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Electric motor question(torque and speed relationship)

  1. Mar 7, 2007 #1
    Hi everyone,

    I know that for a typical dc permanant magnet motor, there is a quadratic relationship between the power and torque,the same goes for power and speed. The max power occur at 1/2 the stall torque,this is also true at 1/2 the no load speed.

    Can someone please tell me what is the difference between a constant torque and a constant speed motor? While choosing the right motor for an application(in my case a motor powered screw jack), I only look at the motor's power rating,voltage,max current,stall load and no load speed, i did not see in catalogs for dc motor, whether it is a constant torque or a constant speed motor.

    Thank you.
     
  2. jcsd
  3. Mar 7, 2007 #2
    I've worked with DC motors for quite a while now.

    DC motors are the simplest type; given a constant voltage, they run to a steady speed and steady current (electric torque= constant * current for DC motors).

    If you need to control the torque (current), you use a current sensor (a resistor) as feedback to control the current.

    If you need to control the speed, you use a speed sensor (tachometer, encoder etc) as a feedback to control the speed.
     
  4. Mar 7, 2007 #3
    The thing is, regarding my design project(motor powered screw jack). I was wondering how the motor would behave.
    I have someone told me that if im using a constant torque motor and the load is low, then the motor would draw less current than at its maximum capability.
    The constant speed type on the other hand, when the load is low, the load would move much faster than the optimum speed for max power/optimum load.
    Is this statement true? or did i get the wrong idea...
     
  5. Mar 8, 2007 #4
    "The thing is, regarding my design project(motor powered screw jack). I was wondering how the motor would behave.
    I have someone told me that if im using a constant torque motor and the load is low, then the motor would draw less current than at its maximum capability.
    The constant speed type on the other hand, when the load is low, the load would move much faster than the optimum speed for max power/optimum load.
    Is this statement true? or did i get the wrong idea.."

    Well, what I'm really saying is there's no such thing as a "Constant torque" dc motor or a "Constant speed" dc motor; it just depends on how you drive the motor.

    Here's the equations of a dc motor, operated with a fixed field (ie a permanent magnet) at steady state:

    V= R*i + K*w
    K*i = Motor torque = B*w + TLoad

    Where
    V is the terminal voltage applied to the motor
    i is the armature current
    w the rotational velocity in rad/s
    B is the viscous friction
    K the motor backemf/torque constant
    R the armature winding resistance
    TLoad the load torque you put on the motor shaft.

    To answer your question, certainly, if you place a larger load TLoad on the motor, the current draw (i) will become higher.

    If you tried to operate your motor at a fixed speed, and when the load is low, you might not get the best power efficiency.

    I'm taking it in your application (a tool), you might not want to use any sensors and just drive the motor with a fixed voltage. In that case, you are at the mercy of the system itself (i.e. the equations above).

    For your application (a motor powered screw jack) I'm guessing more torque than speed is important.

    In this case, you have 2 options:

    1) get a motor with a large torque constant, K.

    even better,
    2) gear the motor, with the small # turns on the motor shaft, and the larger # turns on the tool side. This way, your motor spins fast, but the torque on the tool side is N*K*i, but the speed on the tool side is w/N, where N is the gear ratio.


    So, your answer your question, the statement is true, but you might be looking at the problem the wrong way.
     
  6. Mar 8, 2007 #5
    I think you may be talking about series and shunt wound motors. This relates if I recall to the way the field coil is wired.
    Shunt wound motors will increase speed according to the load. A car starter motor does this as it gives low speed high tourque on starting and then as the load drops off the speed increases rapidly to engine turning speed. this is why you never connect a starter motor not in a car as the motor will try and achieve infinite speed if the load is 0 and tear itself apart in the process.
    Series wound motors give a constant speed related to voltage and the current increases as load increases to the stall speed.
     
  7. Mar 8, 2007 #6
    You have it backwards. A starter motor in a car is series wound and is NOT self regulating concerning speed. A shunt wound motor has little starting torque but regulates its speed.
     
  8. Mar 8, 2007 #7
    Thank you so much for the information! it really helps!
     
  9. Jan 27, 2010 #8
    hi, can u pl tell me how to increase the torque.
    i have a motor with 6kg/cm torque fixed at the centre and i have to use the gear train to drive the screw rod of length one meter. the center motor shud drive tis screw rod. the motor and the rod are connected by a gear train. i need 10kg/cm at the screw rods. diagonally opposite. so can u pl suggest me the gears used and the no.of teeth used for the idler driver and the driven gears, and can u please tell me the pitch of the screw rod so that the nut moves 30cm in 7sec.
     
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