Discussion in 'Homework Help' started by Manjaks, Dec 17, 2011.

1. ### Manjaks Thread Starter New Member

May 2, 2010
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Hi, I have 2 questions for which I can not find an answer (I hope you will be so kind to help me)

1) An asynchrounous motor with phase rotor is started and it works under nominal load. But rotor frequency is approximately 2 times smaller than nominal frequency. What could be the problem and why?

In my opinion the additional resistances in this case are not turned off completely and it causes rotor current to be smaller, therefore torque is also smaller and speed is dropping... Can this be true?

2) If an asycnhrounous motor is working properly under nominal load, and suddenly 1 phase dissapears in stator windings, what will happen? (for example, rotor changes it`s frequency, or motor stops)

Again, in my opinion nothing will happen, because then current in other two phases will get stronger and provide same torque as in 3-phase...

Thanks!

2. ### jegues Well-Known Member

Sep 13, 2010
735
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For 1), it's because the induced voltage in the rotor is much less, and thus the frequency is smaller.

The frequency of induced voltage in the rotor is proportional to the slip.

3. ### strantor AAC Fanatic!

Oct 3, 2010
4,302
1,989
FIRE!
When the motor is spinning, it is consuming a certain amount of watts. The bigger portion of these watts are turned into rotational energy, but a portion is turned into heat. Normally this is not a problem; a fan and/or heat sink is in place to dissipate the heat. When the phase is lost, there is no more rotating field, so the motor stops rotating, but current is still flowing through the remaining 2 windings. You still have the same, or more, current flowing through these 2 windings, except now, this electrical energy is not being transformed into any mechanical energy, it is all heat energy. You essentially make a little oven inside the motor, and it WILL catch fire. very quickly. This is why it is important to put overload protection on 3phase motors. I have miswired 3phase motors before, only connected 2 phases, and the smoke escapes inside of 5 seconds.

4. ### crutschow Expert

Mar 14, 2008
13,496
3,373
If the rotor frequency (not current) is lower than normal that indicates the slip is less than normal (rotor frequency is proportional to slip). The only reason I can think for that is that the motor voltage is higher than normal which would tend to reduce the amount of slip.

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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This might be of interest with respect to the loss of one phase ...

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6. ### strantor AAC Fanatic!

Oct 3, 2010
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from the attached document in post #5:

I've never seen a motor continue to operate when single phased. has anybody here?

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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Here's another more comprehensive set of notes that suggest that ongoing operation on two phases is possible but with likely unfavorable consequences. It's unlikely that a 3-phase induction motor would start with only two phases connected. Although, I've seen a comment somewhere that suggests that it is possible.

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8. ### jegues Well-Known Member

Sep 13, 2010
735
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My professor also mentioned in class that once the motor is operating it is possible to have sustained operation on only two phases.

9. ### crutschow Expert

Mar 14, 2008
13,496
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Single-phase induction motors operate quite nicely of course, so it would seem possible for a 3-phase induction motor to continue to operate with 2-phases, especially if the load is light.

10. ### strantor AAC Fanatic!

Oct 3, 2010
4,302
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the difference is that the single phase motor has the winding arranged 90 degrees out of phase - the second winding shifted 90 degrees by the capacitor. The 3phase motor has the windings arranged 120 degrees out of phase. trying to start the 3phase motor with only single phase would be like trying to take off on a bicycle that had the pedals 120 degrees apart instead of 180 degrees apart; actually, it would be harder than that, because you are only allowed to use your legs to push straight down (maybe imagine a clown trying to ride this 120 degree bicycle on stilts).
now, it was mentioned that if the motor was already in operation, it is possible to remove a phase and the motor keeps spinning. I can see that, maybe, although I have never tried it in real life, it's probably possible, and would probably be possible with 120degree seperated biciycle as well, if the freewheel mechanism was disabled so that inertia can bring the pedals back around to where you can push them again.

11. ### jimkeith Active Member

Oct 26, 2011
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Under some conditions upon phase loss, a 3 phase motor will continue to deliver sufficient torque to continue to rotate, however, at an increased slip--the motor might also stall and really soak up the power. In general, protection of some sort is recommended--the pdf file indicated in post #5 is a good reference.

When a phase is lost directly at the motor, it will generally stall--when a phase is lost at a large installation, the result is a complex witches brew of all sorts of loads interacting with each other--in this situation for a specific motor, there may be 3 phases present, but not balanced in voltage and not 120° apart--so motor performance is not predictable.

12. ### crutschow Expert

Mar 14, 2008
13,496
3,373
That's only true for single-phase capacitor run motors. Capacitor or repulsion start motors disable the start winding when up to speed. The theory is a little complex but a single-phase motor can be viewed as having two counter-rotating fields. The undesired counter rotating fields is suppressed by the induced field from the rotor leaving a rotating field in the desired direction that keeps the rotor rotating.

13. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Any spatially distributed motor phase winding mmf can be viewed as the addition of two contra-rotating flux waves. In a balanced 3-phase winding case, the 120° offset in both spatial / angular position and electrical excitation results in addition of the 'forward' rotating components of each phase and cancellation of the 'backward' rotating components.

So one can envisage even in the absence of one of the 3 phases, the remaining 2 phase components can still produce a diminished 'forward' rotating flux wave. As crutschow's comments indicate, a careful analysis of the situation - particularly in the phase loss situation - is somewhat complex.

14. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
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I'd say Yes, as you have complete variability in torque over the rated range, which will translate into speed variability. Of course your load characteristics determine the relationship of torque/speed.

Current esculates in an attempt to maintain speed, resulting in esculating temperatures. If the 'nominal' load is under about 50% of rated load, you may not notice any change. 'Technically' the answer is much more complex, and I often refer to this document for insight;
http://www04.abb.com/global/seitp/seitp202.nsf/0/41cbf93732b79663c125761f00500f5f/\$file/Vol.7.pdf
You'll find info specific to your question towards the end.

15. ### BillB3857 Senior Member

Feb 28, 2009
2,402
348
As was mentioned earlier, 3 phase motors in a large installation will continue to run with the power source of one phase disabled. When several 3 phase machines are operating from a common 3 phase bus system, and most of them are running, if only one bus fuse blows, it probably won't even be noticed. Single machines may even be shut down and restarted without problems. However, when nearly all of the machines are shut down, there will be a problem with them starting up again. The fact is that a 3 phase motor that is running will tend to "generate" the missing phase. There are "rotary phase converter" units that couple a 3 phase motor to a single phase motor. By exciting two lines of the 3 phase motor and spinning it with the single phase motor, 3 phase output is available. Google "rotary phase converter" to learn more. This is just one interesting "read"...........http://www.woodweb.com/knowledge_base/Building_a_rotary_phase_converter.html