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​Ia​ = 4 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 1.5 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 1.5 N·m/AIa = 4 A
T = N·m
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = 15 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 15 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 240 VIa = 15 ARa = 0.8 Ω
E = V
3.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 0.75 kWn = 1200 rpm
T = N·m
4.Medium
+−120 VRa​ = 1.5 ΩIa​ = 8 AM+−EωT

A DC motor draws 8 A from a 120 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 35 W. Find its efficiency η.

V = 120 VIa = 8 ARa = 1.5 ΩProt = 35 W
η = %
5.Hard
+−24 VRa​ = 3 ΩM+−EωT

A small DC motor has an armature resistance of 3 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 3 Ω
Pmax = W
6.Hard
+−240 VRst​ = ?Ra​ = 0.5 Ω10 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 240 VRa = 0.5 ΩImax = 10 A
Rst = Ω

Answer key

  1. 1. T = 6 N·m
  2. 2. E = 228 V
  3. 3. T = 5.968 N·m
  4. 4. η = 86.35 %
  5. 5. Pmax = 48 W
  6. 6. Rst = 23.5 Ω

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