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 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 10 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 2 Ω

I_start = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.1 N·m/A. Find the torque T it develops at an armature current of 2 A.

k = 0.1 N·m/AI_a = 2 A

T = N·m

3.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

4.Medium

A DC motor draws 6 A from a 24 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 11 W. Find its efficiency η.

V = 24 VI_a = 6 AR_a = 0.25 ΩP_rot = 11 W

η = %

5.Hard

A DC motor (k = 1.5 V·s/rad, R_a = 2 Ω) on a fixed 110 V supply drives a load of 3 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 110 Vk = 1.5 V·s/radR_a = 2 ΩT = 3 N·mn = 300 rpm

R_s = Ω

6.Hard

A separately excited DC motor on 24 V (R_a = 0.25 Ω) runs at 1800 rpm with an armature current of 5 A. The load changes and the current becomes 2 A, with the field unchanged. Find the new speed n₂.

V = 24 VR_a = 0.25 Ωn₁ = 1800 rpmI_a1 = 5 AI_a2 = 2 A

n₂ = rpm

Answer key

1. I_start = 110 A
2. T = 0.2 N·m
3. n₀ = 1050 rpm
4. η = 86.11 %
5. R_s = 29.44 Ω
6. n₂ = 1859 rpm

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