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​ = 0.2 ΩIa​ = 50 AM+−En = ?T

A DC motor with k = 1.2 V·s/rad and an armature resistance of 0.2 Ω runs from 110 V and draws 50 A. Find its speed n in rpm.

V = 110 VRa = 0.2 ΩIa = 50 Ak = 1.2 V·s/rad
n = rpm
2.Easy
+−24 VRa​ = 1.5 ΩIa​ = 2 AM+−En = ?T

A DC motor with k = 0.1 V·s/rad and an armature resistance of 1.5 Ω runs from 24 V and draws 2 A. Find its speed n in rpm.

V = 24 VRa = 1.5 ΩIa = 2 Ak = 0.1 V·s/rad
n = rpm
3.Medium
+−120 VRa​ = 0.4 ΩIa​ = 15 AM+−EωT

A DC motor draws 15 A from a 120 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 68 W. Find its efficiency η.

V = 120 VIa = 15 ARa = 0.4 ΩProt = 68 W
η = %
4.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 5.5 kWn = 2900 rpm
T = N·m
5.Hard
+−120 VRa​ = 0.8 ΩM+−EωT

A small DC motor has an armature resistance of 0.8 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 0.8 Ω
Pmax = W
6.Hard
+−24 VRa​ = 1 ΩM+−EωT

A small DC motor has an armature resistance of 1 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 Ω
Pmax = W

Answer key

  1. 1. n = 795.8 rpm
  2. 2. n = 2005 rpm
  3. 3. η = 91.22 %
  4. 4. T = 18.11 N·m
  5. 5. Pmax = 4500 W
  6. 6. Pmax = 144 W

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