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
+−110 VRa​ = 1.5 ΩIa​ = 10 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 10 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 110 VIa = 10 ARa = 1.5 Ω
E = V
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 8 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 240 VIa = 8 ARa = 1.5 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.1 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.1 V·s/rad
n0 = rpm
4.Medium
+−48 VRa​ = 0.25 ΩIa​ = 12 AM+−EωT

A DC motor draws 12 A from a 48 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 22 W. Find its efficiency η.

V = 48 VIa = 12 ARa = 0.25 ΩProt = 22 W
η = %
5.Hard
+−220 VD = ?Ra​ = 0.2 ΩM+−En = 1000 rpmT = 60 N·m

A DC chopper feeds a motor (k = 1.5 V·s/rad, Ra = 0.2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 60 N·m.

Vdc = 220 Vk = 1.5 V·s/radRa = 0.2 ΩT = 60 N·mn = 1000 rpm
D = %
6.Hard
+−120 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2.5 Ω
Pmax = W

Answer key

  1. 1. E = 95 V
  2. 2. E = 228 V
  3. 3. n0 = 2292 rpm
  4. 4. η = 89.93 %
  5. 5. D = 75.04 %
  6. 6. Pmax = 1440 W

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