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​M+−E = 43.2 Vn = 3000 rpmT

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 43.2 V. Find the motor constant k in V·s/rad.

E = 43.2 Vn = 3000 rpm
k = V·s/rad
2.Easy
+−220 VRa​ = 2 ΩIa​ = ?M+−E = 196 VωT

A DC motor on a 220 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 196 V. Find the armature current Ia.

V = 220 VE = 196 VRa = 2 Ω
Ia = A
3.Medium
+−220 VRa​ = 2 ΩIa​ = 5 AM+−EωT

A DC motor draws 5 A from a 220 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 42 W. Find its efficiency η.

V = 220 VIa = 5 ARa = 2 ΩProt = 42 W
η = %
4.Medium
+−220 VRa​ = 0.4 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 254 W. Find its efficiency η.

V = 220 VIa = 20 ARa = 0.4 ΩProt = 254 W
η = %
5.Hard
+−48 VRs​ = ?Ra​ = 0.5 ΩM+−En = 500 rpmT = 4.8 N·m

A DC motor (k = 0.6 V·s/rad, Ra = 0.5 Ω) on a fixed 48 V supply drives a load of 4.8 N·m. Find the series resistance Rs that slows it down to 500 rpm.

V = 48 Vk = 0.6 V·s/radRa = 0.5 ΩT = 4.8 N·mn = 500 rpm
Rs = Ω
6.Hard
+−120 VRst​ = ?Ra​ = 0.25 Ω15 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 120 VRa = 0.25 ΩImax = 15 A
Rst = Ω

Answer key

  1. 1. k = 0.1375 V·s/rad
  2. 2. Ia = 12 A
  3. 3. η = 91.64 %
  4. 4. η = 90.59 %
  5. 5. Rs = 1.573 Ω
  6. 6. Rst = 7.75 Ω

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