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 permanent-magnet DC motor has the torque constant k = 0.25 N·m/A. Find the torque T it develops at an armature current of 5 A.

k = 0.25 N·m/AI_a = 5 A

T = N·m

2.Easy

A DC motor turns at 2000 rpm, and its back-EMF at that speed is 225 V. Find the motor constant k in V·s/rad.

E = 225 Vn = 2000 rpm

k = V·s/rad

3.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 1.5 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 = 1.5 ΩI_FL = 2 A

SR = %

4.Medium

A DC motor (k = 1 N·m/A, R_a = 0.4 Ω) runs from 240 V and drives a load torque of 25 N·m. Find its speed n in rpm.

V = 240 Vk = 1 N·m/AR_a = 0.4 ΩT = 25 N·m

n = rpm

5.Hard

A separately excited DC motor on 220 V (R_a = 0.4 Ω) runs at 1440 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 = 220 VR_a = 0.4 Ωn₁ = 1440 rpmI_a1 = 30 AI_a2 = 18 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 1.2 V·s/rad, R_a = 0.2 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 400 rpm with a load of 48 N·m.

V_dc = 240 Vk = 1.2 V·s/radR_a = 0.2 ΩT = 48 N·mn = 400 rpm

D = %

Answer key

1. T = 1.25 N·m
2. k = 1.074 V·s/rad
3. SR = 6.667 %
4. n = 2196 rpm
5. n₂ = 1473 rpm
6. D = 24.28 %

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