Faults on three-phase motors

[oconn]

I'm trying to find some information on the effects of phase loss and line loss on three-phase motors.
Any information greatly appreciated.
Thanks

 

[dpc]

What do you see as the difference between phase loss and line loss?

 

[oconn]

I suppose i'm looking for the effects of phase loss on a star connected motor and a delta connected motor, as phase voltage on a 400v star system is 230v,but line voltage is 400v. On a delta system both line voltage and phase voltage are 400v. (in the uk)

 

[itsmoked]

In all cases the torque drops and the motor begins drawing excessive current. If the motor is started in this condition either the protection will operate or the motor will be destroyed.

Are you looking for something specific?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oconn]

Thanks for the replies, i'm just looking for the general effects.

 

[dpc]

The internal connection of the motor is probably not a factor one way or the other.

Loss of one phase while the motor is running causes severe heating in the motor due to negative sequence currents. The motor will generally keep running, but as itsmoked says, either the overloads are going to operate (if you are lucky) or the motor is going to fail dramatically.

 

[davidbeach]

The motor can be considered a black box with three leads. Whether the stuff inside the black box is connected in wye or in delta is immaterial nor can it be discerned without opening the box. What ever you do to the black box from the outside, your results are independent of the winding connection inside.

 

[waross]

The action of the motor has been fairly well described, but the system also has an effect on the action of the motor.
The motor will run with one phase missing but the back EMF will still be generated on all three phases. This is how rotary phase convertors work. If one phase has low voltage, the induction generator action will try to bring the voltage up on that phase and overheat while trying. If the phase is missing there is no load and no current to over heat the motor.
Now consider a plant where one main fuse has failed. The motor will continue to run on single phase and the induction generator effect will try to supply the load with the missing phase back to the failed fuse. A lot of large motors with no load may simply keep the voltage up on the bad phase without overheating. Think sawmill with the large motors left running during a coffee break.
This situation will not arise often, but illustrates the importance of looking at the whole system rather than just the motor. If the phase is lost in the motor controller the action of the motor may not be the same as if a main fuse or a distribution panel fuse has failed. If the phase is lost in the controller and the motor is under a light load there may not be any problems until the next time that the motor tries to start. Even if the controls are on the failed phase, induction generator action may keep the controls energized.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VladimirM]

Let us assume the following situation:
- a three-phase motor is running with two phases only (one of the phases was opened somehow after it was successfully started) and its speed is approx rated (no starting issues),
- its stator current is: Line1 = 100% of FLA, Line2 = 100% of FLA, Line3 = 0% of FLA,
- no thermal and/or overload protection installed for this motor.

How do You think: will this motor be overheated in this mode? Does it depends on stator internal connection circuit: star or delta? Why?

 

[itsmoked]

I believe it will overload and burn out. Why? Because I don't believe it will be FLA=100% it will be FLA=160% or something similar. It will lose speed, the back emf will drop, and the current will go up dramatically. This is my expectation since any and all refrigeration compressors burn up and die within a few minutes of being single phased.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

I should've included negative sequence currents are also part of the overheating problem, as dpc has already pointed out.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

In the case where the load on the motor is low, it will continue to operate without over heating and damage.
As the load is increased, the heating is also increased and the potential for failure increases.

In the case of 100% + 100% + 0%, there will be a difference in the heating of the windings as in a star connected motor, the current through winding 1 and winding 2 will be 100% and the current through winding 3 will be 0%.
In the case of a delta winding, the current through winding 1,2 will be higher than rated and so more heating will occur.

Best regards,

Mark Empson
L M Photonics Ltd

 

[VladimirM]

Itsmoked: agree to negative sequence and strongly disagree to 160% and losing its speed. When the load is low or in the idle mode there can't be 160% even if the phase failure occurs. The speed also can't change much in the idle mode since the rotor tends to run after the positively rotating part of flux.

Marke: agree to all.


Anyway, I tried to find the answer in several books and found nothing. I wonder whether there is a common answer for all motors or maybe some motors tend to burn in this case while others don't?

 

[itsmoked]

If you have a fully loaded motor, not an idling motor, and you you yank a phase it will slow down. That's my experience. But most my experience is with refrigeration and motors in refrigeration are loaded to the limit most of the time. They lose speed because they lose torque. Current goes up on the remaining phases.

Keith Cress
kcress -

http://www.flaminsystems.com