Novice question about high and low speed motors
Why some low speed electrical drives employ a high-speed motor and some form of mechanical speed reduction rather than direct drive motor. Is it because of size?
motor
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
add a comment |
Why some low speed electrical drives employ a high-speed motor and some form of mechanical speed reduction rather than direct drive motor. Is it because of size?
motor
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
2
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
1
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53
add a comment |
Why some low speed electrical drives employ a high-speed motor and some form of mechanical speed reduction rather than direct drive motor. Is it because of size?
motor
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
Why some low speed electrical drives employ a high-speed motor and some form of mechanical speed reduction rather than direct drive motor. Is it because of size?
motor
motor
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
asked Dec 31 '18 at 12:31
ArtyomShepArtyomShep
123
123
2
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
1
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53
add a comment |
2
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
1
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53
2
2
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
1
1
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53
add a comment |
1 Answer
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Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
add a comment |
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1 Answer
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1 Answer
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Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
add a comment |
Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
add a comment |
Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
Power is the product of torque and speed. If you want high power at low speed, then you need high torque.
In an electric motor, the torque is basically a function of the radius of the rotor, its surface area, and the magnetic field intensity between the rotor and the stator.
You can play with all three of these variables, but there are definite limits on how much field intensity you can get with reasonable materials. Therefore, you end up needing either a long motor or a large-diameter motor, both of which require lots of extra (expensive) material to produce.
This is why it usually works out best to go back to the first equation and raise the speed of the motor by means of a gearbox. This gives you the required output speed and torque while allowing the motor speed to be higher and its torque to be lower in order to get the required power.
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
answered Dec 31 '18 at 16:25
Dave Tweed♦Dave Tweed
118k9145256
118k9145256
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
add a comment |
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
"but there are definite limits on how much field intensity you can get with reasonable materials", I assume you're talking about magnetic saturation here?
– Harry Svensson
Dec 31 '18 at 17:50
2
2
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
@HarrySvensson: Actually, no. There is an air gap between the stator and the rotor that largely prevents saturation. But this gap has a minimum width that is mostly determined by the practicalities of manufacturing the motor -- things like basic machine tolerances, bearing runout, thermal expansion, vibration, etc. The width of this gap puts a limit on how intense (or effective) the field can be.
– Dave Tweed♦
Dec 31 '18 at 21:02
add a comment |
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2
Because you will find it difficult to get either an AC or DC motor capable of rotating at very low speeds (less than 30rpm for example) and still produce high levels of torque.
– David777
Dec 31 '18 at 14:04
@David777 And the question is why it is difficult.
– Harry Svensson
Dec 31 '18 at 15:33
Put another way from how @David777 put it: for a given basic motor design (induction, DC field wound, DC permanent magnet with a given magnet material, etc.), the torque you can get out of the motor is roughly proportional to the amount of power dissipated in the motor. The power you can get out of the motor at a given torque is proportional to how fast you can turn it. So it's usually most economical to turn the motors as fast as the bearings will let you, and gear it down.
– TimWescott
Dec 31 '18 at 16:11
1
@HarrySvensson Sorry Harry, I thought I did answer why, by saying that electric motors are generally high speed rotating machines and torque is greatly reduced by lowering their rotational speed, as Tim and DaveTweed also said. But please feel free to expand on my answer if you want as we are supposed to be helping the OP.
– David777
Dec 31 '18 at 17:28
@David777 Yes, you explained the first level of why (which doesn't answer much on its own), and Dave Tweed went to the second level of why, which was why of your why (which is what OP is actually asking about). I don't know if it was due to my comment, or if Dave is just that good. My money is on Dave being that good. - So far you haven't given an answer David, you've made two comments.
– Harry Svensson
Dec 31 '18 at 17:53