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 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 12 A.

k = 1.5 N·m/AI_a = 12 A

T = N·m

2.Easy

A DC motor runs from a 220 V supply and draws an armature current of 8 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 220 VI_a = 8 AR_a = 1.5 Ω

E = V

3.Medium

A DC motor draws 10 A from a 240 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 68 W. Find its efficiency η.

V = 240 VI_a = 10 AR_a = 1.5 ΩP_rot = 68 W

η = %

4.Medium

A DC motor (k = 1.5 V·s/rad, R_a = 1.2 Ω) runs from 120 V. At full load it draws 12 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 1.5 V·s/radR_a = 1.2 ΩI_FL = 12 A

SR = %

5.Hard

A separately excited DC motor on 48 V (R_a = 1 Ω) runs at 1200 rpm with an armature current of 4 A. The load changes and the current becomes 2.4 A, with the field unchanged. Find the new speed n₂.

V = 48 VR_a = 1 Ωn₁ = 1200 rpmI_a1 = 4 AI_a2 = 2.4 A

n₂ = rpm

6.Hard

A separately excited DC motor on 24 V (R_a = 0.2 Ω) runs at 1000 rpm with an armature current of 15 A. The load changes and the current becomes 9 A, with the field unchanged. Find the new speed n₂.

V = 24 VR_a = 0.2 Ωn₁ = 1000 rpmI_a1 = 15 AI_a2 = 9 A

n₂ = rpm

Answer key

1. T = 18 N·m
2. E = 208 V
3. η = 90.92 %
4. SR = 13.64 %
5. n₂ = 1244 rpm
6. n₂ = 1057 rpm

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