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 220 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 210 V. Find the armature current I_a.

V = 220 VE = 210 VR_a = 2 Ω

I_a = A

2.Easy

A DC motor turns at 750 rpm, and its back-EMF at that speed is 210 V. Find the motor constant k in V·s/rad.

E = 210 Vn = 750 rpm

k = V·s/rad

3.Medium

A DC motor with k = 1 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 120 Vk = 1 V·s/rad

n₀ = rpm

4.Medium

A DC motor draws 10 A from a 240 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 115 W. Find its efficiency η.

V = 240 VI_a = 10 AR_a = 1 ΩP_rot = 115 W

η = %

5.Hard

A separately excited DC motor (k = 0.8 V·s/rad at full field, R_a = 0.25 Ω, V = 240 V) drives a constant load torque of 24 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 240 Vk = 0.8 V·s/radR_a = 0.25 ΩT = 24 N·mΦ = 70 %

n = rpm

6.Hard

A separately excited DC motor on 120 V (R_a = 0.5 Ω) runs at 1000 rpm with an armature current of 12 A. The load changes and the current becomes 6 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.5 Ωn₁ = 1000 rpmI_a1 = 12 AI_a2 = 6 A

n₂ = rpm

Answer key

1. I_a = 5 A
2. k = 2.674 V·s/rad
3. n₀ = 1146 rpm
4. η = 91.04 %
5. n = 3910 rpm
6. n₂ = 1026 rpm

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