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​ = 0.4 ΩIa​ = 50 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 50 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 220 VIa = 50 ARa = 0.4 Ω
E = V
2.Easy
+−110 VRa​ = 2 ΩIa​ = 2 AM+−EωT

A DC motor runs from 110 V with an armature current of 2 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 2 ARa = 2 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 15 kWn = 2000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)0.6 N·mn0​n = ?

A DC motor (k = 0.15 N·m/A, Ra = 1.5 Ω) runs from 48 V and drives a load torque of 0.6 N·m. Find its speed n in rpm.

V = 48 Vk = 0.15 N·m/ARa = 1.5 ΩT = 0.6 N·m
n = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 0.4 ΩM+−En = 800 rpmT = 15 N·m

A DC motor (k = 1 V·s/rad, Ra = 0.4 Ω) on a fixed 120 V supply drives a load of 15 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 120 Vk = 1 V·s/radRa = 0.4 ΩT = 15 N·mn = 800 rpm
Rs = Ω
6.Hard
+−120 VRa​ = 0.25 ΩIa​: 50 → 25 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.25 Ω) runs at 1200 rpm with an armature current of 50 A. The load changes and the current becomes 25 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.25 Ωn1 = 1200 rpmIa1 = 50 AIa2 = 25 A
n2 = rpm

Answer key

  1. 1. E = 200 V
  2. 2. Pdev = 212 W
  3. 3. T = 71.62 N·m
  4. 4. n = 2674 rpm
  5. 5. Rs = 2.015 Ω
  6. 6. n2 = 1270 rpm

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