Final answer:
Induction motors do not produce torque with DC input, as they require an AC input to function. A DC motor does produce torque with DC input, and the torque adjusts based on back EMF as the rotor moves.
Step-by-step explanation:
An induction motor designed to operate on alternating current (AC) will not produce torque with a direct current (DC) input when the rotor is stationary because it relies on the changing magnetic field to induce current in the rotor and create torque. However, it won't generate torque with a DC input even when the rotor is moving, as it requires an alternating field to function properly. On the other hand, a DC motor will produce torque both when the rotor is stationary and when it is moving. The starting torque is typically higher and then decreases to a stable operating torque as the motor reaches its operating speed due to back electromotive force (EMF) that opposes the applied voltage.
To answer the question related to the percentage change in torque when permanent magnets lose strength, if a motor's permanent magnets lose 5.0% of their strength, the torque will also decrease by 5.0% because torque in a magnetic motor is directly proportional to magnetic field strength. To return the torque to its original value, the current would need to be increased by 5.0%. This is because torque is also directly proportional to the current in the motor; thus, a loss in magnet strength can be compensated by an increase in current.