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 750 rpm, and its back-EMF at that speed is 106 V. Find the motor constant k in V·s/rad.

E = 106 Vn = 750 rpm

k = V·s/rad

2.Easy

A DC motor runs from a 110 V supply and draws an armature current of 30 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 110 VI_a = 30 AR_a = 0.4 Ω

E = V

3.Medium

A DC motor (k = 0.75 V·s/rad, R_a = 2 Ω) runs from 220 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 = 220 Vk = 0.75 V·s/radR_a = 2 ΩI_FL = 6 A

SR = %

4.Medium

A DC motor draws 30 A from a 110 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 246 W. Find its efficiency η.

V = 110 VI_a = 30 AR_a = 0.25 ΩP_rot = 246 W

η = %

5.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 1 Ω) on a fixed 240 V supply drives a load of 4.8 N·m. Find the series resistance R_s that slows it down to 800 rpm.

V = 240 Vk = 0.6 V·s/radR_a = 1 ΩT = 4.8 N·mn = 800 rpm

R_s = Ω

6.Hard

A DC motor (k = 1.2 V·s/rad, R_a = 2 Ω) on a fixed 110 V supply drives a load of 4.8 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 110 Vk = 1.2 V·s/radR_a = 2 ΩT = 4.8 N·mn = 300 rpm

R_s = Ω

Answer key

1. k = 1.35 V·s/rad
2. E = 98 V
3. SR = 5.769 %
4. η = 85.73 %
5. R_s = 22.72 Ω
6. R_s = 16.08 Ω

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