3. The Retarding Element.—This part acts as a load on the induction motor and enables the adjustment of its speed to normal limits. In order that the speed shall be proportional to the driving torque, which varies with the watts in the circuit, it is necessary that the torque of the retarding device be proportional to the speed. For this reason a short circuited constant field generator, consisting of a metal disc rotating between permanent magnet poles, has been generally adopted.
Ques. How is the retarding torque produced?
Ans. Eddy currents are induced in the disc in rotating through the magnetic field which, according to Lenz's law, oppose the force that produces them, thus developing a retarding torque.
Ques. How is the constant field for the retarding disc produced?
Ans. By permanent magnets.
The retarding disc may be the same disc used for the moving element, in which case the meter field acts on one edge while the permanent magnet field acts on the edge diametrically opposite. This arrangement simplifies the number of parts and saves space and weight of moving element.
Ques. What error is likely to be introduced by the retarding element?
Ans. If the strength of the permanent magnets change from any cause, the retarding torque will be changed and the calibration of the meter rendered inaccurate.
Ques. How may the strength of the permanent magnets be changed?