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 = 0.1 N·m/A. Find the torque T it develops at an armature current of 50 A.

k = 0.1 N·m/AI_a = 50 A

T = N·m

2.Easy

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 21.6 V. Find the motor constant k in V·s/rad.

E = 21.6 Vn = 1200 rpm

k = V·s/rad

3.Medium

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 228 W. Find its efficiency η.

V = 220 VI_a = 20 AR_a = 1.5 ΩP_rot = 228 W

η = %

4.Medium

A DC motor (k = 0.15 N·m/A, R_a = 0.8 Ω) runs from 24 V and drives a load torque of 0.6 N·m. Find its speed n in rpm.

V = 24 Vk = 0.15 N·m/AR_a = 0.8 ΩT = 0.6 N·m

n = rpm

5.Hard

A DC motor (k = 0.1 V·s/rad, R_a = 0.2 Ω) on a fixed 24 V supply drives a load of 0.8 N·m. Find the series resistance R_s that slows it down to 900 rpm.

V = 24 Vk = 0.1 V·s/radR_a = 0.2 ΩT = 0.8 N·mn = 900 rpm

R_s = Ω

6.Hard

A DC chopper feeds a motor (k = 1.5 V·s/rad, R_a = 0.8 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 18 N·m.

V_dc = 120 Vk = 1.5 V·s/radR_a = 0.8 ΩT = 18 N·mn = 300 rpm

D = %

Answer key

1. T = 5 N·m
2. k = 0.1719 V·s/rad
3. η = 81.18 %
4. n = 1324 rpm
5. R_s = 1.622 Ω
6. D = 47.27 %

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