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​Ia​ = 40 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.8 N·m/A. Find the torque T it develops at an armature current of 40 A.

k = 0.8 N·m/AIa = 40 A
T = N·m
2.Easy
+−110 VRa​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 110 V supply. Its normal full-load current is 40 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.25 Ω
Istart = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.75 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 0.75 V·s/rad
n0 = rpm
4.Medium
+−240 VRa​ = 0.5 ΩIa​ = 15 AM+−EωT

A DC motor draws 15 A from a 240 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 105 W. Find its efficiency η.

V = 240 VIa = 15 ARa = 0.5 ΩProt = 105 W
η = %
5.Hard
+−120 VRs​ = ?Ra​ = 1.5 ΩM+−En = 900 rpmT = 3.2 N·m

A DC motor (k = 0.4 V·s/rad, Ra = 1.5 Ω) on a fixed 120 V supply drives a load of 3.2 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 120 Vk = 0.4 V·s/radRa = 1.5 ΩT = 3.2 N·mn = 900 rpm
Rs = Ω
6.Hard
+−48 VRst​ = ?Ra​ = 0.25 Ω25 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 48 VRa = 0.25 ΩImax = 25 A
Rst = Ω

Answer key

  1. 1. T = 32 N·m
  2. 2. Istart = 440 A
  3. 3. n0 = 2801 rpm
  4. 4. η = 93.96 %
  5. 5. Rs = 8.788 Ω
  6. 6. Rst = 1.67 Ω

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