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 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 115 V. Find the armature current I_a.

V = 120 VE = 115 VR_a = 0.2 Ω

I_a = A

2.Easy

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.5 Ω runs from 240 V and draws 40 A. Find its speed n in rpm.

V = 240 VR_a = 0.5 ΩI_a = 40 Ak = 0.8 V·s/rad

n = rpm

3.Medium

A DC motor draws 10 A from a 120 V supply. Its armature resistance is 1.2 Ω and its rotational (friction, windage and iron) losses are 54 W. Find its efficiency η.

V = 120 VI_a = 10 AR_a = 1.2 ΩP_rot = 54 W

η = %

4.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

5.Hard

A separately excited DC motor on 120 V (R_a = 0.25 Ω) runs at 1000 rpm with an armature current of 30 A. The load changes and the current becomes 45 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.25 Ωn₁ = 1000 rpmI_a1 = 30 AI_a2 = 45 A

n₂ = rpm

6.Hard

A separately excited DC motor (k = 0.75 V·s/rad at full field, R_a = 0.8 Ω, V = 120 V) drives a constant load torque of 6 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.75 V·s/radR_a = 0.8 ΩT = 6 N·mΦ = 75 %

n = rpm

Answer key

1. I_a = 25 A
2. n = 2626 rpm
3. η = 85.5 %
4. n₀ = 3056 rpm
5. n₂ = 966.7 rpm
6. n = 1892 rpm

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