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​ = 0.2 ΩIa​ = ?M+−E = 116 VωT

A DC motor on a 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 116 V. Find the armature current Ia.

V = 120 VE = 116 VRa = 0.2 Ω
Ia = A
2.Easy
+−220 VRa​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 4 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 2 Ω
Istart = A
3.Medium
T (N·m)n (rpm)4 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 0.25 Ω) runs from 48 V and drives a load torque of 4 N·m. Find its speed n in rpm.

V = 48 Vk = 0.4 N·m/ARa = 0.25 ΩT = 4 N·m
n = rpm
4.Medium
+−110 VRa​ = 0.4 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 110 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 102 W. Find its efficiency η.

V = 110 VIa = 20 ARa = 0.4 ΩProt = 102 W
η = %
5.Hard
+−120 VD = ?Ra​ = 0.8 ΩM+−En = 500 rpmT = 2 N·m

A DC chopper feeds a motor (k = 0.5 V·s/rad, Ra = 0.8 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 500 rpm with a load of 2 N·m.

Vdc = 120 Vk = 0.5 V·s/radRa = 0.8 ΩT = 2 N·mn = 500 rpm
D = %
6.Hard
+−24 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 2.5 Ω
Pmax = W

Answer key

  1. 1. Ia = 20 A
  2. 2. Istart = 110 A
  3. 3. n = 1086 rpm
  4. 4. η = 88.09 %
  5. 5. D = 24.48 %
  6. 6. Pmax = 57.6 W

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