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
+−120 VRa​ = 0.2 ΩIa​ = 30 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 30 A. Its armature resistance is 0.2 Ω. Find the back-EMF E.

V = 120 VIa = 30 ARa = 0.2 Ω
E = V
2.Easy
+−240 VRa​ = 1 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 8 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 240 VIa = 8 ARa = 1 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.4 V·s/rad
n0 = rpm
4.Medium
+−48 VRa​ = 0.2 ΩIa​ = 10 AM+−EωT

A DC motor draws 10 A from a 48 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 23 W. Find its efficiency η.

V = 48 VIa = 10 ARa = 0.2 ΩProt = 23 W
η = %
5.Hard
+−110 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 1.5 Ω
Pmax = W
6.Hard
+−120 VRst​ = ?Ra​ = 0.5 Ω60 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance Rst that limits the starting current to 60 A.

V = 120 VRa = 0.5 ΩImax = 60 A
Rst = Ω

Answer key

  1. 1. E = 114 V
  2. 2. E = 232 V
  3. 3. n0 = 2626 rpm
  4. 4. η = 91.04 %
  5. 5. Pmax = 2017 W
  6. 6. Rst = 1.5 Ω

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