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+−En = ?T

A DC motor with k = 0.3 V·s/rad and an armature resistance of 1.5 Ω runs from 110 V and draws 10 A. Find its speed n in rpm.

V = 110 VRa = 1.5 ΩIa = 10 Ak = 0.3 V·s/rad
n = rpm
2.Easy
+−240 VRa​ = 1.2 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 1.2 Ω runs from 240 V and draws 25 A. Find its speed n in rpm.

V = 240 VRa = 1.2 ΩIa = 25 Ak = 0.6 V·s/rad
n = rpm
3.Medium
+−110 VRa​ = 0.5 ΩIa​ = 25 AM+−EωT

A DC motor draws 25 A from a 110 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 146 W. Find its efficiency η.

V = 110 VIa = 25 ARa = 0.5 ΩProt = 146 W
η = %
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 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
6.Hard
T (N·m)n (rpm)0.24 N·m600 rpm

A DC motor with k = 0.12 V·s/rad and Ra = 1 Ω must drive a load of 0.24 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.12 V·s/radRa = 1 ΩT = 0.24 N·mn = 600 rpm
V = V

Answer key

  1. 1. n = 3024 rpm
  2. 2. n = 3342 rpm
  3. 3. η = 83.33 %
  4. 4. η = 93.96 %
  5. 5. Pmax = 1440 W
  6. 6. V = 9.54 V

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