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 turns at 3000 rpm, and its back-EMF at that speed is 232 V. Find the motor constant k in V·s/rad.

E = 232 Vn = 3000 rpm

k = V·s/rad

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.5 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.5 N·m/AI_a = 4 A

T = N·m

3.Medium

A DC motor (k = 0.75 N·m/A, R_a = 0.8 Ω) runs from 240 V and drives a load torque of 15 N·m. Find its speed n in rpm.

V = 240 Vk = 0.75 N·m/AR_a = 0.8 ΩT = 15 N·m

n = rpm

4.Medium

A DC motor (k = 0.5 V·s/rad, R_a = 1 Ω) runs from 120 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 0.5 V·s/radR_a = 1 ΩI_FL = 4 A

SR = %

5.Hard

A separately excited DC motor on 48 V (R_a = 0.8 Ω) runs at 1800 rpm with an armature current of 4 A. The load changes and the current becomes 8 A, with the field unchanged. Find the new speed n₂.

V = 48 VR_a = 0.8 Ωn₁ = 1800 rpmI_a1 = 4 AI_a2 = 8 A

n₂ = rpm

6.Hard

A separately excited DC motor on 240 V (R_a = 0.25 Ω) runs at 1800 rpm with an armature current of 30 A. The load changes and the current becomes 18 A, with the field unchanged. Find the new speed n₂.

V = 240 VR_a = 0.25 Ωn₁ = 1800 rpmI_a1 = 30 AI_a2 = 18 A

n₂ = rpm

Answer key

1. k = 0.7385 V·s/rad
2. T = 2 N·m
3. n = 2852 rpm
4. SR = 3.448 %
5. n₂ = 1671 rpm
6. n₂ = 1823 rpm

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