Why no resistance required when motor is running @ high speed? (1 Viewer)

[unfold]

Hey peeps,
this is straight out of 2001 HSC physics paper..and i'm realli confused on it! i thought resistance IS required when motor is running at high speed so as to prevent it from going too fast or overheating..but NOOoooO! some generous..please help un-muddle this for me... THANkyou guyz!

Question 21 (3 marks)

A fan that ventilates an underground mine is run by a very large d.c. electric motor. This motor is connected in series with a variable resistor to protect the windings in the coil. When the motor is starting up, the variable resistor is adjusted to have a large resistance. The resistance is then lowered slowly as the motor increases to its operating speed.

Explain why no resistance is required when the motor is running at high speed, but a substantial resistance is needed when the motor is starting up.

 

[rama_v]

A DC motor suffers from the fact that there is a back emf - a force that acts in the opposite direction of the motor's rotation due to the "Generator effect". This generator effect is caused by the rotation of the coil inside a magnetic field, and its direction is opposite to that of the current flow (due to Lenz's law).

When the motor is running at high speed no external resistance is required because the coil would generate a large back emf since there is a large change in flux as the coil cuts through the magnetic field at high speed, hence generating a back emf which acts as a resistance itself. Therefore there is no need for an external resistor at high speeds. At low speeds though the back emf is lower as the speed of the motor is lower, and an external resistor is required to prevent the coil from overheating and blowing out.

 

[Libbster]

Also remember Faraday's Law: the induced emf around the loop is proportional to the rate at which the flux through the loop is changing. So when the motor is starting up, it cuts flux slowly, thus there is a high 'forward current' (for lack of a better word ) but little back current to balance this forward current, and thus, the windings could easily burn out.

However, once the motor is running at high speed, it is quickly cutting magnetic flux, thus the back current is larger, allowing the motor the run without a resistor.