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​ = 0.25 ΩIa​ = 30 AM+−EωT

A DC motor runs from 120 V with an armature current of 30 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 30 ARa = 0.25 Ω
Pdev = W
2.Easy
+−240 VRa​ = 1.2 ΩIa​ = 8 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 1.2 Ω runs from 240 V and draws 8 A. Find its speed n in rpm.

V = 240 VRa = 1.2 ΩIa = 8 Ak = 1.5 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)1.8 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 0.2 Ω) runs from 24 V and drives a load torque of 1.8 N·m. Find its speed n in rpm.

V = 24 Vk = 0.3 N·m/ARa = 0.2 ΩT = 1.8 N·m
n = rpm
4.Medium
+−110 VRa​ = 1 ΩIa​ = 6 AM+−EωT

A DC motor draws 6 A from a 110 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 19 W. Find its efficiency η.

V = 110 VIa = 6 ARa = 1 ΩProt = 19 W
η = %
5.Hard
+−48 VRst​ = ?Ra​ = 0.2 Ω80 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 48 VRa = 0.2 ΩImax = 80 A
Rst = Ω
6.Hard
+−24 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.5 Ω
Pmax = W

Answer key

  1. 1. Pdev = 3375 W
  2. 2. n = 1467 rpm
  3. 3. n = 725.7 rpm
  4. 4. η = 91.67 %
  5. 5. Rst = 0.4 Ω
  6. 6. Pmax = 96 W

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