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
+−220 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 220 V supply. Its normal full-load current is 12 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 0.8 Ω
Istart = A
2.Easy
+−24 VRa​ = 0.8 ΩIa​ = ?M+−E = 22.4 VωT

A DC motor on a 24 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 22.4 V. Find the armature current Ia.

V = 24 VE = 22.4 VRa = 0.8 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 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 = 0.75 N·m/ARa = 1 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.75 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.75 V·s/rad
n0 = rpm
5.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
6.Hard
T (N·m)n (rpm)24 N·m300 rpm

A DC motor with k = 2 V·s/rad and Ra = 1 Ω must drive a load of 24 N·m at 300 rpm. Find the armature voltage V it needs.

k = 2 V·s/radRa = 1 ΩT = 24 N·mn = 300 rpm
V = V

Answer key

  1. 1. Istart = 275 A
  2. 2. Ia = 2 A
  3. 3. Tstall = 180 N·m
  4. 4. n0 = 1528 rpm
  5. 5. Pmax = 1513 W
  6. 6. V = 74.83 V

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