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ETO ETO Exam papers 📅 Jul 2025

Exam Question

(a) What design factor limits the maximum torque of a d-c motor? (6)

(b) A shunt motor runs on no load at 700 r/min off a 440 V supply. The resistance of the shunt circuit is 240Ω. The following table gives the relationship between the flux and the shunt current:

Shunt Current (A)

 0.5

 0.75

 1.0

 1.25

 1.5

 1.75

 2.0

 Flux per Pole (mWb)

6.0

8.0

9.4

10.2

10.8

11.2

11.5

Calculate the additional resistance required in the shunt circuit to raise the no-load speed to 1000 r/min. (10)

Reference Answer

(a) The maximum torque of a DC motor is primarily limited by the maximum current it can safely handle. This maximum current is, in turn, determined by the motor's ability to dissipate the heat generated by the current (heat = I²R). The type of insulation used and the cooling method employed significantly influence the motor's heat dissipation capacity. Therefore, the key design factors limiting maximum torque are:
The motor's windings can only withstand a certain amount of current before overheating.
The effectiveness of the motor's cooling system (e.g., forced air, liquid cooling) directly impacts its ability to handle high currents and thus high torque.
The quality and heat resistance of the insulation used in the motor's windings determine how much heat the motor can tolerate before the insulation degrades.

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