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​ = 2 ΩIa​ = ?M+−E = 232 VωT

A DC motor on a 240 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 232 V. Find the armature current Ia.

V = 240 VE = 232 VRa = 2 Ω
Ia = A
2.Easy
+−48 VRa​ = 0.5 ΩIa​ = ?M+−E = 45 VωT

A DC motor on a 48 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 45 V. Find the armature current Ia.

V = 48 VE = 45 VRa = 0.5 Ω
Ia = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.15 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.15 V·s/rad
n0 = rpm
4.Medium
+−220 VRa​ = 1 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 200 W. Find its efficiency η.

V = 220 VIa = 20 ARa = 1 ΩProt = 200 W
η = %
5.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
6.Hard
+−240 VD = ?Ra​ = 1.2 ΩM+−En = 500 rpmT = 12 N·m

A DC chopper feeds a motor (k = 1.2 V·s/rad, Ra = 1.2 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 500 rpm with a load of 12 N·m.

Vdc = 240 Vk = 1.2 V·s/radRa = 1.2 ΩT = 12 N·mn = 500 rpm
D = %

Answer key

  1. 1. Ia = 4 A
  2. 2. Ia = 6 A
  3. 3. n0 = 1528 rpm
  4. 4. η = 86.36 %
  5. 5. Pmax = 96 W
  6. 6. D = 31.18 %

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