Question On Three Phase Motor
01-27-2005, 04:56 PM
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Question On Three Phase Motor
At work today was replaceing a 3-Phase motor that was wired in Delta. The replacement motor was wired Y. The question is when I moved the jumpers from Y to Delta and started it up it blew the fuses.
Put it back to Y and it ran fine. I thought that the Delta / Y pattern was set by the transformer and needed to remain the same on the motor. Just hopeing someone can explain it to me. Anyway hope everyone had a better day at work than I. Thanks for answering all my other questions before I asked. This is a great site to be a member of, and thanks again.
Put it back to Y and it ran fine. I thought that the Delta / Y pattern was set by the transformer and needed to remain the same on the motor. Just hopeing someone can explain it to me. Anyway hope everyone had a better day at work than I. Thanks for answering all my other questions before I asked. This is a great site to be a member of, and thanks again.
01-27-2005, 07:43 PM
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Well, This CAN get to be a very complex subject depending on all kinds of other factors!
In a nutshell, it all depends on what voltage the replacement motor was designed and wound for regardless if arrives Delta or Wye connected.
For instance if the replacement 3ph motor is a 208vac 3ph Wye connected motor, it CAN be reconnected in Delta configuration to operate on 120Vac 3ph...
For most applications it doesn't really matter if a motor is wye or delta if the connected voltage rating matches the source voltage.
But if a 480vac Wye connected motor is reconnected delta (230vac)yet operating on 480vac, it will blow fuses because it is operating on way too much voltage for the windings.
To complicate this, sometimes replacement motors are equipped with more winding combinations to allow operation on multiple voltages and are a crazy zig-zag combination of windings!!!
On the other hand, transformers being wye or delta can be a BIG deal especially if you are planning on switching and paralleling with other transformers. This is due to a 30 electrical degree phase shift between a Delta-Wye transformer compared to a Wye-Wye transformer EVEN IF THEY HAVE THE SAME PRIMARY AND SECONDARY VOLTAGES!!! Parallel the two and breakers blow!!!!
There are all kinds of drawbacks and bennies to distribution transformer secondaries being Wye or Delta connected...
Frequently, Wye connected secondaries in industrial plant distribution xfmrs will have their zero sequence connection(center tap of a Wye winding) grounded through a BIG resistor mounted on the outside of the transformer. This helps reduce ground fault currents when a motor for instance "grounds out".. It sure cuts down on melting of iron inside the motor by greatly limiting the arc fault current. Yet this resistor still keeps the secondary centertap essentially "grounded" at all times except during faults.
Delta transformer secondaries can be useful for the redundancy they provide especially where mission critical loads are powered. Navy ships frequently have 440vac 3ph DELTA distribution systems. ("dirty delta")
If one leg of a transformer blows, it can be rewired out from the circuit and the 3phase power can still be delivered to a piece of equipment (at ~2/3 the transformer capacity). Not pretty, but it works...
Ungrounded delta distribution systems can cause some WEIRD problems. If a ground fault occurs in one phase of a load (For instance: the A-phase winding on a motor) Nothing happens until a SECOND motor gets a ground fault on DIFFERENT phase!! BANG!!! two motors short to ground!!!
Nikola Tesla came up with the concept of the PolyPhase electrical system. It is really fascinating all the useful connections that can be made on 3ph systems!!!
There is WAY more about these systems than can be related over a text forum like this...
Sorry, if I got a "little" long winded...
Keith
In a nutshell, it all depends on what voltage the replacement motor was designed and wound for regardless if arrives Delta or Wye connected.
For instance if the replacement 3ph motor is a 208vac 3ph Wye connected motor, it CAN be reconnected in Delta configuration to operate on 120Vac 3ph...
For most applications it doesn't really matter if a motor is wye or delta if the connected voltage rating matches the source voltage.
But if a 480vac Wye connected motor is reconnected delta (230vac)yet operating on 480vac, it will blow fuses because it is operating on way too much voltage for the windings.
To complicate this, sometimes replacement motors are equipped with more winding combinations to allow operation on multiple voltages and are a crazy zig-zag combination of windings!!!
On the other hand, transformers being wye or delta can be a BIG deal especially if you are planning on switching and paralleling with other transformers. This is due to a 30 electrical degree phase shift between a Delta-Wye transformer compared to a Wye-Wye transformer EVEN IF THEY HAVE THE SAME PRIMARY AND SECONDARY VOLTAGES!!! Parallel the two and breakers blow!!!!
There are all kinds of drawbacks and bennies to distribution transformer secondaries being Wye or Delta connected...
Frequently, Wye connected secondaries in industrial plant distribution xfmrs will have their zero sequence connection(center tap of a Wye winding) grounded through a BIG resistor mounted on the outside of the transformer. This helps reduce ground fault currents when a motor for instance "grounds out".. It sure cuts down on melting of iron inside the motor by greatly limiting the arc fault current. Yet this resistor still keeps the secondary centertap essentially "grounded" at all times except during faults.
