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 with an armature resistance of 0.2 Ω is switched directly onto a 24 V supply. Its normal full-load current is 15 A. Find the armature current I_start at the moment of starting.

V = 24 VR_a = 0.2 Ω

I_start = A

2.Easy

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 22.4 V. Find the motor constant k in V·s/rad.

E = 22.4 Vn = 3000 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.6 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 220 Vk = 0.6 V·s/rad

n₀ = rpm

4.Medium

A DC motor with k = 1 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 110 Vk = 1 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 0.5 Ω, V = 120 V) drives a constant load torque of 18 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 1.5 V·s/radR_a = 0.5 ΩT = 18 N·mΦ = 75 %

n = rpm

6.Hard

A separately excited DC motor on 24 V (R_a = 0.8 Ω) runs at 900 rpm with an armature current of 4 A. The load changes and the current becomes 1 A, with the field unchanged. Find the new speed n₂.

V = 24 VR_a = 0.8 Ωn₁ = 900 rpmI_a1 = 4 AI_a2 = 1 A

n₂ = rpm

Answer key

1. I_start = 120 A
2. k = 0.0713 V·s/rad
3. n₀ = 3501 rpm
4. n₀ = 1050 rpm
5. n = 950.7 rpm
6. n₂ = 1004 rpm

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