DC motor min field and max speed

This topic contains 3 replies, has 2 voices, and was last updated by John C John C 3 months, 2 weeks ago.

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  • #9921

    Luther Davis
    Participant

    Can you tell me how the min field effects max speed?


  • #9922
    John C
    John C
    Galco Representative

    Hello Luther, great question.

    Ok so lets say we have a DC motor rated for 1750RPM (Base Speed at 500Vdc) and the mfg says it will go to 2500 absolute max speed. It has a Shunt wound Field winding rated for 300Vdc @ 1.70 amps cold, and a min field current rating of 0.88 amps. It is at this 0.88 amp field current level that the motor can run at its maximum speed of 2500RPM (these are not actual values but rather just for example).
    So lets discuss briefly how that works. A DC motor will spin rated Base Speed (1750 in this case) at 500 volts and full field strength of 1.7 amps (notice I reference the field current not the voltage 300vdc). DC motor Field Controllers are effectively current regulators not voltage. When a DC motor gets hot the field resistance goes up significantly (like 180 ohms cold can turn into 200+ when hot and this will make the current and torque go down too). If the resistance goes up the current goes down unless you have a field current regulator type supply. All application with a DC motor going above Base speed must have tach feedback and a field regulator.


  • #9923

    Luther Davis
    Participant

    Ok well I think I got it, but why does the current down to make the speed go up?
    That just seems backwards to me.


  • #9924
    John C
    John C
    Galco Representative

    Very good question Luther,

    So a DC motor starts running at low speed and speeds up to rated base speed, 1750. Now at this point the field strength is a full and held constant at 1.7amps from 0 to rated base speed, even as the motor heat’s up the current stays constant. The Armature speeds up cutting lines of flux produced by the field windings in the frame of the motor. As the Armature speeds up more and more towards base speed the voltage goes up with it directly proportionally until we reach the full 500 volt rating at 1750 RPM. But 500Vdc is the max voltage rating already, we still have to get to 2500 right? So we decrease the field current a little at a time so the RPM can come up and keep the motor at 500 volts even. This is known as Constant Horsepower operation, the RPM goes up, the Field current goes down and the Armature voltage stays constant to our max speed of 2500 RPM.


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