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 worksheetPractise 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
+−120 VRa​ = 0.5 ΩIa​ = 15 AM+−EωT

A DC motor runs from 120 V with an armature current of 15 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 15 ARa = 0.5 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.4 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 0.3 V·s/rad and an armature resistance of 0.4 Ω runs from 120 V and draws 25 A. Find its speed n in rpm.

V = 120 VRa = 0.4 ΩIa = 25 Ak = 0.3 V·s/rad
n = rpm
3.Medium
+−220 VRa​ = 1.5 ΩIa​ = 8 AM+−EωT

A DC motor draws 8 A from a 220 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 50 W. Find its efficiency η.

V = 220 VIa = 8 ARa = 1.5 ΩProt = 50 W
η = %
4.Medium
T (N·m)n (rpm)25 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 0.4 Ω) runs from 240 V. At full load it draws 25 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 1 V·s/radRa = 0.4 ΩIFL = 25 A
SR = %
5.Hard
T (N·m)n (rpm)0.24 N·m800 rpm

A DC motor with k = 0.12 V·s/rad and Ra = 1.2 Ω must drive a load of 0.24 N·m at 800 rpm. Find the armature voltage V it needs.

k = 0.12 V·s/radRa = 1.2 ΩT = 0.24 N·mn = 800 rpm
V = V
6.Hard
+−110 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 2 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 2 Ω
Pmax = W

Answer key

  1. 1. Pdev = 1688 W
  2. 2. n = 3501 rpm
  3. 3. η = 91.7 %
  4. 4. SR = 4.348 %
  5. 5. V = 12.45 V
  6. 6. Pmax = 1513 W

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