Restarting a Delta Motor After One Winding Opens

The delta motor should even start - with one winding is open. I don't know why it shuold not re start in this condition

The rotation of the EM Field will still be there - with a jump - so the currents in the phases (remaining) may be un-equal and this may create the problem of over-loading of individual phases.

One similar article I could get by a quick search, (no use of quoting my old rusted book , and any way i have to search the relevant chapters on motor faults ) I hope this will be enough.

http://www.progress-energy.com/custservice/carcig/resourcectr/presentations/MotorProtection_VoltageUnbalance.pdf

Refer the page 4 of this article.

"The motor will continue to drive the load till it burns out or properly sized relays take it off line"

Sorry-- didn't mean to post here.

You say it starts then fails shortly...then cooks the overoad...windings etc.... all okay...High resitance across contacts on the contactor or relay when energized will do this...or a low voltage leg on the 3 phase...Should not even start on two phases...will "growl" and single phase with no movement..an overload is a fancy fuse...you can still cook them......In my experience , if a motor moves and then gives me an issue...it drops a phase , has increased amps due to a poor connection , or mechanicals bind causing resistance and increased load causing the problem...

ok 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?

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.

Please refer to my post #1 and the referred link

Do you want to repeat the same explanations ?

There are only 4 posts can you not go through them before asking a question ?

Also if a single phase is out don't you think two more are still available ?

sb,

Yes, thank you I did read your post and the attached link. The link only discusses single-phasing when one phase of supply voltage is lost due to blown fuse, etc, it doesn't go over the theory of a MOTOR winding open with all 3 phase voltages available.

I'm sorry I didn't completely understand your first post, your sentence structure was a little strange:

"The delta motor should even start - with one winding is open. I don't know why it shuold not re start in this condition"

I wasn't originally sure what you were trying to say and wanted some other opinions as well. Anyway thank you for responding.

Sorry, but sb has misinterpreted. IF THE MOTOR IS ALREADY RUNNING, it will continue to run with an open phase, but it will not re-start with only 2 of the 3 phases. The current and therefore the magnetic fields have no rotation element to it. Others have said this, I am just trying to counter the misinterpretation.

So back to your problem; there are a couple of possibilities I can think of off the top of my head:

1. The problem is two fold; you have something that is causing an overload in Phase C, AND your trip mechanism inside of the OL relay is welded, so the motor starter never drops out and the overload condition eventually melts the heater element. It is, after all, a "heater" element!
2. You have a bad / loose connection between the contactor and the overload on Phase C. The added heat from the high resistance of that connection is burning up your heater element. But since the current THROUGH the heater element is not necessarily high, it is not getting down into the relay operating mechanism to cause it to to disengage and trip.
3. You HAD a loose connection on the heater element itself, then when they replaced it, the connection points on the OL relay are now so badly damaged that you can no longer get a good solid connection when you mount the heater element.
4. Your motor has a bad connection, but it only shows up when it heats up. So when your guys test it, the circuit looks good, but when the motor runs under load, it heats up and causes an overload. This is a likely co-factor in #1 above.

I would start by first getting rid of that overload relay and replace it with a Solid State version. No chance of resistance / welding problems with those. A good one will also be able to pick up a phase current imbalance, which can be useful in determining the orginal root cause of the fault.

JRaef,

Thank you for your reply. However the thermal O/L relay DID open and trip the contactor, just not before the heater burnt up. Either way the O/L apparently did not trip soon enough.

To me it's like which came first, the chicken or the egg? Did a bad motor cause the excessive currents which burnt up the overload heater? OR did the heater burning open and the O/L not tripping soon enough CAUSE the motor to be damaged? I don't know what the intial problem was. My recommendation was to change out the motor being that it had bad ohm readings, and replace the entire O/L relay not just the heater. (Unfortunately a solid state device is not feasable for us for the relatively low importance of this overhead door drive motor) We will see if the problem will be solved when those get changed out.

ALSO, to get back to electrical theory, JRaef are you stating that this motor could not restart on 2 phases?

SB seems to support the theory that a Delta could conceivably restart with all the line voltages present but with one motor winding opened up. That was my original question.

This is what someone else told me:

"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."

Does this seem possible?

Thanks for your replies.

If you think on other side the Y may start and not delta

Let us say the phases are

R = V,0⁰

B = V,120⁰

Y = V,240⁰

The motor does not know it, and hence the voltage across RB is (say Y has opened)

= (v,0⁰) - (V,120⁰) and this applies between two phases in series, without any phase shift and thus no rotating field, no toqrue at all.

But you do get it in Y connection , with neutral

Hope i am correct (brushing up what i learnt ages back)