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 runs from 220 V with an armature current of 12 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 12 AR_a = 2 Ω

P_dev = W

2.Easy

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 108 V. Find the motor constant k in V·s/rad.

E = 108 Vn = 1500 rpm

k = V·s/rad

3.Medium

A DC motor draws 5 A from a 48 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 6 W. Find its efficiency η.

V = 48 VI_a = 5 AR_a = 1 ΩP_rot = 6 W

η = %

4.Medium

A DC motor (k = 0.8 N·m/A, R_a = 0.4 Ω) runs from 220 V and drives a load torque of 16 N·m. Find its speed n in rpm.

V = 220 Vk = 0.8 N·m/AR_a = 0.4 ΩT = 16 N·m

n = rpm

5.Hard

A DC chopper feeds a motor (k = 0.75 V·s/rad, R_a = 1 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 1500 rpm with a load of 7.5 N·m.

V_dc = 240 Vk = 0.75 V·s/radR_a = 1 ΩT = 7.5 N·mn = 1500 rpm

D = %

6.Hard

A DC chopper feeds a motor (k = 0.25 V·s/rad, R_a = 0.2 Ω) from a 48 V DC bus. The average armature voltage is D·48 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 3 N·m.

V_dc = 48 Vk = 0.25 V·s/radR_a = 0.2 ΩT = 3 N·mn = 1000 rpm

D = %

Answer key

1. P_dev = 2352 W
2. k = 0.6875 V·s/rad
3. η = 87.08 %
4. n = 2531 rpm
5. D = 53.25 %
6. D = 59.54 %

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