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Electrical service for a VFD motor

  1. Oct 9, 2012 #1
    I get very confused with AC electrical service, i'm a mechanical engineer so these things just confuse me. DC makes perfect sense to me.


    I need to supply a single phase motor with 230V (the specs say 230V +/- 10%), so that would make my tolerance 207V to 253V. The motor is for a variable speed pump that can be adjusted by a variable frequency drive to adjust the RPM output. It has terminals for Line1, Line2, and Neutral

    Anyway, the building has 3 wires, L1, L2, and N. When I measure across L1 and N, or L2 and N, I get 120V. When I measure across L1 and L2 I get 208V.

    So in theory I am withing the tolerance using 208V single phase, but the motor is starting to receive an over-current alarm lately and shuts down (it has operated for 6 months before with no problems or alarms). Any idea why this would start happening now?

    Also, how would I get 230V single phase power? And where does this 208V come from? I assuming the 208V is coming from a Wye Three-Phase system, and for some reason I only have two legs and a neutral at my point of work. In theory, I could get 240V single phase from a Delta Three-Phase configuration by going across poles, right? But in order to hit 230V on the dot, I would need a transformer I think?

    I'm just worried that since I'm on the low side of my voltage tolerance it's been drawing a lot of current to the motor and thus maybe burnt it out quicker than it normally would have lasted. If the motor started to degrade from this, wouldn't it increase the resistance over time and try to draw even more current? Maybe that's why the alarm started recently?

    Any help or comments would be awesome.

  2. jcsd
  3. Oct 11, 2012 #2
    What you have is a three face 208 V from y secondary transformer, one leg is missing. If you want 230V you will need a transformer, either 120 to 240 or 208 to 240, I think the first one is far more common.
    However, I don't think that that is your problem, you are overloading the motor, look for mechanical problems on your pump or bearring problems.
  4. Oct 11, 2012 #3
    So the thrird leg would give me 3-phase service right? I am just using two legs of 3-phase wye power?

    I'll check for mechanical problems. Thanks for the tip. I'm hoping it's something like that since technically I am in the operating voltage range. Even though, they recommend closer to 230V. I'm guessing the pump was made for 120/240V single-phase service where there are two lines in phase.

    I have to ask this too however at the risk of sounding stupid, because I don't know this stuff well-

    How come if I measure across Line to Netural in a 120V/240V single phase setup I get 120V, but the Line to Line would be 240V? Because in this setup I have down here where I am tapping off two of the three Lines in this Wye configuration, if I measure Line to Netural I also get 120V, but Line to Line is 208V.

    Is this because in the 120/240 residential setup, the two lines are in the same phase, but in the three-phase setup they are in different phases? Why does this change the potential?

  5. Oct 11, 2012 #4


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    Looks to me like you have a High-leg delta setup.

  6. Oct 11, 2012 #5
    I don't think I have Delta, because then if I measured pole to pole, I would get 240, but I only get 208. If I go pole to neutral I get 120
  7. Oct 12, 2012 #6
    Hello Nlaham :

    I agree with OldRookie - I am quite certain you have 2 Phases of a 3 Phase Wye. The Set up is 120V Line to Neutral and 208V L-L

    - Keep in mind you are adding vectors (120 Deg between V A-N and V B-N but you are only measuring magnitudes. As for the math - you are measuring the Voltage of Phase A RELATIVE to Phase B & the Phases are 120Deg out of phase So from Ph A to Ph B (208V) = If you could measure the halfway point between A and B - to represent that part of the Voltage vector in line with A to B it would be 120 V * Cos (30 deg ) = 103.9V - the 30 Deg is the angle between The A-B Measurement and the A to N measurement. IAs a Mech E - the vectors should make sense.

    As for the Overcurrent Tripping - there could be a number of issues - since you report the Drive/Motor/Pump are rated for 240 Single Phase and it is only getting 208 - within Spec --- but too close for comfort. The VFD rectifies the Incoming voltage to DC and then basically synthesizes real 3 Phase ( I am assuming) power. When you rectify the Incoming voltage the DC voltage is dependent on the incoming voltage, and the 3Ph Output can not REALLY make AC voltage higher then the input ( typical industrial drive) - so the motor will also only see 208V - not the 240 it is rated for ( VFD can cheat a little here and make a non-pure sinusoid - and get a little higher RMS voltage - but the Peak of the waveform is limited - this causes distortion ( harmonics ) to the motor - adding extra heating in the motor - etc.

