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 a 24 V supply and draws an armature current of 5 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 24 VI_a = 5 AR_a = 0.5 Ω

E = V

2.Easy

A DC motor on a 240 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 228 V. Find the armature current I_a.

V = 240 VE = 228 VR_a = 1.5 Ω

I_a = A

3.Medium

A DC motor draws 5 A from a 24 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 9 W. Find its efficiency η.

V = 24 VI_a = 5 AR_a = 0.25 ΩP_rot = 9 W

η = %

4.Medium

A DC motor (k = 0.12 V·s/rad, R_a = 1.2 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 = 0.12 V·s/radR_a = 1.2 ΩI_FL = 2 A

SR = %

5.Hard

A separately excited DC motor on 24 V (R_a = 0.5 Ω) runs at 1000 rpm with an armature current of 6 A. The load changes and the current becomes 3 A, with the field unchanged. Find the new speed n₂.

V = 24 VR_a = 0.5 Ωn₁ = 1000 rpmI_a1 = 6 AI_a2 = 3 A

n₂ = rpm

6.Hard

A DC motor with k = 1.2 V·s/rad and R_a = 2 Ω must drive a load of 6 N·m at 750 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radR_a = 2 ΩT = 6 N·mn = 750 rpm

V = V

Answer key

1. E = 21.5 V
2. I_a = 8 A
3. η = 87.29 %
4. SR = 11.11 %
5. n₂ = 1071 rpm
6. V = 104.2 V

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