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
+−48 VRa​ = 0.2 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 8 A. Its armature resistance is 0.2 Ω. Find the back-EMF E.

V = 48 VIa = 8 ARa = 0.2 Ω
E = V
2.Easy
+−220 VRa​Ia​ = 8 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.12 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.12 N·m/AIa = 8 A
T = N·m
3.Medium
T (N·m)n (rpm)24 N·mn0​n = ?

A DC motor (k = 0.6 N·m/A, Ra = 0.5 Ω) runs from 240 V and drives a load torque of 24 N·m. Find its speed n in rpm.

V = 240 Vk = 0.6 N·m/ARa = 0.5 ΩT = 24 N·m
n = rpm
4.Medium
T (N·m)n (rpm)30 N·mn0​nFL​

A DC motor (k = 0.75 V·s/rad, Ra = 0.2 Ω) runs from 240 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 0.75 V·s/radRa = 0.2 ΩIFL = 40 A
SR = %
5.Hard
+−220 VRst​ = ?Ra​ = 1.5 Ω25 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 1.5 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 220 VRa = 1.5 ΩImax = 25 A
Rst = Ω
6.Hard
T (N·m)n (rpm)15 N·m500 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 0.5 Ω must drive a load of 15 N·m at 500 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 0.5 ΩT = 15 N·mn = 500 rpm
V = V

Answer key

  1. 1. E = 46.4 V
  2. 2. T = 0.96 N·m
  3. 3. n = 3501 rpm
  4. 4. SR = 3.448 %
  5. 5. Rst = 7.3 Ω
  6. 6. V = 49.27 V

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