A coil rotating in a magnetic field generates an induced electromotive force. If the external circuit of the coil is closed, an induced current will be generated in the coil. The magnetic field will generate an amperometric force on the induced current, creating a moment of force opposite to the original direction of rotation, which dampens the rotation of the coil. The following two methods demonstrate the electromagnetic damping effect on a sensitive ammeter and a motor, respectively, when the short circuit is connected.
Method 1
Purpose To demonstrate the short-circuit protection of a sensitive ammeter.
Equipment Sensitive ammeter, wire, etc.
Operation
(1) The sensitive ammeter is shaken so that the pointer has a large swing. After stopping the shaking, you can observe the pointer to swing many times, after a certain time to stop down.
(2) Shake the sensitive ammeter again so that it has a large swing amplitude. Immediately in the two terminals connected to a wire (short circuit), can be found that the pointer swing rapidly reduced than not connected to the short circuit when the swing time is much shorter. This is due to the coil connected to the pointer in the magnetic field when swinging the induced current, the coil is subject to the role of the resistance moment formed by the Ampere force, so that the pointer swing rapidly attenuated. This provides damping protection.
(3) and then shake the sensitive ammeter has been connected to the short circuit, the visible pointer swing is very small, and quickly stop. The reason is the same operation (2).
Explanation
(1) usually JD409 or JD409-1 type sensitive ammeter damping time is less than 4S, because this sensitive ammeter moving coil aluminum frame is closed, there has been a certain damping effect. So this demonstration is best to use the old sensitive ammeter (internal moving coil aluminum frame is not closed), the demonstration of short-circuit damping effect is better.
(2) This experiment shows that the sensitive ammeter is not used, should be added to the two terminals on the short circuit, in order to achieve the role of damping protection. Prevent in the process of moving or transportation, the ammeter is subjected to vibration, the pointer amplitude is too large and was hit by the bend or the tip of the shaft off and so on.
Method II
Objective To demonstrate the method of short-circuit braking of an electric motor.
Equipment Toy motor, single-pole two-position switch, dry cell battery, wire, etc.
Operation
(1) Connect the toy motor, two dry batteries, and the single-pole, two-position switch with wires as shown.
(2)Trigger the single-knife double-position switch to a, the motor is high-speed rotation. Cut off the power supply, visible motor power failure, still can keep rotating for a long time. Make a note of the time from power cut to complete stoppage.
(3) again switch to a, the motor rotates at high speed, the single-knife double-position switch to b. The motor is found to stop rotating quickly. With operation (2) in marked contrast. This is because the motor rotor has been rotating at high speed, after cutting off the power supply, is still rotating at high speed in the magnetic field, the rotor will produce induced electromotive force. If the external circuit is closed at this time (e.g. switch is turned to b), an induced current is generated in the circuit, which is then equivalent to a generator. The rotor coil with the induced current is subjected to the braking effect of the amperometric torque, which will cause the rotation to stop quickly. Therefore, this time the short circuit outside the motor plays the role of electromagnetic damping of the rotor.