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
+−240 VRa​ = 1 ΩIa​ = ?M+−E = 210 VωT

A DC motor on a 240 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 210 V. Find the armature current Ia.

V = 240 VE = 210 VRa = 1 Ω
Ia = A
2.Easy
+−240 VRa​ = 1 ΩIa​ = ?M+−E = 225 VωT

A DC motor on a 240 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 225 V. Find the armature current Ia.

V = 240 VE = 225 VRa = 1 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 2 Ω is connected to 120 V. Find its stall torque Tstall, the torque at zero speed.

V = 120 Vk = 0.5 N·m/ARa = 2 Ω
Tstall = N·m
4.Medium
+−240 VRa​ = 0.8 ΩIa​ = 10 AM+−EωT

A DC motor draws 10 A from a 240 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 139 W. Find its efficiency η.

V = 240 VIa = 10 ARa = 0.8 ΩProt = 139 W
η = %
5.Hard
+−240 VD = ?Ra​ = 1.2 ΩM+−En = 600 rpmT = 7.2 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 1.2 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 7.2 N·m.

Vdc = 240 Vk = 0.6 V·s/radRa = 1.2 ΩT = 7.2 N·mn = 600 rpm
D = %
6.Hard
+−110 VRst​ = ?Ra​ = 0.8 Ω10 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 110 VRa = 0.8 ΩImax = 10 A
Rst = Ω

Answer key

  1. 1. Ia = 30 A
  2. 2. Ia = 15 A
  3. 3. Tstall = 30 N·m
  4. 4. η = 90.88 %
  5. 5. D = 21.71 %
  6. 6. Rst = 10.2 Ω

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