Delta transformer secondaries can be useful for the redundancy they provide especially where mission critical loads are powered. Navy ships frequently have 440vac 3ph DELTA distribution systems. ("dirty delta")
If one leg of a transformer blows, it can be rewired out from the circuit and the 3phase power can still be delivered to a piece of equipment (at ~2/3 the transformer capacity). Not pretty, but it works...
Ungrounded delta distribution systems can cause some WEIRD problems. If a ground fault occurs in one phase of a load (For instance: the A-phase winding on a motor) Nothing happens until a SECOND motor gets a ground fault on DIFFERENT phase!! BANG!!! two motors short to ground!!!
Nikola Tesla came up with the concept of the PolyPhase electrical system. It is really fascinating all the useful connections that can be made on 3ph systems!!!
There is WAY more about these systems than can be related over a text forum like this...
Sorry, if I got a "little" long winded...
Keith
01-27-2005, 07:49 PM
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In a delta configuration the three phases act as a floating ground. That is to say that as the sine wave increaces on one leg the other two legs act as a ground for it. In a Y configuration the three phases converge on the ground in the center of the "Y". At least this is the way I understand it.
The part I don't really understand is how the motor windings can make a diffrence since I thought that a motor had to be wired for the transformer that was supplying it the power. I was thinking that we had some motors that could be wired either way and work. Maybe I'm wrong and it's all in the motor windings.
The part I don't really understand is how the motor windings can make a diffrence since I thought that a motor had to be wired for the transformer that was supplying it the power. I was thinking that we had some motors that could be wired either way and work. Maybe I'm wrong and it's all in the motor windings.
01-27-2005, 08:28 PM
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Thanks Kevin that was probbaly it. The incoming voltage is 480V 3~. I don't understand all the specifics with the delta / Y windings I suppose and I'm not well versed in converting 3~ voltage to leg to ground voltage without my meter. I know that 480V converts to 277V. I didn't know that it made a diffrence in leg to ground voltage if it was delta or Y.
01-27-2005, 10:49 PM
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This is going to be hard to explain without a diagram.
Sometimes the terminal blocks are wired wrong from the factory. I have had this happen several times. If you wire it up Wye, no problem...wire it up Delta and you have a dead short.
You have three windings in the motor. If you use a continuity tester, you can find pairs of wired that are the two ends of each winding. They CANNOT be directly across from one another on the terminal block. If they are, it will work fine in Wye, dead short in Delta.
You should have a terminal block with six connections on it, like this.
4 5 6
o o o
o o o
1 2 3
Make sure all the windings are connected to the block in this fashion:
Connect one wire from winding #1 to block position 1, and the other end to block position 5.
Connect one wire from winding #2 to block position 2, and the other end to block position 6.
Connect one wire from winding #3 to block position 3, and the other end to block position 4.
To make it a delta connection, put the bridge straps so that 1 &4 are connected, 2 &5 are connected, and 3 & 6 are connected. A wye connection would connect terminals 4,5,&6 all together.
Last, connect the three phases of power to terminals 1, 2, &3. Hook up ground to it's terminal, usually on the body of the motor.
If the motor is rotating the wrong direction, switch places with any two power wires.
Sometimes the terminal blocks are wired wrong from the factory. I have had this happen several times. If you wire it up Wye, no problem...wire it up Delta and you have a dead short.
You have three windings in the motor. If you use a continuity tester, you can find pairs of wired that are the two ends of each winding. They CANNOT be directly across from one another on the terminal block. If they are, it will work fine in Wye, dead short in Delta.
You should have a terminal block with six connections on it, like this.
4 5 6
o o o
o o o
1 2 3
Make sure all the windings are connected to the block in this fashion:
Connect one wire from winding #1 to block position 1, and the other end to block position 5.
Connect one wire from winding #2 to block position 2, and the other end to block position 6.
Connect one wire from winding #3 to block position 3, and the other end to block position 4.
To make it a delta connection, put the bridge straps so that 1 &4 are connected, 2 &5 are connected, and 3 & 6 are connected. A wye connection would connect terminals 4,5,&6 all together.
Last, connect the three phases of power to terminals 1, 2, &3. Hook up ground to it's terminal, usually on the body of the motor.
If the motor is rotating the wrong direction, switch places with any two power wires.
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01-28-2005, 09:12 AM
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Duner, I've run across some unusual power dist systems onboard an aircraft carrier museum ship.
Radio Room lighting panels had 120vac 3ph DELTA!!! Try as you might, there was no 208 to run a larger A/C unit (secondaries of the lighting transformer power distribution buss were connected DELTA. 120vac 3ph.
In negotiating with the mgmt of the museum ship over trying to get a donated A/C unit installed, they stressed that the ships electrician had to make sure the prospective 120vac A/C unit had NO continuity from the neutral lead back to case ground as the h and n lead had to go across 2 phases (an ungrounded "dirty delta" 120vac lighting power source...) If it had a bond between the neut to case ground, a fault elsewhere on a different 120vac phase ,could cause a major problem for the poor A/C unit.
