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Thread: Shunt DC motor question

  1. #51
    Join Date
    May 2005
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    Run-Away Motor.

    I agree that if the field of a shunt wound D.C. motor with no load is lost, the motor speed ramps up without limit until the commutator segments start flying away.

    This is a very dangerous situation for any persons or equipment in the line of the commutator.

    I saw this happened while I was still in school at I.I.T. in Chicago when a student accidentally opened the field of a shunt would motor of around 10 or 15 Hp. The motor started accelerating VERY rapidly. Fortunately the lab instructor was able to dive toward the distribution panel and pull one of the patch coards feeding the motor.

    I took a stand behind a steel I-beam as soon as I heard the motor speeding up.

    BTW, transit motors (busses, locomotives, etc.) are always series wound since this gives very good accelerating torque. In the case of busses and rapid transit trains, series resistors are used to accelerate the motor(s). In the case of locomotives, the field current to the generator is used to lower the voltage to the motor for starting from zero speed. As the motor accelerates, it's back EMF increases the effective impedance of the motor so less series resistance can be used until the motor is in "shunt" (across the line).

  2. #52
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    Feb 2019
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    Ward Leonard System

    There is an older DC motor control system called Ward Leonard. On a shunt DC motor, the speed of the motor is controlled by varying the armature voltage while applying full field voltage, up to base (nameplate) speed. This is a constant torque mode. Once reaching base speed, the controller will then start to reduce the field voltage to increase the motor speed above base speed. This is now a constant HP mode (speed increases while torque capability reduces). If the field is reduced too much and the load is light enough, the motor could easily overspeed and destroy itself. This arrangement could be found on elevator and printing press controls. Elevator controls could also to the opposite, overexcite the field (applying higher than nameplate voltage) for short periods of time when running the motor slowly, such as when leveling the car to the floor. As a number of you have stated, reducing the field voltage reduces back emf (essentially the motor acting partially as a generator). Back emf opposes the applied voltage (similar to placing batteries in series the wrong way). With less back emf, the armature current increases, causing the motor to speed up.

  3. #53
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    A use of Ward Leonard drives was with mine winders.

    Using a slipring motor with partial resistance in the rotor circuit allowed a degree of slip. The main generator set had a flywheel between the motor and generator (one I know of the flywheel was 14 tons).
    Accelerating the cage the flywheel gave up kinetic energy, braking the cage it absorbed energy. Not very efficient but it worked as it prevented sudden loads on the pit head supply.

    They aren’t new “efficient” drives but they served their purpose at the time.
    The reports of my death are greatly exaggerated.

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