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
+−240 VRa​M+−E = 232 Vn = 1000 rpmT

A DC motor turns at 1000 rpm, and its back-EMF at that speed is 232 V. Find the motor constant k in V·s/rad.

E = 232 Vn = 1000 rpm
k = V·s/rad
2.Easy
+−120 VRa​ = 1 ΩIa​ = 5 AM+−EωT

A DC motor runs from 120 V with an armature current of 5 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 5 ARa = 1 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 1.5 kWn = 1450 rpm
T = N·m
4.Medium
T (N·m)n (rpm)0.16 N·mn0​n = ?

A DC motor (k = 0.08 N·m/A, Ra = 0.8 Ω) runs from 24 V and drives a load torque of 0.16 N·m. Find its speed n in rpm.

V = 24 Vk = 0.08 N·m/ARa = 0.8 ΩT = 0.16 N·m
n = rpm
5.Hard
T (N·m)n (rpm)0.8 N·m400 rpm

A DC motor with k = 0.4 V·s/rad and Ra = 1 Ω must drive a load of 0.8 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.4 V·s/radRa = 1 ΩT = 0.8 N·mn = 400 rpm
V = V
6.Hard
+−220 VRa​ = 0.25 ΩIa​: 30 → 7.5 AM+−En = ?T

A separately excited DC motor on 220 V (Ra = 0.25 Ω) runs at 900 rpm with an armature current of 30 A. The load changes and the current becomes 7.5 A, with the field unchanged. Find the new speed n2.

V = 220 VRa = 0.25 Ωn1 = 900 rpmIa1 = 30 AIa2 = 7.5 A
n2 = rpm

Answer key

  1. 1. k = 2.215 V·s/rad
  2. 2. Pdev = 575 W
  3. 3. T = 9.879 N·m
  4. 4. n = 2674 rpm
  5. 5. V = 18.76 V
  6. 6. n2 = 923.8 rpm

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