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 = 0.6 V·s/rad and an armature resistance of 0.2 Ω runs from 110 V and draws 40 A. Find its speed n in rpm.

V = 110 VR_a = 0.2 ΩI_a = 40 Ak = 0.6 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 25 A.

k = 0.05 N·m/AI_a = 25 A

T = N·m

3.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 1.2 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.3 V·s/radR_a = 1.2 ΩI_FL = 2 A

SR = %

4.Medium

A DC motor (k = 0.3 N·m/A, R_a = 0.8 Ω) runs from 48 V and drives a load torque of 1.5 N·m. Find its speed n in rpm.

V = 48 Vk = 0.3 N·m/AR_a = 0.8 ΩT = 1.5 N·m

n = rpm

5.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 0.2 Ω, V = 110 V) drives a constant load torque of 60 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 110 Vk = 1.5 V·s/radR_a = 0.2 ΩT = 60 N·mΦ = 75 %

n = rpm

6.Hard

A separately excited DC motor on 48 V (R_a = 0.4 Ω) runs at 1500 rpm with an armature current of 5 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 48 VR_a = 0.4 Ωn₁ = 1500 rpmI_a1 = 5 AI_a2 = 10 A

n₂ = rpm

Answer key

1. n = 1623 rpm
2. T = 1.25 N·m
3. SR = 5.263 %
4. n = 1401 rpm
5. n = 843.2 rpm
6. n₂ = 1435 rpm

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