MMF across an air gap in a motor

Discussion in 'Homework Help' started by amilton542, Nov 13, 2010.

1. amilton542 Thread Starter Active Member

Nov 13, 2010
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64
hi ive recently been reading a book called motors and drives and have attempted the review questions as i think its relevant for my course, but im struggling to work out the formula to calculate MMF across an air gap in a motor. do you add together the reluctance of the air gap and iron together and minus it from the total MMF?
Also the reluctance of air is μo= 4xpie to the power of ten -7 but all it says for the reluctance of iron is μfe ,fe meaning ferromagnetic. is reluctance of iron non existent as ive bin given no answer to what i should type in a calculater for μfe.
formula for MMF across an air gap and what i should type in to my calculater for μfe would be great

2. Kermit2 AAC Fanatic!

Feb 5, 2010
3,791
945
The iron(when not saturated magnetically) will have a negligable effect on the MMF of the circuit. The majority of the MMF will be developed pushing the field across the air gap.

Like two resistors in an electric circuit. one very low ohm. one very high ohm. most of the electric potential will be developed across the high value resistor. If the metal in the core is well below saturation then you can figure the MMF in the gap almost as if the iron had no effect. (very low resistance)

If this isn't helping then could you explain the problem a little more?

3. amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
ok, heres the question
A magnetic circuit of uniform cross sectional area has two air gaps of 0.5mm and 1mm respectively in series. The exciting winding provides an MMF of 1200 Ampere turns. Estimate the MMF across each of the air gaps and flux density. ive worked out the flux density across each of the air gaps and got 3.02 T for the 0.5mm gap and 1.51 T across the 1mm gap, do i need to apply the magnetic ohms law equation to solve this because to work out the reluctance of the gap i need the area but ive only got the gap (g), many thanks

4. mik3 Senior Member

Feb 4, 2008
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You don't need to know the exact value of the cross sectional area since it is the same for both air gaps and it will cancel out.

5. amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
what formula would you use to calculate the mmf drop then? would you be able to tell me your answers and how you got them, many thanks

6. mik3 Senior Member

Feb 4, 2008
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Just represent it as a magnetic circuit and calculate the MMF across the two air gaps like the voltage across two series resistors. You don't need to know the exact value of the cross sectional area. Just represent it with letter A and it will cancel out.

7. amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
ok i got an mmf of 400 and an mmf of 800,now im thinkin the mmf potential will increase the bigger the gap like voltage through a large resistor, but dont understand what u mean about cancelling each other out. how will they?

8. amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
i get you now,yes the area does cancel out because its the same for both sides, thanks alot.

9. mik3 Senior Member

Feb 4, 2008
4,846
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A bit of thinking helps

10. amilton542 Thread Starter Active Member

Nov 13, 2010
494
64
indeedy,i may hav some more questions for u at some point. motors and drives by austin hughes, best book on motors ive ever picked up

Feb 4, 2008
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63