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 12 A.

k = 0.12 N·m/AI_a = 12 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 12 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 with k = 2 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 220 Vk = 2 V·s/rad

n₀ = rpm

4.Medium

A DC motor draws 20 A from a 240 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 176 W. Find its efficiency η.

V = 240 VI_a = 20 AR_a = 1 ΩP_rot = 176 W

η = %

5.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 0.5 Ω, V = 240 V) drives a constant load torque of 60 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 240 Vk = 1.5 V·s/radR_a = 0.5 ΩT = 60 N·mΦ = 75 %

n = rpm

6.Hard

A DC motor (k = 0.12 V·s/rad, R_a = 1.5 Ω) on a fixed 48 V supply drives a load of 0.24 N·m. Find the series resistance R_s that slows it down to 1200 rpm.

V = 48 Vk = 0.12 V·s/radR_a = 1.5 ΩT = 0.24 N·mn = 1200 rpm

R_s = Ω

Answer key

1. T = 1.44 N·m
2. I_start = 120 A
3. n₀ = 1050 rpm
4. η = 88 %
5. n = 1811 rpm
6. R_s = 14.96 Ω

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