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 = 0.8 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 0.8 N·m/AI_a = 25 A

T = N·m

2.Easy

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 24 V supply. Its normal full-load current is 5 A. Find the armature current I_start at the moment of starting.

V = 24 VR_a = 0.2 Ω

I_start = A

3.Medium

A DC motor draws 25 A from a 240 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 341 W. Find its efficiency η.

V = 240 VI_a = 25 AR_a = 0.5 ΩP_rot = 341 W

η = %

4.Medium

A DC motor draws 30 A from a 220 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 185 W. Find its efficiency η.

V = 220 VI_a = 30 AR_a = 0.5 ΩP_rot = 185 W

η = %

5.Hard

A separately excited DC motor on 220 V (R_a = 1 Ω) runs at 900 rpm with an armature current of 25 A. The load changes and the current becomes 12.5 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 1 Ωn₁ = 900 rpmI_a1 = 25 AI_a2 = 12.5 A

n₂ = rpm

6.Hard

A small DC motor has an armature resistance of 2 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 Ω

P_max = W

Answer key

1. T = 20 N·m
2. I_start = 120 A
3. η = 89.11 %
4. η = 90.38 %
5. n₂ = 957.7 rpm
6. P_max = 1800 W

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