DC motors worksheet with answers

Free printable DC motors worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

A DC motor on a 240 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 225.6 V. Find the armature current I_a.

V = 240 VE = 225.6 VR_a = 1.2 Ω

I_a = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.75 N·m/AI_a = 4 A

T = N·m

3.Medium

A DC motor with k = 0.6 N·m/A and R_a = 2 Ω is connected to 120 V. Find its stall torque T_stall, the torque at zero speed.

V = 120 Vk = 0.6 N·m/AR_a = 2 Ω

T_stall = N·m

4.Medium

A DC motor with k = 0.1 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 24 Vk = 0.1 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 2 V·s/rad at full field, R_a = 1.5 Ω, V = 220 V) drives a constant load torque of 24 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 220 Vk = 2 V·s/radR_a = 1.5 ΩT = 24 N·mΦ = 80 %

n = rpm

6.Hard

A separately excited DC motor on 110 V (R_a = 1.5 Ω) runs at 1440 rpm with an armature current of 10 A. The load changes and the current becomes 5 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 1.5 Ωn₁ = 1440 rpmI_a1 = 10 AI_a2 = 5 A

n₂ = rpm

Answer key

1. I_a = 12 A
2. T = 3 N·m
3. T_stall = 36 N·m
4. n₀ = 2292 rpm
5. n = 1179 rpm
6. n₂ = 1554 rpm

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