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.

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1.Easy
+−220 VRa​ = 1.2 ΩIa​ = 15 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 15 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 220 VIa = 15 ARa = 1.2 Ω
E = V
2.Easy
+−120 VRa​ = 0.25 ΩIa​ = 40 AM+−EωT

A DC motor runs from 120 V with an armature current of 40 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 40 ARa = 0.25 Ω
Pdev = W
3.Medium
+−110 VRa​ = 0.25 ΩIa​ = 50 AM+−EωT

A DC motor draws 50 A from a 110 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 195 W. Find its efficiency η.

V = 110 VIa = 50 ARa = 0.25 ΩProt = 195 W
η = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 0.8 V·s/rad
n0 = rpm
5.Hard
+−110 VRst​ = ?Ra​ = 0.2 Ω20 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 110 VRa = 0.2 ΩImax = 20 A
Rst = Ω
6.Hard
+−24 VRst​ = ?Ra​ = 0.25 Ω10 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 24 VRa = 0.25 ΩImax = 10 A
Rst = Ω

Answer key

  1. 1. E = 202 V
  2. 2. Pdev = 4400 W
  3. 3. η = 85.09 %
  4. 4. n0 = 2626 rpm
  5. 5. Rst = 5.3 Ω
  6. 6. Rst = 2.15 Ω

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