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
+−240 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 240 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.8 Ω
Istart = A
2.Easy
+−220 VRa​ = 0.25 ΩIa​ = 30 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 30 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 220 VIa = 30 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.4 V·s/rad
n0 = rpm
4.Medium
+−48 VRa​ = 0.25 ΩIa​ = 6 AM+−EωT

A DC motor draws 6 A from a 48 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 8 W. Find its efficiency η.

V = 48 VIa = 6 ARa = 0.25 ΩProt = 8 W
η = %
5.Hard
+−110 VRs​ = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 5 N·m

A DC motor (k = 0.5 V·s/rad, Ra = 1.2 Ω) on a fixed 110 V supply drives a load of 5 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 110 Vk = 0.5 V·s/radRa = 1.2 ΩT = 5 N·mn = 750 rpm
Rs = Ω
6.Hard
T (N·m)n (rpm)14.4 N·m600 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 0.5 Ω must drive a load of 14.4 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 0.5 ΩT = 14.4 N·mn = 600 rpm
V = V

Answer key

  1. 1. Istart = 300 A
  2. 2. E = 212.5 V
  3. 3. n0 = 1146 rpm
  4. 4. η = 94.1 %
  5. 5. Rs = 5.873 Ω
  6. 6. V = 81.4 V

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