    SOOOO -- if the motor/ pump are near max load - possibly slightly overloaded, and the line voltage is 208 and then perhaps takes an additional 10% dip - the motor then needs so much current to maintain the power output - this will cause a trip.

    Depending on the VFD you may be able to "lie" to it and change the configuration to make it think the Motor is really smaller than it is - and not drive the motor so hard. - but then you will loose pump capacity. If you NEED all of the pump capacity - you can also get a "Boost Transformer" that will have a 208 tap input and a 240 tap to feed the VFD. Sizing this I would consult the Drive Manufacturer.
  8. Oct 12, 2012 #7
    Thanks that makes a lot of sense. I believe what you are saying is probably what is happening. The transformers to go from two-line 208V to two-line 230V or 240V seem to be very expensive however. I will consult with the pump manufacturer and see. I will also see if I lower the VFD to a small output, maybe half the speed of max output, if the pump will trip.

  9. Oct 12, 2012 #8
    Hello Again Nick:

    Actually these are called buck/boost transformers because they can go ether way - I am reading between the lines and am guessing that you have 240 Single Phase to 3 Phase VFD to your motor - correct. With the largest of these typically 3 HP - I would estimate the transformer you would need to be about $350 -- really not too much compared to the trouble of the trips - and probably any other fixes.

    Last edited: Oct 12, 2012
  10. Oct 12, 2012 #9
    I believe you are correct sir. The motor's max output I think is about 3HP (3450 rpms) and it is described as a three-phase six pole motor. It is also described in the manual as a Permanent Magnet Synchronous Motor (PMSM).

    Input is described as 230VAC (single phase), 50/60Hz, at 3200 Watts. It requires a two-pole 20 amp breaker for circuit protection.

    So I think everything you assumed is about right.

    Would you be able to direct me to a couple sources so that I can start looking around at these buck/boost transformers?

    Thanks for all the help guys.
  11. Oct 12, 2012 #10

    I based my pricing on automationdirect.com - they only goto 2.0KVA ($200) - you need 2.5 to 3 - my guess. I do not know where you are located - but am seeing a few even on e-bay - but > than 2 KVA get pretty rare - so my earlier budget may be off a little. Even for 2x the $ - I think this may be the best bet - if oyu only have one system like this. Most of the larger electrical transformer manufacturers make these and the drives companies offer them as well - for the exact reason you are facing.
  12. Oct 15, 2012 #11
    I agree that adding a transformer wil get you the 240 you need. As for the voltage to ground, square root of three is used, as 208 divided by square root of three is 120.
    Now if you like to try something before you buy a transformer I will sugest increase acceleration and decelaration times and disable dynamic brake as the most symple modification to the settings on the VFD. I don't know if there is a feedback control, if there is, reduce the proportional gain to 1. Good luck
  13. Oct 16, 2012 #12
    Thanks you guys for the information. I have learned quite a few things through this. I'll let you know how it works out.

  14. Oct 16, 2012 #13
    We had a service technician look at the pump today and I found out the motor indeed needs to be replaced. I haven't seen the motor, but I'm guessing it is burnt out. This would explain why it was getting the over current alarm, but my colleague still maintains it burnt out in less than a year because we had 208V on it, and not 230V, even though the manufacturer has told me directly, it can accept as low as 207V.

    Luckily this motor is still under warranty and is being replaced, however, I'm thinking for this next motor we should spring for this transformer so it doesn't burn out in another year.
  15. Oct 16, 2012 #14
    Remember it is 208 at the time when you measured it, depending on your power quality overall a few voltage dips even for extended periods of time could have had an effect. Also the VFD could be distorting the waveform at the low voltage ( trying to put out 240), causing additional heating in the windings - so still check its settings.
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