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 1 Ω is switched directly onto a 120 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 120 VR_a = 1 Ω

I_start = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.5 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.5 N·m/AI_a = 15 A

T = N·m

3.Medium

A DC motor draws 15 A from a 110 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 57 W. Find its efficiency η.

V = 110 VI_a = 15 AR_a = 1 ΩP_rot = 57 W

η = %

4.Medium

A DC motor draws 30 A from a 110 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 94 W. Find its efficiency η.

V = 110 VI_a = 30 AR_a = 0.2 ΩP_rot = 94 W

η = %

5.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 1.5 Ω) on a fixed 110 V supply drives a load of 4.8 N·m. Find the series resistance R_s that slows it down to 400 rpm.

V = 110 Vk = 0.8 V·s/radR_a = 1.5 ΩT = 4.8 N·mn = 400 rpm

R_s = Ω

6.Hard

A DC motor with k = 0.3 V·s/rad and R_a = 0.4 Ω must drive a load of 6 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radR_a = 0.4 ΩT = 6 N·mn = 750 rpm

V = V

Answer key

1. I_start = 120 A
2. T = 7.5 N·m
3. η = 82.91 %
4. η = 91.7 %
5. R_s = 11.25 Ω
6. V = 31.56 V

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