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 0.5 Ω is switched directly onto a 24 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 24 VR_a = 0.5 Ω

I_start = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 1.2 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 1.2 N·m/AI_a = 25 A

T = N·m

3.Medium

A DC motor draws 50 A from a 220 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 500 W. Find its efficiency η.

V = 220 VI_a = 50 AR_a = 0.4 ΩP_rot = 500 W

η = %

4.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 1 Ω) runs from 48 V. At full load it draws 5 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.6 V·s/radR_a = 1 ΩI_FL = 5 A

SR = %

5.Hard

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 120 VR_a = 2.5 Ω

P_max = W

6.Hard

A separately excited DC motor on 220 V (R_a = 1 Ω) runs at 1440 rpm with an armature current of 12 A. The load changes and the current becomes 4.8 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 1 Ωn₁ = 1440 rpmI_a1 = 12 AI_a2 = 4.8 A

n₂ = rpm

Answer key

1. I_start = 48 A
2. T = 30 N·m
3. η = 86.36 %
4. SR = 11.63 %
5. P_max = 1440 W
6. n₂ = 1490 rpm

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