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​ = 1.5 ΩIa​ = 4 AM+−EωT

A DC motor runs from 120 V with an armature current of 4 A. The armature resistance is 1.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 4 ARa = 1.5 Ω
Pdev = W
2.Easy
+−110 VRa​ = 1 ΩIa​ = ?M+−E = 106 VωT

A DC motor on a 110 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 106 V. Find the armature current Ia.

V = 110 VE = 106 VRa = 1 Ω
Ia = A
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 7.5 kWn = 3000 rpm
T = N·m
4.Medium
+−220 VRa​ = 1.5 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 304 W. Find its efficiency η.

V = 220 VIa = 20 ARa = 1.5 ΩProt = 304 W
η = %
5.Hard
+−120 VRst​ = ?Ra​ = 1.5 Ω10 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 1.5 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 120 VRa = 1.5 ΩImax = 10 A
Rst = Ω
6.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

Answer key

  1. 1. Pdev = 456 W
  2. 2. Ia = 4 A
  3. 3. T = 23.87 N·m
  4. 4. η = 79.45 %
  5. 5. Rst = 10.5 Ω
  6. 6. Pmax = 2017 W

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