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| Synchronous motor rotor excitation effect on current draw |
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| Simon:
I have come across this latest question in my assignment. --- Quote ---A 30 kW star-connected synchronous motor is supplied from a 415 volt 50 Hz source. When fully loaded and running at unity power factor the input power is 35 kW. The excitation current under these conditions is 10 A. What will be the value of input power if the load remains constant and the excitation (i) is reduced to 5 A (ii) is increased to 20 A? --- End quote --- I am baffled. Firstly why state both 30KW and 35KW. What the motor is designed for is irrelevant as the question is not about will it break. It can't be a KW versus VA thing because this is at a power factor of 1. While I find a lot of reference around about generically under and over exciting the rotor I have not found any explanation on how I would calculate the stator currents from the information given. Obviously the course material was it's usual vacuous confusing rubbish with 1/4 just waffling and 1/4 showing examples totally different to this question. I would think that I would need a decent understanding of the magnetic's in this case and know the change in load angle. Can I assume that with a change in excitation current there will be a proportionate change in back EMF that will in turn change the stator currents so that IXs lineV and back e.m.f V phasors all add up again but this assumes no movements in those phasors which I am sure will happen and I can't see how I would calculate the angles. |
| jbb:
Hmm Looking at the question, I think there isn’t enough information for a detailed analysis. I therefore suggest that you make (and state!) an assumption or two. And I also note that the powers they gave you were in kW. |
| Simon:
Oh they never talk VA, I have to work out from the subtle wording if they mean W or VA. 70% the challenge with this course is dealing with it's own failings rather than trying to learn the subject. I have got to the point of being too scared to pick a new module fearful of the disasters it may hold. I want to be judged on my own mistakes, not theirs. What assumption can i make? is my idea of back emf proportional to excitation correct? ish? |
| T3sl4co1l:
Hmm, guessing: 1. The "30kW" figure is the mechanical rating and load, so, "40HP" if it were over here. 2. 35kW is the electrical consumption, so, 35kVA = 35kW (PF = 1) is the input and presumably the added 5kW is losses. 3. The asked-for "input power" is Apparent Power (VA), and PF < 1 under the specified conditions. Phase angle varies with rotor current but I don't remember by how much, and also what that does for maximum torque output, if half field at full load is likely to stall the motor or what. That point may also simply be ignored here. In any case, I think the expected result is VAs goes up, capacitive or inductive respectively (or the other way around, I forget), while the real power remains ~same (because the load is assumed constant). If you're curious, I had: Electrical Power and Controls, Skvarenina and DeWitt, Pearson Prentice Hall, 2nd ed. (2004). I remember some problems like this, but not the formulas for them. I don't remember them being nearly as nonsensical as your assignments have been... Tim |
| Simon:
Yes at power factor 1 the current in the stator is minimum. So either scenario will produce an increase in current. Ah hang on they say "input power" this sounds like a trick question. If by power input they mean watts not VA then presumably that is unchanged as the VA will be what changes with the lower power factor. |
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