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​ = 0.8 ΩIa​ = 5 AM+−EωT

A DC motor runs from 120 V with an armature current of 5 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 5 ARa = 0.8 Ω
Pdev = W
2.Easy
+−220 VRa​ = 0.4 ΩIa​ = 25 AM+−EωT

A DC motor runs from 220 V with an armature current of 25 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 25 ARa = 0.4 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 2 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)0.2 N·mn0​nFL​

A DC motor (k = 0.1 V·s/rad, Ra = 1 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 = 0.1 V·s/radRa = 1 ΩIFL = 2 A
SR = %
5.Hard
+−24 VRs​ = ?Ra​ = 0.25 ΩM+−En = 800 rpmT = 0.6 N·m

A DC motor (k = 0.1 V·s/rad, Ra = 0.25 Ω) on a fixed 24 V supply drives a load of 0.6 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 24 Vk = 0.1 V·s/radRa = 0.25 ΩT = 0.6 N·mn = 800 rpm
Rs = Ω
6.Hard
T (N·m)n (rpm)2.5 N·m1200 rpm

A DC motor with k = 0.5 V·s/rad and Ra = 1.2 Ω must drive a load of 2.5 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.5 V·s/radRa = 1.2 ΩT = 2.5 N·mn = 1200 rpm
V = V

Answer key

  1. 1. Pdev = 580 W
  2. 2. Pdev = 5250 W
  3. 3. n0 = 1146 rpm
  4. 4. SR = 9.091 %
  5. 5. Rs = 2.354 Ω
  6. 6. V = 68.83 V

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