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
+−220 VRa​ = 1.5 ΩIa​ = ?M+−E = 212.5 VωT

A DC motor on a 220 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 212.5 V. Find the armature current Ia.

V = 220 VE = 212.5 VRa = 1.5 Ω
Ia = A
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 1.5 Ω
Istart = A
3.Medium
+−240 VRa​ = 1.2 ΩIa​ = 8 AM+−EωT

A DC motor draws 8 A from a 240 V supply. Its armature resistance is 1.2 Ω and its rotational (friction, windage and iron) losses are 111 W. Find its efficiency η.

V = 240 VIa = 8 ARa = 1.2 ΩProt = 111 W
η = %
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 0.75 kWn = 960 rpm
T = N·m
5.Hard
+−220 VD = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 12 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 1.2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 12 N·m.

Vdc = 220 Vk = 1 V·s/radRa = 1.2 ΩT = 12 N·mn = 750 rpm
D = %
6.Hard
+−110 VD = ?Ra​ = 1.2 ΩM+−En = 300 rpmT = 12 N·m

A DC chopper feeds a motor (k = 1.5 V·s/rad, Ra = 1.2 Ω) from a 110 V DC bus. The average armature voltage is D·110 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 12 N·m.

Vdc = 110 Vk = 1.5 V·s/radRa = 1.2 ΩT = 12 N·mn = 300 rpm
D = %

Answer key

  1. 1. Ia = 5 A
  2. 2. Istart = 160 A
  3. 3. η = 90.22 %
  4. 4. T = 7.46 N·m
  5. 5. D = 42.25 %
  6. 6. D = 51.57 %

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