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 k = 0.25 V·s/rad and an armature resistance of 0.5 Ω runs from 24 V and draws 4 A. Find its speed n in rpm.

V = 24 VR_a = 0.5 ΩI_a = 4 Ak = 0.25 V·s/rad

n = rpm

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.05 N·m/A. Find the torque T it develops at an armature current of 50 A.

k = 0.05 N·m/AI_a = 50 A

T = N·m

3.Medium

A DC motor (k = 0.12 N·m/A, R_a = 1 Ω) runs from 48 V and drives a load torque of 0.6 N·m. Find its speed n in rpm.

V = 48 Vk = 0.12 N·m/AR_a = 1 ΩT = 0.6 N·m

n = rpm

4.Medium

A DC motor with k = 0.75 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 120 Vk = 0.75 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor on 220 V (R_a = 0.4 Ω) runs at 900 rpm with an armature current of 40 A. The load changes and the current becomes 16 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 0.4 Ωn₁ = 900 rpmI_a1 = 40 AI_a2 = 16 A

n₂ = rpm

6.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 2 Ω, V = 120 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 120 Vk = 0.4 V·s/radR_a = 2 ΩT = 2.4 N·mΦ = 70 %

n = rpm

Answer key

1. n = 840.3 rpm
2. T = 2.5 N·m
3. n = 3422 rpm
4. n₀ = 1528 rpm
5. n₂ = 942.4 rpm
6. n = 3508 rpm

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