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 turns at 1000 rpm, and its back-EMF at that speed is 216 V. Find the motor constant k in V·s/rad.

E = 216 Vn = 1000 rpm

k = V·s/rad

2.Easy

A DC motor runs from 110 V with an armature current of 10 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power P_dev.

V = 110 VI_a = 10 AR_a = 0.8 Ω

P_dev = W

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 1.2 Ω) runs from 240 V. At full load it draws 8 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 1 V·s/radR_a = 1.2 ΩI_FL = 8 A

SR = %

4.Medium

A DC motor draws 50 A from a 220 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 630 W. Find its efficiency η.

V = 220 VI_a = 50 AR_a = 0.2 ΩP_rot = 630 W

η = %

5.Hard

A DC motor (k = 0.4 V·s/rad, R_a = 0.25 Ω) on a fixed 120 V supply drives a load of 16 N·m. Find the series resistance R_s that slows it down to 800 rpm.

V = 120 Vk = 0.4 V·s/radR_a = 0.25 ΩT = 16 N·mn = 800 rpm

R_s = Ω

6.Hard

A DC motor (k = 1 V·s/rad, R_a = 0.25 Ω) on a fixed 220 V supply drives a load of 30 N·m. Find the series resistance R_s that slows it down to 800 rpm.

V = 220 Vk = 1 V·s/radR_a = 0.25 ΩT = 30 N·mn = 800 rpm

R_s = Ω

Answer key

1. k = 2.063 V·s/rad
2. P_dev = 1020 W
3. SR = 4.167 %
4. η = 89.73 %
5. R_s = 1.912 Ω
6. R_s = 4.291 Ω

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