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
+−120 VRa​ = 0.8 ΩIa​ = 6 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 0.8 Ω runs from 120 V and draws 6 A. Find its speed n in rpm.

V = 120 VRa = 0.8 ΩIa = 6 Ak = 1.5 V·s/rad
n = rpm
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = 30 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.4 Ω runs from 110 V and draws 30 A. Find its speed n in rpm.

V = 110 VRa = 0.4 ΩIa = 30 Ak = 0.6 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 7.5 kWn = 1800 rpm
T = N·m
4.Medium
+−220 VRa​ = 0.5 ΩIa​ = 50 AM+−EωT

A DC motor draws 50 A from a 220 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 780 W. Find its efficiency η.

V = 220 VIa = 50 ARa = 0.5 ΩProt = 780 W
η = %
5.Hard
+−220 VRs​ = ?Ra​ = 1.5 ΩM+−En = 1500 rpmT = 11.25 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 1.5 Ω) on a fixed 220 V supply drives a load of 11.25 N·m. Find the series resistance Rs that slows it down to 1500 rpm.

V = 220 Vk = 0.75 V·s/radRa = 1.5 ΩT = 11.25 N·mn = 1500 rpm
Rs = Ω
6.Hard
+−24 VRst​ = ?Ra​ = 0.2 Ω50 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 50 A.

V = 24 VRa = 0.2 ΩImax = 50 A
Rst = Ω

Answer key

  1. 1. n = 733.4 rpm
  2. 2. n = 1560 rpm
  3. 3. T = 39.79 N·m
  4. 4. η = 81.55 %
  5. 5. Rs = 5.313 Ω
  6. 6. Rst = 0.28 Ω

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