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 on a 240 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 230 V. Find the armature current I_a.

V = 240 VE = 230 VR_a = 0.5 Ω

I_a = A

2.Easy

A DC motor with k = 0.25 V·s/rad and an armature resistance of 0.25 Ω runs from 48 V and draws 12 A. Find its speed n in rpm.

V = 48 VR_a = 0.25 ΩI_a = 12 Ak = 0.25 V·s/rad

n = rpm

3.Medium

A DC motor with k = 0.75 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 0.75 V·s/rad

n₀ = rpm

4.Medium

A DC motor draws 10 A from a 110 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 85 W. Find its efficiency η.

V = 110 VI_a = 10 AR_a = 0.4 ΩP_rot = 85 W

η = %

5.Hard

A DC motor (k = 0.75 V·s/rad, R_a = 0.2 Ω) on a fixed 110 V supply drives a load of 18.75 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 110 Vk = 0.75 V·s/radR_a = 0.2 ΩT = 18.75 N·mn = 600 rpm

R_s = Ω

6.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 1 Ω) on a fixed 220 V supply drives a load of 6.4 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 220 Vk = 0.8 V·s/radR_a = 1 ΩT = 6.4 N·mn = 300 rpm

R_s = Ω

Answer key

1. I_a = 20 A
2. n = 1719 rpm
3. n₀ = 3056 rpm
4. η = 88.64 %
5. R_s = 2.315 Ω
6. R_s = 23.36 Ω

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