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 120 V supply and draws an armature current of 4 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 120 VI_a = 4 AR_a = 2 Ω

E = V

2.Easy

A DC motor with an armature resistance of 1 Ω is switched directly onto a 48 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 48 VR_a = 1 Ω

I_start = A

3.Medium

A DC motor (k = 0.75 V·s/rad, R_a = 0.5 Ω) runs from 48 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.75 V·s/radR_a = 0.5 ΩI_FL = 4 A

SR = %

4.Medium

A DC motor draws 40 A from a 220 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 400 W. Find its efficiency η.

V = 220 VI_a = 40 AR_a = 0.5 ΩP_rot = 400 W

η = %

5.Hard

A DC motor with k = 1 V·s/rad and R_a = 1.5 Ω must drive a load of 8 N·m at 500 rpm. Find the armature voltage V it needs.

k = 1 V·s/radR_a = 1.5 ΩT = 8 N·mn = 500 rpm

V = V

6.Hard

A separately excited DC motor on 120 V (R_a = 0.2 Ω) runs at 1000 rpm with an armature current of 20 A. The load changes and the current becomes 5 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.2 Ωn₁ = 1000 rpmI_a1 = 20 AI_a2 = 5 A

n₂ = rpm

Answer key

1. E = 112 V
2. I_start = 48 A
3. SR = 4.348 %
4. η = 86.36 %
5. V = 64.36 V
6. n₂ = 1026 rpm

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