model answer request (1 Viewer)

[barnyard]

(question is from north sydney girls high 2020 trial)
can someone write up a model answer for this pretty please bc there isnt one in the marking scheme and ive had a mind blank where the only thing i can think of is adding more armatures and the whole variable resistor thing which is already in the question
also maybe im slow but i thought u cant get rid of back emf so????? like literally what could i even say

q: Explain improvements that can be made to motors to minimise the issues with torque and back emf. (8 marks)

here's the marking scheme too for an 8/8 answer:
Student outlines 4 improvements to the DC motor drawn above and details the Physics principle that governs this improvement and how this principle plays role. Student must include at least 1 improvement that addresses torque and 1 improvement that addresses back emf.
OR
Student describes 3 improvements to the DC motor drawn above and explains the Physics principle that governs this improvement and details how this principle plays role (diagrams). Student must include at least 1 improvement that addresses torque and 1 improvement that addresses back emf.

 

[coolcat6778]

This question sucks so bad

 

[wizzkids]

That question shows lack of understanding of Physics. Yes, you are correct; the back emf in a DC motor is inherent in the design and it can't be eliminated. In fact, it is vital to stopping the motor from over-revving and blowing up. The purpose of adding a series resistor in a DC motor is to reduce the in-rush current when the motor is starting from stationary. When the motor is stationary, the back-emf is zero and hence the current is limited only by the DC resistance of the armature winding, which is kept deliberately low to reduce energy losses as heat in the armature. The DC motor has maximum torque at low speeds.
What did the rest of the question say? What is the issue with torque? Not enough torque, too much torque, or not sufficiently uniform?
The relevant equation is
To increase torque, you can increase any of n, B, I or A. One issue you could discuss about torque is that the number of windings on the armature and the number of segments on the commutator can be increased to make a multi-pole motor. Multi-pole motors have more uniform torque during one revolution of the motor. In fact, practical electric drill motors typically have 9 or 11 poles to smooth-out the torque and reduce noise and vibration, but you can't stop the back emf.