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Can delta motor re-start after one winding opens?

  1. Jan 21, 2009 #1
    There is an overhead door at work that will start to open but then stops. I don't know all the details because I haven't been out to the door yet, but the information has been given to me by one of my guys.

    The door is driven by a 3-phase 480 volt motor. I don't know anymore information than that. I don't know the hp or if its delta or wye connected or anything else, I have not seen it. The guys tell me they don't see anything mechanically wrong with the door, and that it operates smoothly by the manual chain.

    I am told that the C-phase thermal overload element was found completely fried. I found this to be unusual being that the overload relay's purpose is to open up the control circuit to protect the motor, in this case the heater couldn't even protect itself! This is a bi-metal type OL set at 115%. So aside from assuming the OL relay took too long to trip, they did the following checks:

    They took Meggar readings phase to ground from the load terminals and the insulation for all phases was acceptable. They did a continuity check phase to phase and said there was continuity.

    The workers proceeded to replace JUST the C-phase thermal element with a new one.(without replacing the entire O/L relay) They started the motor and the same thing happened, the motor started, the door started to open and then stopped, it tripped on thermal OL, but yet again the C-phase O/L heater was fried.

    They took resistance readings again this time from the motor with the line connections determed. They paid particular attention to the exact ohm readings this time and got 20,20,40. The electric brake was determed as well, and was not a factor into these readings. One of the guys explains the readings by saying it is a delta motor with one winding burnt open. I asked him if he could read the nameplate, but he could not.

    I understand that a single-phased motor will continue to run once started due to inertia and the one phase being energized, BUT I though it could NOT re-start. His theory was that an "opened" delta could re-start if it had all three-phases supplied to it, it would still energize 2 out of 3 phases, energizing enough poles to give it the necessary magnetic fields.

    Can anyone explain some theory behind single-phasing, and if it is possible to restart a delta-wound motor that has had one winding burnt open?
     
  2. jcsd
  3. Jan 22, 2009 #2
    Can anyone with an engineering background explain to me if it this theoretically possible?

    I understand a motor won't start when one phase of voltage being supplied from the source is lost due to a blown fuse, a bad contact or even a destroyed overload heater. The motor will only have a single phase combination of voltage availabe and has no phase differential to start spinning.

    What I want to understand is:

    *if all 3 phases of the supply voltage are balanced and available to the motor

    AND

    *the motor is a Delta with one winding open, will the 3-phase voltages going through the two good motor winding phases be sufficient to create the phase differential and starting torque to get the rotor spinning?

    (if it was wye with one open winding it would not allow current to flow in 2 of the 3 phase combinations, due to the one open winding being in series with the two good windings. I don't think this would allow any phase differential)

    Despite the fact the currents will increase and trip the overload device and bad things in general would happen, I just want to know if this theoretically possible.
     
  4. Jan 22, 2009 #3
    I don't think it is possible for a 3 phase motor to start on its own with one phase missing.
     
  5. Jan 23, 2009 #4

    stewartcs

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    A three-phase motor will not start with one line open. If the motor is running when single phasing occurs it will continue to run as long as the shaft load is less than about 80 percent rated load and the remaining single-phase voltage is normal (rotation of the rotor produces a quadrature field that maintains rotation). However, excessive vibration and rapid increase in the phase current will damage the motor if it is allowed to continue to run.

    If you have an open winding on a delta-connected motor, the motor may be started since you still have a phase difference between the two remaining windings. This of course depends on the load. Note that the line is not open but the winding is. Since the three phases are available and assuming coil A is opened, phases A to C will energize coil C at some phase angle, and phase B to C will energize coil B at a different phase angle (120 degrees apart).

    However, a wye-connected motor won’t start with an open line or open winding.

    Hope this helps.

    CS
     
  6. Jan 23, 2009 #5
    CS,

    Thank you for your reply. Almost everyone I talked to instantly defaulted to "a 3-phase motor CAN'T re-start once single-phased."

    Thank you for clearing up the distinction in theory between a loss of a line phase and the loss of a motor winding.
     
  7. Oct 1, 2009 #6
    Not entirely true...

    An open delta system (either by design, or by failure of one transformer in a 3-bank system) will still provide all 3-phases to the motor, only at a loss of power.

    There are 2 ways to determine the amount of power available in this situation: the capacity would be approximately 87% of the power rating of the two functioning transformers (assuming identical size) or 57% of the power originally available when all three transformers are working.

    So, as long as the circuit path from the transformers to the motor is continuous (and the load is within limits), the motor will still start.
     
  8. Oct 1, 2009 #7

    stewartcs

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    No, we're talking about the line connection going from the supply to the motor winding, not the line connection to the primary on the transformer.

    If you have an open-wye/open-delta connection at a transformer, you can certainly get three-phase power from two single phase transformers. However, this is not the same situation as we are describing here.

    Losing one phase (line) to the motor from the supply (i.e. after the transformer secondary) will cause single-phasing. No rotating magnetic field will be generated and the motor will not start.

    CS
     
  9. Oct 1, 2009 #8

    Agreed. I was just pointing out that there are different ways of looking at a loss of phase. For example: an open motor winding, an open transformer winding, or an open connection between the two.

    As I stated before, "as long as the circuit path from the transformers to the motor is continuous (and the load is within limits), the motor will still start."
     
  10. Oct 1, 2009 #9
    Correct, and the fact that the phase-to-phase dc resistance measurements were 20, 20, and 40 ohms proves that the motor was wired as a delta, not a wye configuration, and had one coil open.
    Bob S
     
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