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
+−24 VRa​ = 0.5 ΩIa​ = 4 AM+−E = ?ωT

A DC motor runs from a 24 V supply and draws an armature current of 4 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 24 VIa = 4 ARa = 0.5 Ω
E = V
2.Easy
+−24 VRa​ = 0.2 ΩIa​ = ?M+−E = 21 VωT

A DC motor on a 24 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 21 V. Find the armature current Ia.

V = 24 VE = 21 VRa = 0.2 Ω
Ia = A
3.Medium
+−240 VRa​ = 0.8 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 240 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 224 W. Find its efficiency η.

V = 240 VIa = 20 ARa = 0.8 ΩProt = 224 W
η = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.2 N·m/A and Ra = 1.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.2 N·m/ARa = 1.2 Ω
Tstall = N·m
5.Hard
+−240 VD = ?Ra​ = 2 ΩM+−En = 600 rpmT = 1.5 N·m

A DC chopper feeds a motor (k = 0.75 V·s/rad, Ra = 2 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 1.5 N·m.

Vdc = 240 Vk = 0.75 V·s/radRa = 2 ΩT = 1.5 N·mn = 600 rpm
D = %
6.Hard
+−240 VRst​ = ?Ra​ = 1 Ω80 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 240 VRa = 1 ΩImax = 80 A
Rst = Ω

Answer key

  1. 1. E = 22 V
  2. 2. Ia = 15 A
  3. 3. η = 88.67 %
  4. 4. Tstall = 8 N·m
  5. 5. D = 21.3 %
  6. 6. Rst = 2 Ω

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