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 48 V with an armature current of 4 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 4 AR_a = 1.2 Ω

P_dev = W

2.Easy

A DC motor with k = 0.75 V·s/rad and an armature resistance of 0.4 Ω runs from 48 V and draws 15 A. Find its speed n in rpm.

V = 48 VR_a = 0.4 ΩI_a = 15 Ak = 0.75 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.5 V·s/rad, R_a = 0.8 Ω) runs from 110 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.5 V·s/radR_a = 0.8 ΩI_FL = 6 A

SR = %

4.Medium

A DC motor draws 40 A from a 240 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 416 W. Find its efficiency η.

V = 240 VI_a = 40 AR_a = 0.8 ΩP_rot = 416 W

η = %

5.Hard

A DC motor (k = 0.75 V·s/rad, R_a = 2 Ω) on a fixed 110 V supply drives a load of 3 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 110 Vk = 0.75 V·s/radR_a = 2 ΩT = 3 N·mn = 300 rpm

R_s = Ω

6.Hard

A small DC motor has an armature resistance of 1.2 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 24 VR_a = 1.2 Ω

P_max = W

Answer key

1. P_dev = 172.8 W
2. n = 534.8 rpm
3. SR = 4.563 %
4. η = 82.33 %
5. R_s = 19.61 Ω
6. P_max = 120 W

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