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
+−48 VRa​ = 0.25 ΩIa​ = 25 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 25 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 48 VIa = 25 ARa = 0.25 Ω
E = V
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = 20 AM+−EωT

A DC motor runs from 240 V with an armature current of 20 A. The armature resistance is 1.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 20 ARa = 1.5 Ω
Pdev = W
3.Medium
+−240 VRa​ = 0.5 ΩIa​ = 40 AM+−EωT

A DC motor draws 40 A from a 240 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 528 W. Find its efficiency η.

V = 240 VIa = 40 ARa = 0.5 ΩProt = 528 W
η = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.5 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 1.5 V·s/rad
n0 = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 1.2 ΩM+−En = 400 rpmT = 5 N·m

A DC motor (k = 1 V·s/rad, Ra = 1.2 Ω) on a fixed 120 V supply drives a load of 5 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 120 Vk = 1 V·s/radRa = 1.2 ΩT = 5 N·mn = 400 rpm
Rs = Ω
6.Hard
+−240 VD = ?Ra​ = 1.5 ΩM+−En = 750 rpmT = 4.8 N·m

A DC chopper feeds a motor (k = 0.8 V·s/rad, Ra = 1.5 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 4.8 N·m.

Vdc = 240 Vk = 0.8 V·s/radRa = 1.5 ΩT = 4.8 N·mn = 750 rpm
D = %

Answer key

  1. 1. E = 41.75 V
  2. 2. Pdev = 4200 W
  3. 3. η = 86.17 %
  4. 4. n0 = 1401 rpm
  5. 5. Rs = 14.42 Ω
  6. 6. D = 29.93 %

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