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 240 V with an armature current of 15 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power P_dev.

V = 240 VI_a = 15 AR_a = 1 Ω

P_dev = W

2.Easy

A DC motor with k = 1.5 V·s/rad and an armature resistance of 0.4 Ω runs from 240 V and draws 20 A. Find its speed n in rpm.

V = 240 VR_a = 0.4 ΩI_a = 20 Ak = 1.5 V·s/rad

n = rpm

3.Medium

A DC motor draws 10 A from a 110 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 80 W. Find its efficiency η.

V = 110 VI_a = 10 AR_a = 1 ΩP_rot = 80 W

η = %

4.Medium

A DC motor with k = 0.4 N·m/A and R_a = 1.5 Ω is connected to 110 V. Find its stall torque T_stall, the torque at zero speed.

V = 110 Vk = 0.4 N·m/AR_a = 1.5 Ω

T_stall = N·m

5.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 0.5 Ω) on a fixed 120 V supply drives a load of 9.6 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 120 Vk = 0.8 V·s/radR_a = 0.5 ΩT = 9.6 N·mn = 600 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 1000 rpm with a load of 12 N·m.

V_dc = 220 Vk = 0.6 V·s/radR_a = 0.25 ΩT = 12 N·mn = 1000 rpm

D = %

Answer key

1. P_dev = 3375 W
2. n = 1477 rpm
3. η = 83.64 %
4. T_stall = 29.33 N·m
5. R_s = 5.311 Ω
6. D = 30.83 %

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