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Why is the air gap between the stator and rotor on a motor kept as small as it is?

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2

$begingroup$

Between the stator windings and rotor is a small air gap. What would happen if this gap were larger than it is? Surely the magnetic field set up by energizing the stator windings would still cut the rotor and induce an emf across it. So what effect does this air gap have on the performance of the motor?

motor three-phase inductance induction-motor emf

share|improve this question

edited Jul 9 at 16:08

Charles Cowie

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asked Jul 9 at 15:28

Ph3ngPh3ng

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$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Note that it is not always kept small. Increasing the airgap is a way to reduce armature reaction without having to increase the amount of permanent magnet material. This allows for higher peak torque at the expense of efficiency. Depending on the application this may be an acceptable tradeoff.
  $endgroup$
  – Jon
  Jul 9 at 16:09
  

  
  
  
  

add a comment | 

2

$begingroup$

Between the stator windings and rotor is a small air gap. What would happen if this gap were larger than it is? Surely the magnetic field set up by energizing the stator windings would still cut the rotor and induce an emf across it. So what effect does this air gap have on the performance of the motor?

motor three-phase inductance induction-motor emf

share|improve this question

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Note that it is not always kept small. Increasing the airgap is a way to reduce armature reaction without having to increase the amount of permanent magnet material. This allows for higher peak torque at the expense of efficiency. Depending on the application this may be an acceptable tradeoff.
  $endgroup$
  – Jon
  Jul 9 at 16:09
  

  
  
  
  

add a comment | 

$begingroup$

Between the stator windings and rotor is a small air gap. What would happen if this gap were larger than it is? Surely the magnetic field set up by energizing the stator windings would still cut the rotor and induce an emf across it. So what effect does this air gap have on the performance of the motor?

motor three-phase inductance induction-motor emf

share|improve this question

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

$endgroup$

share|improve this question

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

share|improve this question

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

edited Jul 9 at 16:08

Charles Cowie

23.4k11 gold badge1818 silver badges4242 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

asked Jul 9 at 15:28

Ph3ngPh3ng

10355 bronze badges

2 Answers
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13

$begingroup$

Air has a much higher reluctance (the magnetic equivalent of resistance) than the magnetic materials used in the motor. The smaller the air gap is, the lower that reluctance, and thus the higher the magnetic flux (which is the magnetic analog of current), allowing the motor to work more efficiently and at a higher power.

Smaller air gaps also minimize leakage flux, which means more of the flux that is produced actually does something, rather than just going through the air and doing nothing to help the motor work.

share|improve this answer

answered Jul 9 at 15:31

HearthHearth

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$endgroup$

add a comment | 

0

$begingroup$

Let's say you design a motor to operate at a given field strength. Most of the magnetic path reluctance comes from the air-gap. Therefore the larger the air-gap, the more H field and therefore length of magnetic material you need to establish the field, so the more expensive the motor is to make. The size and weight also go up as the amount of magnetic material.

Some motors seem to have quite a large air-gap. That comes down to cost as well. A small air-gap needs precision machining, which becomes prohibitively expensive for very small gaps. There's an optimum size of gap for any size and target market of motor that minimises the total magnet and machining costs.

Note that although this is saying the same sort of thing as Hearth's answer, it comes from a different direction.

share|improve this answer

answered Jul 9 at 17:07

Neil_UKNeil_UK

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add a comment | 

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13

$begingroup$

Air has a much higher reluctance (the magnetic equivalent of resistance) than the magnetic materials used in the motor. The smaller the air gap is, the lower that reluctance, and thus the higher the magnetic flux (which is the magnetic analog of current), allowing the motor to work more efficiently and at a higher power.

Smaller air gaps also minimize leakage flux, which means more of the flux that is produced actually does something, rather than just going through the air and doing nothing to help the motor work.

share|improve this answer

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

$endgroup$

add a comment | 

13

$begingroup$

Air has a much higher reluctance (the magnetic equivalent of resistance) than the magnetic materials used in the motor. The smaller the air gap is, the lower that reluctance, and thus the higher the magnetic flux (which is the magnetic analog of current), allowing the motor to work more efficiently and at a higher power.

Smaller air gaps also minimize leakage flux, which means more of the flux that is produced actually does something, rather than just going through the air and doing nothing to help the motor work.

share|improve this answer

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

$endgroup$

add a comment | 

$begingroup$

Air has a much higher reluctance (the magnetic equivalent of resistance) than the magnetic materials used in the motor. The smaller the air gap is, the lower that reluctance, and thus the higher the magnetic flux (which is the magnetic analog of current), allowing the motor to work more efficiently and at a higher power.

Smaller air gaps also minimize leakage flux, which means more of the flux that is produced actually does something, rather than just going through the air and doing nothing to help the motor work.

share|improve this answer

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

$endgroup$

share|improve this answer

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

answered Jul 9 at 15:31

HearthHearth

6,90811 gold badge1717 silver badges5151 bronze badges

0

$begingroup$

Let's say you design a motor to operate at a given field strength. Most of the magnetic path reluctance comes from the air-gap. Therefore the larger the air-gap, the more H field and therefore length of magnetic material you need to establish the field, so the more expensive the motor is to make. The size and weight also go up as the amount of magnetic material.

Some motors seem to have quite a large air-gap. That comes down to cost as well. A small air-gap needs precision machining, which becomes prohibitively expensive for very small gaps. There's an optimum size of gap for any size and target market of motor that minimises the total magnet and machining costs.

Note that although this is saying the same sort of thing as Hearth's answer, it comes from a different direction.

share|improve this answer

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges

$endgroup$

add a comment | 

0

$begingroup$

Let's say you design a motor to operate at a given field strength. Most of the magnetic path reluctance comes from the air-gap. Therefore the larger the air-gap, the more H field and therefore length of magnetic material you need to establish the field, so the more expensive the motor is to make. The size and weight also go up as the amount of magnetic material.

Some motors seem to have quite a large air-gap. That comes down to cost as well. A small air-gap needs precision machining, which becomes prohibitively expensive for very small gaps. There's an optimum size of gap for any size and target market of motor that minimises the total magnet and machining costs.

Note that although this is saying the same sort of thing as Hearth's answer, it comes from a different direction.

share|improve this answer

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges

$endgroup$

add a comment | 

$begingroup$

Let's say you design a motor to operate at a given field strength. Most of the magnetic path reluctance comes from the air-gap. Therefore the larger the air-gap, the more H field and therefore length of magnetic material you need to establish the field, so the more expensive the motor is to make. The size and weight also go up as the amount of magnetic material.

Some motors seem to have quite a large air-gap. That comes down to cost as well. A small air-gap needs precision machining, which becomes prohibitively expensive for very small gaps. There's an optimum size of gap for any size and target market of motor that minimises the total magnet and machining costs.

Note that although this is saying the same sort of thing as Hearth's answer, it comes from a different direction.

share|improve this answer

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges

$endgroup$

share|improve this answer

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges

answered Jul 9 at 17:07

Neil_UKNeil_UK

83.5k22 gold badges8585 silver badges193193 bronze badges