The normal 440vac delta power was unavailable as it was relocated for heaters for the museum guests.
The voltage relationship between a given set of 3ph windings connected delta/ Wye is 1.53. (Square root of 3)
208vac is 1.53 times 120vac
480v phase-to-phase is 1.53 times 277vac to ground.
Darn it!... I need to find a good internet URL that visually describes what I am increasingly unable to articulate over a keyboard!!!
I'll look for one to see if I can post its URL when I get back from my day trip this evening...
Hang in there, it aint that hard once you get the relationship of the 3 leg voltages when they are varying 120 electrical degrees apart...
Keith
Radio Room lighting panels had 120vac 3ph DELTA!!! Try as you might, there was no 208 to run a larger A/C unit (secondaries of the lighting transformer power distribution buss were connected DELTA. 120vac 3ph.
In negotiating with the mgmt of the museum ship over trying to get a donated A/C unit installed, they stressed that the ships electrician had to make sure the prospective 120vac A/C unit had NO continuity from the neutral lead back to case ground as the h and n lead had to go across 2 phases (an ungrounded "dirty delta" 120vac lighting power source...) If it had a bond between the neut to case ground, a fault elsewhere on a different 120vac phase ,could cause a major problem for the poor A/C unit.
The normal 440vac delta power was unavailable as it was relocated for heaters for the museum guests.
The voltage relationship between a given set of 3ph windings connected delta/ Wye is 1.53. (Square root of 3)
208vac is 1.53 times 120vac
480v phase-to-phase is 1.53 times 277vac to ground.
Darn it!... I need to find a good internet URL that visually describes what I am increasingly unable to articulate over a keyboard!!!
I'll look for one to see if I can post its URL when I get back from my day trip this evening...
Hang in there, it aint that hard once you get the relationship of the 3 leg voltages when they are varying 120 electrical degrees apart...
Keith
01-29-2005, 08:22 AM
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dunner, I Know that I amworking on specialized equipment but we do have 35V 3~ motors in my plant. They are small and really don't carry too much of a load but at 35V it's about 2 or 3 amps each.
cp, thanks for the short lesson on windings, maybe the water is flowing and clearing out the mud from my eyes.
quote "Square root of 3= 1.53 is only true for very small values of 3."
Do you mean only low voltages say up to 660 and is not correct at 15,000?
cp, thanks for the short lesson on windings, maybe the water is flowing and clearing out the mud from my eyes.
quote "Square root of 3= 1.53 is only true for very small values of 3."
Do you mean only low voltages say up to 660 and is not correct at 15,000?
The square root of 3 is 1.732, not 1.53 as SoTexRattler posted.
01-30-2005, 07:56 AM
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120 volt 3 phase
Originally posted by duner
connect the motor according to name plate. and in all my years i've never heard of 120v 3phase power
connect the motor according to name plate. and in all my years i've never heard of 120v 3phase power
01-30-2005, 03:27 PM
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Sqrt 3 DOES = 1.73 after all!!! CP: I typed that in and thought... Yeah thats right.. DUH!
That's what I get for trying to use my memory in a hurry!
K.
01-30-2005, 04:02 PM
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Well each of us has a 3ph Wye connnected ALTERNATOR installed on our CTD's.
It just has a 3ph full wave bridge rectifier hooked to the 3 phases.
Some of the ANCIENT alternators for 6volt car electrical systems were connected DELTA.
They used huge external selenium rectifiers instead of those tiny silicon diodes like we have nowadays! Those old Motorola tube type police radios, red light(s) and motor driven sirens drew a bunch of current!!
Jughead: On the old military generator, it was probably made specifically for lighting. 120vac no matter how you connected it.
Unusual that it was military surplus and it DIDN't have a gazillion different voltage connections!!! Probably hard connected that way so they could load the heck out of one or two legs and still get away with it.
I believe you said you were able to go in and cut apart the 3 windings and reconnect Wye, is that right?
I have been looking on the web for a good "graphical" description and associated text for the concept behind polyphase electrical systems. Most of the promising sites require registration or are promo's for engineering textbook sale$...
I'll keep looking and post teh URL's here when I find the right one.
K.
It just has a 3ph full wave bridge rectifier hooked to the 3 phases.
Some of the ANCIENT alternators for 6volt car electrical systems were connected DELTA.
They used huge external selenium rectifiers instead of those tiny silicon diodes like we have nowadays! Those old Motorola tube type police radios, red light(s) and motor driven sirens drew a bunch of current!!
Jughead: On the old military generator, it was probably made specifically for lighting. 120vac no matter how you connected it.
Unusual that it was military surplus and it DIDN't have a gazillion different voltage connections!!! Probably hard connected that way so they could load the heck out of one or two legs and still get away with it.
I believe you said you were able to go in and cut apart the 3 windings and reconnect Wye, is that right?
I have been looking on the web for a good "graphical" description and associated text for the concept behind polyphase electrical systems. Most of the promising sites require registration or are promo's for engineering textbook sale$...
I'll keep looking and post teh URL's here when I find the right one.
K.