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 = 0.75 N·m/A. Find the torque T it develops at an armature current of 12 A.

k = 0.75 N·m/AI_a = 12 A

T = N·m

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.2 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 0.2 N·m/AI_a = 6 A

T = N·m

3.Medium

A DC motor draws 6 A from a 110 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 19 W. Find its efficiency η.

V = 110 VI_a = 6 AR_a = 1 ΩP_rot = 19 W

η = %

4.Medium

A DC motor draws 2 A from a 48 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 5 W. Find its efficiency η.

V = 48 VI_a = 2 AR_a = 1 ΩP_rot = 5 W

η = %

5.Hard

A DC motor (k = 0.15 V·s/rad, R_a = 1.2 Ω) on a fixed 24 V supply drives a load of 0.3 N·m. Find the series resistance R_s that slows it down to 1000 rpm.

V = 24 Vk = 0.15 V·s/radR_a = 1.2 ΩT = 0.3 N·mn = 1000 rpm

R_s = Ω

6.Hard

A DC chopper feeds a motor (k = 0.6 V·s/rad, R_a = 0.25 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 900 rpm with a load of 12 N·m.

V_dc = 220 Vk = 0.6 V·s/radR_a = 0.25 ΩT = 12 N·mn = 900 rpm

D = %

Answer key

1. T = 9 N·m
2. T = 1.2 N·m
3. η = 91.67 %
4. η = 90.63 %
5. R_s = 2.946 Ω
6. D = 27.98 %

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