follow an ideal constant power curve.
For any given drive frequency and torque, the motor will have an ideal operating voltage at which it is most efficient. So on top of the real world 'non-constant load' breaking our approximation, you have real world 'non-constant efficiency' breaking the approximation.
A real world induction motor at its nominal operating voltage and _no load_ probably draws somewhere between 30% and 60% of its nominal full load current. This is almost all reactive current (no real power delivered), and is drawn to create the motor magnetic field.
The relationship between the magnetizing current flow and the applied voltage is very non-linear, and strongly depends upon the saturation of the iron. With 480V applied to a 240V motor, I would expect so much magnetizing current to flow that I am very surprised that no OCPD or overload popped.
Is it possible that the motor was in fact wired for 480V, but with a coil reversed, or with the rotation reversed?
-Jon
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
A motor wired for 240V, but hooked up to 480V will run at a much higher speed. The higher speed will cause the impellar to cavitate. This in turn will cause the seals to be damaged, hence a leaking pump.
crossman
Senior Member
- Location
- Southeast Texas
Most likely we are talking about a 3-phase squirrel-cage induction motor. The actual motor speed cannot exceed the synchronous speed. Typical induction motor slip will be around 4%. Supplying 480 to an induction motor connected for 240 is not going to increase the speed hardly at all.
crossman
Senior Member
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- Southeast Texas
Okay for the rest of you....
Given: a 480 volt 3-phase 10 HP induction motor is connected correctly for 480 volts. The motor is connected to a large fan and the motor is drawing 9 amps with a supply voltage of 480.
If I change the voltage to 470, does the current go up or down?
If I change the voltage to 450, does the current go up or down?
If I change the voltage to 490, does the current go up or down?
If I change the voltage to 510, does the current go up or down?
Given: a 480 volt 3-phase 10 HP induction motor is connected correctly for 480 volts. The motor is connected to a large fan and the motor is drawing 9 amps with a supply voltage of 480.
If I change the voltage to 470, does the current go up or down?
If I change the voltage to 450, does the current go up or down?
If I change the voltage to 490, does the current go up or down?
If I change the voltage to 510, does the current go up or down?
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
crossman:
Because I am not involved in fan design I can not provide a good answer.
Fan power and torque increase with speed given other conditions are constant. Induction motor speed drops with increasing load. Maximum motor torque will drop with voltage, however actual torque supplied is a function of the load. Magnetizing current will increase with input voltage. Also I believe that many fan motors will be designed with low resistance rotors for maximum efficiency. Thus, relatively low slip.
Here is my guess. Above 480 current increases, and below 480 current increases. Or maybe there is a minimum at a different point. Or maybe current is monotonic. If monotonic it would increase with voltage.
.
Because I am not involved in fan design I can not provide a good answer.
Fan power and torque increase with speed given other conditions are constant. Induction motor speed drops with increasing load. Maximum motor torque will drop with voltage, however actual torque supplied is a function of the load. Magnetizing current will increase with input voltage. Also I believe that many fan motors will be designed with low resistance rotors for maximum efficiency. Thus, relatively low slip.
Here is my guess. Above 480 current increases, and below 480 current increases. Or maybe there is a minimum at a different point. Or maybe current is monotonic. If monotonic it would increase with voltage.
.
crossman
Senior Member
- Location
- Southeast Texas
gar said:
Thanks for the reply. My guess is that within the voltage ranges given, the higher the voltage, the higher the current. But that is just a guess.
I would love to hear some more opinions and thoughts.
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