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​ = 1.5 ΩIa​ = ?M+−E = 45 VωT

A DC motor on a 48 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 45 V. Find the armature current Ia.

V = 48 VE = 45 VRa = 1.5 Ω
Ia = A
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 8 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 1.5 Ω
Istart = A
3.Medium
T (N·m)n (rpm)24 N·mn0​nFL​

A DC motor (k = 1.2 V·s/rad, Ra = 0.4 Ω) runs from 220 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 1.2 V·s/radRa = 0.4 ΩIFL = 20 A
SR = %
4.Medium
T (N·m)n (rpm)15 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 0.2 Ω) runs from 110 V and drives a load torque of 15 N·m. Find its speed n in rpm.

V = 110 Vk = 0.75 N·m/ARa = 0.2 ΩT = 15 N·m
n = rpm
5.Hard
+−220 VD = ?Ra​ = 2 ΩM+−En = 300 rpmT = 1.2 N·m

A DC chopper feeds a motor (k = 0.3 V·s/rad, Ra = 2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 1.2 N·m.

Vdc = 220 Vk = 0.3 V·s/radRa = 2 ΩT = 1.2 N·mn = 300 rpm
D = %
6.Hard
+−120 VRs​ = ?Ra​ = 0.8 ΩM+−En = 750 rpmT = 15 N·m

A DC motor (k = 1 V·s/rad, Ra = 0.8 Ω) on a fixed 120 V supply drives a load of 15 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 120 Vk = 1 V·s/radRa = 0.8 ΩT = 15 N·mn = 750 rpm
Rs = Ω

Answer key

  1. 1. Ia = 2 A
  2. 2. Istart = 160 A
  3. 3. SR = 3.774 %
  4. 4. n = 1350 rpm
  5. 5. D = 7.92 %
  6. 6. Rs = 1.964 Ω

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