Can a 3 phase motor start with power running in 2 phases
Can a 3 phase motor start with power running in 2 phases
(OP)
Hi all,
Just a quick simple question. I had a problem with an elevator and the contactor was returned to me. It looks like one of the poles is slightly damanged so it looks like the motor was probably single phasing, causing burn out. My question is can a motor even drive any load with only 2 poles of the contactor making good contact? I know that once a motor is running 3 phases, single phasing will then cause heat damange. But can a motor even start with only two phases in place with power?
Thanks!
Just a quick simple question. I had a problem with an elevator and the contactor was returned to me. It looks like one of the poles is slightly damanged so it looks like the motor was probably single phasing, causing burn out. My question is can a motor even drive any load with only 2 poles of the contactor making good contact? I know that once a motor is running 3 phases, single phasing will then cause heat damange. But can a motor even start with only two phases in place with power?
Thanks!





RE: Can a 3 phase motor start with power running in 2 phases
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Can a 3 phase motor start with power running in 2 phases
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RE: Can a 3 phase motor start with power running in 2 phases
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Can a 3 phase motor start with power running in 2 phases
http:/www.jraef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Can a 3 phase motor start with power running in 2 phases
RE: Can a 3 phase motor start with power running in 2 phases
That, and the fact that on most elevators, the "Close Door" button is only there for Fire Fighters when they use their override key. For the general public, it's basically a placebo. It appears to work only because by the time you get antsy enough to push it, the time delay has elapsed already anyway.
RE: Can a 3 phase motor start with power running in 2 phases
If there's any solid state current control to soften the ride, there should usually be very low currents during break/make. Have several times experienced contactor faults, while elevators was still operating "almost" normal. (Only complaint was starting/stopping "not as smooth as normal".)
Cause was in these cases shorted thyristor on drive, and other symtoms seen was sparking on contactors (should normally be very little on this type drive) and unusually hot motor.
These was as I remember Schindler drives, which also used a 2nd winding-set in motors, for eddy current braking.
RE: Can a 3 phase motor start with power running in 2 phases
If the motor is rotated fast enough by an external force establish a rotating field, it may come up to speed.
The torque will be much reduced.
The exception.
If a motor has power factor condensors connected to the motor terminals and a capacitor fuse is blown on one of the energised phases, leaving the capacitors connected from an energised phase to the motor terminal with no power, the motor will probably start as a capacitor start motor and run at reduce torque.
In all these cases, the motor is assumed to be unloaded or lightly loaded. In the case of a load with torque requirements in excess of the reduced torque, the motor will either stall or run slow and hot.
respectfully
RE: Can a 3 phase motor start with power running in 2 phases
RE: Can a 3 phase motor start with power running in 2 phases
RE: Can a 3 phase motor start with power running in 2 phases
RE: Can a 3 phase motor start with power running in 2 phases
In our workshop, i have tried alot of time testing 3 phase motor on single phase power supply (it happens supplied on rated voltage) especially to test broken rotor bars but the result is the same as what appears above!
The rotor changes or deviates its angular position but not a one complete turn, eventually it hums and rapid heat rises on its frame.
RE: Can a 3 phase motor start with power running in 2 phases
You can also start and run a three phase motor on single phase,you have to create the other phase by using a capacitor,once up to speed the cap is dist,the motor will run happily on single phase.
Remember this is not a substite for 3 phase supply,the motor will not provide the same torque or power,or will overheat
RE: Can a 3 phase motor start with power running in 2 phases
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Can a 3 phase motor start with power running in 2 phases
RE: Can a 3 phase motor start with power running in 2 phases
Two phases of a threee phase group is effectively single phase. Two phases of a quadrature two phase system is definitely two phases. There was a thread about this a while ago, but the search function has hidden it.
http://www.tpub.com/neets/book5/18a.htm gives the idea.
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Sometimes I only open my mouth to swap feet...
RE: Can a 3 phase motor start with power running in 2 phases
Just FYI, there was a time when what you were describing was called a "2 phase" motor, but that was confusing when compared to a true 2 phase motor which is designed to be fed with a true 2 phase (poly-phase) system. I don't know if any 2 phase poly-phase systems still exist, but the motors do and users must adapt 3 phase power systems to feed them. Long ago the motor you are describing was changed to being called a "split phase" to avoid this confusion, although I recently came across a university paper that described it as "2 phase" (U Profs are often slow to change). It is still technically referred to as a single phase motor because it is fed with single phase power.
ScottyUK, I think this was the thread you were referring to.
thread237-146050
http:/www.jraef.com
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RE: Can a 3 phase motor start with power running in 2 phases
I would agree with you there,motors are not produced for two phase supply.
However ALL single phase induction motors are in effect two phase.The other winding is for starting /running and is supplied with current that leads the current in the main winding,this phase shift produces a rotating field,it does not matter what the phase angle is.
Two phases are not effectivly single phase,unless the phase angle was unity,however slight the phase difference you will have a rotaing field which will produce torque,2 phases of a three phase supply will be 120deg shift,this is why (most)3 phase machines start ok.
A two phase supply was quadrature,so the effect was not so good hence the demise of two phase supplies.The performance of 3 phase motor can nearly match a DC machine.
RE: Can a 3 phase motor start with power running in 2 phases
I suppose there is some technical truth to what you say, but the CONVENTION is to call the power that we provide to residences "single phase power" and the motors used in them are "single phase motors", regardless of whether they are connected line-line or line-neutral. In what you are describing, the "2 phases" are never driving the motor simultaneously, it shifts from one to the other when it reaches speed. It is still a 1 phase motor. Referring to them as 2 phase just confuses people who are not as savvy as the rest of us.
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