Can a 3 phase motor start with power running in 2 phases

[noel0589]

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!

 

[itsmoked]

Yes it might start. Might not either. But you could certainly burn it out both running or not. It will burn out not running more quickly but protection is more likely to protect it "well enough". If it is running then badly selected protection may allow its burnout.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

I've never seen one start. Rotate, yes, but get to full speed? No.

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[itsmoked]

I agree. On the case of an elevator going down it might reach running speed. OP hasn't said if this was hydro or cable tho.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Good point. It may start if it was a traction elevator going down. Not hydraulic though.

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[davidbeach]

On the case of an elevator going down it might reach running speed.

It may start if it was a traction elevator going down.

A very common misconception. Traction elevators are counterweighted for half their rated capacity. So in most cases the elevator regenerates going up and requires power to go down. Only when the elevator is heavily loaded does it require power to go up or regenerate power going down.

 

[jraef]

OK, I guess I knew that somewhere in the dark recesses of my memory. I worked for a summer at Chevron as an elevator tech intern at a 50 story high rise in downtown San Francisco, although all of our elevators used big DC motors. I spent most of my time trying not to toss my cookies when looking down the shafts. I mostly learned what I didn't want to do for a living.

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.

 

[Hewlett]

Was this a direct started motor?
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.

 

[waross]

Only two phases is actually single phase as far as the motor is concerned. It is very unusual for a three phase motor to be able to self rotate under phase loss conditions.
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

 

[suriae]

Generally three phase motor that single phased will not start, what you will hear is a humming sound followed by tripping of overload relays. Sustained single phasing without protection system tripping will result to overheating and burning.

 

[aolalde]

On 99 of 100 applications a three phase motor will never start under single phase powering.

 

[stardelta]

With only 2 phases applied they do all sorts of things, but they generally dont get smoothly up to speed. Little ones can, but not always rotate at erratic speeds approaching the design speed with reduced torque and power but they always vibrate and generate noise at the same time and its obvious they are not happy. Larger ones just tend to sit there with no shaft rotation (in fact they tend to lock up) and make buzzing noises until something trips. I guess its all to do with the torque required to overcome the weight and inertia of larger rotors.

 

[powerjunx]

noel,

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.

 

[raydio]

I have found that a three phase motor will start and run on two phases,however it will run hotter,and less starting torque.
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

 

[itsmoked]

just to clarify: two phases = one phase as far as a motor is concerned.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[raydio]

There are many two phase motors around,these are often called capacitor run motors,the capacitor will create the other phase,these are poor starters,with low starting torque,but ok for lawnmowers,fans etc.Capacitor start machines are different altogether,(high starting torque).The capacitor provides phase shift,not nessarly 120 degrees as with 2 phases of a three phase supply,therefore a normal three phase motor should start ok on just two phases,I have never had any problem,in fact you are not always aware that the motor is starting and running on two phases,until it burns out!

 

[ScottyUK]

Keith,

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.

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[jraef]

raydio,
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

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[raydio]

jrael,
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.

 

[jraef]

Well, this has gone way off topic, so let's not waste too much time on it.
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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