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​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 120 V supply. Its normal full-load current is 25 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.4 Ω
Istart = A
2.Easy
+−120 VRa​ = 2 ΩIa​ = 6 AM+−EωT

A DC motor runs from 120 V with an armature current of 6 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 6 ARa = 2 Ω
Pdev = W
3.Medium
+−120 VRa​ = 0.4 ΩIa​ = 15 AM+−EωT

A DC motor draws 15 A from a 120 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 86 W. Find its efficiency η.

V = 120 VIa = 15 ARa = 0.4 ΩProt = 86 W
η = %
4.Medium
T (N·m)n (rpm)0.16 N·mn0​nFL​

A DC motor (k = 0.08 V·s/rad, Ra = 1.5 Ω) runs from 24 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.08 V·s/radRa = 1.5 ΩIFL = 2 A
SR = %
5.Hard
+−110 VRa​ = 2 ΩM+−En = ?T = 1.2 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 2 Ω, V = 110 V) drives a constant load torque of 1.2 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 0.6 V·s/radRa = 2 ΩT = 1.2 N·mΦ = 80 %
n = rpm
6.Hard
+−240 VD = ?Ra​ = 1.5 ΩM+−En = 1200 rpmT = 3.75 N·m

A DC chopper feeds a motor (k = 0.75 V·s/rad, Ra = 1.5 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 1200 rpm with a load of 3.75 N·m.

Vdc = 240 Vk = 0.75 V·s/radRa = 1.5 ΩT = 3.75 N·mn = 1200 rpm
D = %

Answer key

  1. 1. Istart = 300 A
  2. 2. Pdev = 648 W
  3. 3. η = 90.22 %
  4. 4. SR = 14.29 %
  5. 5. n = 2089 rpm
  6. 6. D = 42.39 %

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