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 5 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 5 AR_a = 0.8 Ω

P_dev = W

2.Easy

A DC motor with k = 0.75 V·s/rad and an armature resistance of 0.5 Ω runs from 220 V and draws 25 A. Find its speed n in rpm.

V = 220 VR_a = 0.5 ΩI_a = 25 Ak = 0.75 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 0.5 Ω) runs from 240 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 0.6 V·s/radR_a = 0.5 ΩI_FL = 40 A

SR = %

4.Medium

A DC motor (k = 0.1 V·s/rad, R_a = 1.5 Ω) runs from 24 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 24 Vk = 0.1 V·s/radR_a = 1.5 ΩI_FL = 2 A

SR = %

5.Hard

A small DC motor has an armature resistance of 3 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 110 VR_a = 3 Ω

P_max = W

6.Hard

A DC motor (k = 0.5 V·s/rad, R_a = 0.25 Ω) on a fixed 110 V supply drives a load of 12.5 N·m. Find the series resistance R_s that slows it down to 900 rpm.

V = 110 Vk = 0.5 V·s/radR_a = 0.25 ΩT = 12.5 N·mn = 900 rpm

R_s = Ω

Answer key

1. P_dev = 220 W
2. n = 2642 rpm
3. SR = 9.091 %
4. SR = 14.29 %
5. P_max = 1008 W
6. R_s = 2.265 Ω

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