8.3. Motor Meters. The motor meters can be used in D.C. as well as in A.C. circuits. In
principle the motor meter is a small motor of D.C. or A.C. type whose instantaneous speed of rotation is proportional to the circuit current in case of an ampere-hour meter and to the power of the circuit in case of a watt-hour meter.
The following are the essential parts of the motor meters:
- An operating torque system. It produces a torque and causes the moving system to rotate continuously.
- A braking device. It is usually a permanent magnet, known as brake magnet. This brake magnet induces current in some part of the moving system which in turn produces the braking torque. Thus the braking torque is proportional to the induced currents whereas the induced currents are proportional to the speed of the moving system (and hence the braking torque is proportional to the speed of the moving system (disc). When the braking torque is equal to the driving torque the moving system attains a steady speed.
- Revolution registering device. This device is obtained by having a worm cut on the spindle of the instrument. The worm engages with a pinion and thus drives the train of wheels and registers ampere-hours and watt-hours directly.
Types of motor meters. The various types of motor meters are:
(i) Mercury motor meters
(ii) Commutator motor meters
(iii) Induction motor/energy meters.
- Mercury motor meters and commutator meters are used on D. C. circuits
- Induction meters are used on A.C. circuit.