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
+−110 VRa​ = 0.8 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 12 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 110 VIa = 12 ARa = 0.8 Ω
E = V
2.Easy
+−110 VRa​ = 1 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1 Ω is switched directly onto a 110 V supply. Its normal full-load current is 8 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 1 Ω
Istart = A
3.Medium
+−48 VRa​ = 1.5 ΩIa​ = 4 AM+−EωT

A DC motor draws 4 A from a 48 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 5 W. Find its efficiency η.

V = 48 VIa = 4 ARa = 1.5 ΩProt = 5 W
η = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1 N·m/A and Ra = 1 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 1 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)6 N·m1500 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 1.5 Ω must drive a load of 6 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 1.5 ΩT = 6 N·mn = 1500 rpm
V = V
6.Hard
T (N·m)n (rpm)20 N·m500 rpm

A DC motor with k = 2 V·s/rad and Ra = 0.8 Ω must drive a load of 20 N·m at 500 rpm. Find the armature voltage V it needs.

k = 2 V·s/radRa = 0.8 ΩT = 20 N·mn = 500 rpm
V = V

Answer key

  1. 1. E = 100.4 V
  2. 2. Istart = 110 A
  3. 3. η = 84.9 %
  4. 4. Tstall = 240 N·m
  5. 5. V = 196 V
  6. 6. V = 112.7 V

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