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 with an armature resistance of 0.8 Ω is switched directly onto a 220 V supply. Its normal full-load current is 15 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 0.8 Ω

I_start = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 1.5 N·m/A. Find the torque T it develops at an armature current of 20 A.

k = 1.5 N·m/AI_a = 20 A

T = N·m

3.Medium

A DC motor with k = 1.2 N·m/A and R_a = 1.5 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 1.2 N·m/AR_a = 1.5 Ω

T_stall = N·m

4.Medium

A DC motor with k = 1 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 110 Vk = 1 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor on 240 V (R_a = 2 Ω) runs at 1200 rpm with an armature current of 4 A. The load changes and the current becomes 2 A, with the field unchanged. Find the new speed n₂.

V = 240 VR_a = 2 Ωn₁ = 1200 rpmI_a1 = 4 AI_a2 = 2 A

n₂ = rpm

6.Hard

A separately excited DC motor (k = 0.5 V·s/rad at full field, R_a = 1.2 Ω, V = 110 V) drives a constant load torque of 2 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 110 Vk = 0.5 V·s/radR_a = 1.2 ΩT = 2 N·mΦ = 85 %

n = rpm

Answer key

1. I_start = 275 A
2. T = 30 N·m
3. T_stall = 192 N·m
4. n₀ = 1050 rpm
5. n₂ = 1221 rpm
6. n = 2345 rpm

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