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​ = 0.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.2 Ω
Istart = A
2.Easy
+−120 VRa​ = 2 ΩIa​ = ?M+−E = 104 VωT

A DC motor on a 120 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 104 V. Find the armature current Ia.

V = 120 VE = 104 VRa = 2 Ω
Ia = A
3.Medium
+−240 VRa​ = 0.4 ΩIa​ = 25 AM+−EωT

A DC motor draws 25 A from a 240 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 288 W. Find its efficiency η.

V = 240 VIa = 25 ARa = 0.4 ΩProt = 288 W
η = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.25 N·m/A and Ra = 1 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.25 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−120 VD = ?Ra​ = 0.8 ΩM+−En = 900 rpmT = 6.4 N·m

A DC chopper feeds a motor (k = 0.8 V·s/rad, Ra = 0.8 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 900 rpm with a load of 6.4 N·m.

Vdc = 120 Vk = 0.8 V·s/radRa = 0.8 ΩT = 6.4 N·mn = 900 rpm
D = %
6.Hard
+−220 VRst​ = ?Ra​ = 1.5 Ω100 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 1.5 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 220 VRa = 1.5 ΩImax = 100 A
Rst = Ω

Answer key

  1. 1. Istart = 600 A
  2. 2. Ia = 8 A
  3. 3. η = 91.03 %
  4. 4. Tstall = 6 N·m
  5. 5. D = 68.17 %
  6. 6. Rst = 0.7 Ω

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