Decreasing RPM on induction motor while keeping torque the same

Discussion in 'General Electronics Chat' started by zazas321, Apr 20, 2018.

  1. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Hey Guys. I just wanted confirm if I understand it right. Lets say that I need 1000 Rpm and 10Nm torque. But I have got 3000Rpm motor with a torque of 10Nm at 3000Rpm. If I reduce the frequency by 3 times using variable frequency drive, that would decrease the RPM to 1000 but at the same time increase the torque to 30Nm. 30Nm may be even too high and the motor may stall.

    Looking at the formula:
    τ = (I * V * E *60) / (rpm * 2π)

    What if I reduce the voltage by 3 times aswell. Would that keep the torque the same? For example instead of 230V ac I would have arround 80V AC. Is that how it works guys?
     
  2. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    I am not a motor expert, but I recall that the T is equal the the HP times a constant(I can't recall).....divided by the RPM. At least they used to.

    If that still works.....it means that to reduce RPM and keep same torque.......you need to increase HP.

    But they keep changing everything now........they might have digital controllers to solve this.

    We have motor experts on here...stick around and they'll chime in.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    If you reduce the speed, the motor output torque will essentially remain the same, but the motor output power will be proportionally reduced.
     
    Last edited: Apr 21, 2018
  4. MisterBill2

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    Jan 23, 2018
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    What becomes unfortunate is that it is more complicated than just reducing the frequency, because that changes the impedance of the windings and so the current changes as well. So the easy way to do it is to use a variable speed drive made for your kind of motor. If it is an induction motor things get complicated, but ifit is a series universal motor, the kind with brushes and a commutator, it gets a lot simpler.
     
  5. shortbus

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    Sep 30, 2009
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    This still has to do with your bicycle post, correct? You are missing something with trying to just run the motor slower without using gearing. Motor heat. A motor is made to run at a certain speed, and many are fan cooled, when you slow down a motor as much as you want to do, it will overheat. A motor that isn't fan cooled is also designed to run at a certain speed at a certain temperature and they will run excessively hot if not used at the correct speed too.

    Even a 3phase motor on a VFD is limited on the amount under frequency it can be operated at.

    An electric motor has it's highest power at 'locked rotor', but that has nothing to do with its real power, because it's not turning the shaft. The HP and output torque of an electric motor is measured at it's rated RPM. Running one slower than that will reduce both it's HP and output TQ. But it will use more electrical power, but just turn that power into heat. You need gearing to do what you want. Or a internal combustion motor, because they use a different way of making power and torque.
     
    Last edited: Apr 21, 2018
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  6. -live wire-

    Active Member

    Dec 22, 2017
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    This is not always true. Take, for example, stepper motors. More speed generally means less torque, and less speed means more torque.

    What people do for average power reduction in AC circuits is phase angle control. They do not step down the voltage. PAC is where after the wave hits a zero-point you turn it off then wait a certain amount of time to turn it back on. It is like PWM in DC appliances but a lot more complicated in the circuitry. You may want a microcontroller (like the ATtiny) if you need the power reduction to be linear.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Generally true, but if the drive is able to keep the current at the motor rated value throughout the rpm range, then the torque will remain constant up to the rated limit.
    Max.
     
    Last edited: Apr 21, 2018
  8. cmartinez

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    Jan 17, 2007
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    Electric motors are designed to operate at optimal performance at fixed rpms and load. Any deviation from those parameters, and efficiency will suffer, and that's unavoidable. I once advised a company to stop relying on their VFD so much, and change their 3,600 rpm three-phase 2-poles induction motors to 900 rpm 8-poles instead. They tested my idea, and soon enough they changed all the motors in their machinery. Now not only is their process less troublesome, but their electricity bill went down as well.
     
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  9. -live wire-

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    Dec 22, 2017
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    That is generally true, but you can add feedback to operate at lower RPM and the same or even greater torque. Maybe use a rotary encoder or something. But then you need a microcontroller.
     
  10. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Thank you guys for all the answers! All very helpful! Lets take an example: Considering my previous example; I need 1000 RPM and 10Nm and the motor that I have got is 3000RPM and 10Nm. Lets say I decide to use 3:1 gearing to achieve my required 1000RPM, but at the same time, torque will rise to 30Nm .. What would be the best way to reduce RPM while keeping torque the same
     
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  11. cmartinez

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    Thw very best way would be to use a motor that already performs at the desired rpms. Next to that is the use of a gear reducer, because keep in mind that those things have efficiencies of about 80-85%. So you'd be losing between 15-20% power for that effect.
     
  12. MaxHeadRoom

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    Jul 18, 2013
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    Traditionally the most economical way, if you do not require the full rpm of the motor is to use a Smaller motor that will give you the required torque after the reduction.
    Does it matter if in your case the max torque ability were to be 30N-m?
    Max.
     
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  13. MisterBill2

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    Jan 23, 2018
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    A stepper motor does not qualify as a "regular motor" because it is not a regular motor.
     
  14. -live wire-

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    Dec 22, 2017
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    What do you mean by that?
     
  15. MaxHeadRoom

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    Huh??
    Current = Torque applies to just about all motors.
    Max.
     
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  16. MaxHeadRoom

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    The torque is a product of load (current) if the demand is not there then the torque is relatively lower for a given application.
    Max.
     
  17. crutschow

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    Mar 14, 2008
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    This thread is about induction motors and that's what my answer was referring to.
    A stepper motor is not an induction motor.
     
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  18. MisterBill2

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    Jan 23, 2018
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    Crutschow is totally correct. And an induction motor is not a good choice for driving a bike, nor would a stepper motor work very well.
    The one type of motor that does do well from a stalled state is the series universal motor. Of course the brushless electronicley commutated DC motors in the electric motorcycles that I have seen are very impressive, but they are neither simple nor cheap. An induction motor will need a serious reduction ratio to provide anything close to worthwhile performance. You could achieve a 10:1 reduction in a reasonably sized roller chain drive with a 6 tooth sprocket on the motor and a 60 tooth sprocket on the rear wheel. But you would still be burdened with an induction motor that is really not suitable for the application.
     
  19. MaxHeadRoom

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    Jul 18, 2013
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    I have seen demonstrations of a induction motor and a VFD used for a gantry crane lift, The operator could hold the lift load stationary, just as the old wound rotor motor braked version did.
    I have also read of them being used in elevator applications.
    Max.
     
  20. shortbus

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    Sep 30, 2009
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    Why do you think excessive torque to your needs is a bad thing? I have never seen this as something that is not acceptable. If your expecting a motors torque output to be "exact" I think you will be sadly mistaken and out of luck. Torque in and electric motor drive when it is needed to be limited is usually done by a torque limiting type clutch, one that slips when over torqued. But that is not used thing in most machines.

    This is the same as Max was saying in post #16. It goes to the same being true in an electrical circuit, the load will only use what it needs, extra amperage or torque is just there for when an overload does occur.
     
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