The big question The left arm\'s insulation is completely sanded off, but the ri

Question

The big question The left arm's insulation is completely sanded off, but the right arm's insulation is only half-gone, which means that current to the motor is cut off for half of each rotation (whenever the unsanded half of the right arm is touching the right "leg") Bu why on Earth do we need the current to be off half the time when the motor has to have current to run? Discussand write a summar of your team's take on this question, being of your team's take on this question, being sure to hit these two points: Why/when does the magnet exert a torque on the armature? . What would happen to the motor if the current were always on? 3JPG

Explanation / Answer

when does the magnet exert a torque on the armature

Torque is defined as that force which tends to produce and maintain rotation. The function oftorque in a DC motor is to provide the mechanical output or drive the piece of equipment thatthe DC motor is attached to.When a voltage is applied to aFigure 5 Armature Current in a Basic DC Motormotor, current will flow throughthe field winding, establishing amagnetic field. Current will alsoflow through the armaturewinding, from the negative brushto the positive brush as shown inFigure 5.Since the armature is a current-carrying conductor in a magneticfield, the conductor has a forceexerted on it, tending to move it atright angles to that field. Usingthe left-hand rule for current-carrying conductors, you will seethat the magnetic field on one sideis strengthened at the bottom,while it is weakened on the otherside. Using the right-hand rule formotors, we can see that there is a force exerted on the armature which tends to turn the armaturein the counter-clockwise direction. The sum of the forces, in pounds, multiplied by the radiusof the armature, in feet, is equal to the torque developed by the motor in pound-feet (1b - ft).It is evident that if the armature current were reversed, but the field were the same,torque would be developed in the opposite direction. Likewise, if the field polarity were reversedand the armature remained the same, torque would also be developed in the opposite direction.The force that is developed on a conductor of a motor armature is due to the combined actionof the magnetic fields. The force developed is directly proportional to the strength of the mainfield flux and the strength of the field around the armature conductor. As we know, the fieldstrength around each armature conductor depends on the amount of current flowing through thearmature conductor. Therefore, the torque which is developed by the motor can be determined.

why does the magnet exert a torque on the armature

The armature is the rotating part in which current carrying wires are acted upon by the magnetic field is exerted on the wire, but when current flows through the wire, a magnetic field. The coil will thus rotate until its plane is perpendicular to the magnetic lines The tendency of a force to produce rotation is called torque.

If the field is opened ( a dangerous operation in itself,, because of high voltage due to Lf dif/dt) the armature Votage ( back emf will collapse) and armature current will rise as given by Vapp = Ka dia/dt +Ra*ia +vback(due to remanent magnetism ) with ia=ia0 at no load. The motor decelerates due to friction and windage . The torque due to remanent magnetism will not be significant.

In a DC shunt motor if you cut the supply for the field winding, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of motor.

So if flux drops to 0 theoretically speed will tend to infinity. practically it will damage the windings of the motor and also probably crush the shaft if the fuse doesn't blow out soon enough.

In a DC series motor, if you cut off the supply for the field winding it means you have cut the supply off the motor as the field is in series with the armature. So it will simply come to rest after sometime.

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