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 = 0.12 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.12 N·m/AI_a = 4 A

T = N·m

2.Easy

A DC motor runs from a 110 V supply and draws an armature current of 5 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 110 VI_a = 5 AR_a = 1 Ω

E = V

3.Medium

A DC motor draws 2 A from a 48 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 5 W. Find its efficiency η.

V = 48 VI_a = 2 AR_a = 1.5 ΩP_rot = 5 W

η = %

4.Medium

A DC motor draws 30 A from a 240 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 338 W. Find its efficiency η.

V = 240 VI_a = 30 AR_a = 0.5 ΩP_rot = 338 W

η = %

5.Hard

A separately excited DC motor on 220 V (R_a = 1.5 Ω) runs at 1800 rpm with an armature current of 5 A. The load changes and the current becomes 7.5 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 1.5 Ωn₁ = 1800 rpmI_a1 = 5 AI_a2 = 7.5 A

n₂ = rpm

6.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 0.2 Ω, V = 120 V) drives a constant load torque of 37.5 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 1.5 V·s/radR_a = 0.2 ΩT = 37.5 N·mΦ = 75 %

n = rpm

Answer key

1. T = 0.48 N·m
2. E = 105 V
3. η = 88.54 %
4. η = 89.06 %
5. n₂ = 1768 rpm
6. n = 962 rpm

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