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.

New worksheet Practise online with hints

For teachers: free to print, copy and use in class. Every new worksheet has new values and its own answer key. A link to ExamProblems on your course page is appreciated.

Name: ____________________ Date: ____________

1.Easy

A DC motor with k = 0.6 V·s/rad and an armature resistance of 1.2 Ω runs from 240 V and draws 15 A. Find its speed n in rpm.

V = 240 VR_a = 1.2 ΩI_a = 15 Ak = 0.6 V·s/rad

n = rpm

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.6 N·m/A. Find the torque T it develops at an armature current of 50 A.

k = 0.6 N·m/AI_a = 50 A

T = N·m

3.Medium

A DC motor (k = 1.2 V·s/rad, R_a = 2 Ω) runs from 120 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 1.2 V·s/radR_a = 2 ΩI_FL = 6 A

SR = %

4.Medium

A DC motor (k = 0.4 V·s/rad, R_a = 1.5 Ω) runs from 48 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 = 48 Vk = 0.4 V·s/radR_a = 1.5 ΩI_FL = 4 A

SR = %

5.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 1 Ω) on a fixed 240 V supply drives a load of 16 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 240 Vk = 0.8 V·s/radR_a = 1 ΩT = 16 N·mn = 600 rpm

R_s = Ω

6.Hard

A separately excited DC motor on 240 V (R_a = 0.5 Ω) runs at 900 rpm with an armature current of 25 A. The load changes and the current becomes 50 A, with the field unchanged. Find the new speed n₂.

V = 240 VR_a = 0.5 Ωn₁ = 900 rpmI_a1 = 25 AI_a2 = 50 A

n₂ = rpm

Answer key

1. n = 3533 rpm
2. T = 30 N·m
3. SR = 11.11 %
4. SR = 14.29 %
5. R_s = 8.487 Ω
6. n₂ = 850.5 rpm

Full step-by-step solutions: practise this topic on examproblems.com.