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​ = 1.2 ΩIa​ = 10 AM+−EωT

A DC motor runs from 220 V with an armature current of 10 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 10 ARa = 1.2 Ω
Pdev = W
2.Easy
+−240 VRa​ = 2 ΩIa​ = 6 AM+−EωT

A DC motor runs from 240 V with an armature current of 6 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 6 ARa = 2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)15 N·mn0​nFL​

A DC motor (k = 0.75 V·s/rad, Ra = 0.4 Ω) runs from 120 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 120 Vk = 0.75 V·s/radRa = 0.4 ΩIFL = 20 A
SR = %
4.Medium
T (N·m)n (rpm)6 N·mn0​nFL​

A DC motor (k = 0.4 V·s/rad, Ra = 0.2 Ω) runs from 48 V. At full load it draws 15 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.4 V·s/radRa = 0.2 ΩIFL = 15 A
SR = %
5.Hard
+−24 VRs​ = ?Ra​ = 0.5 ΩM+−En = 400 rpmT = 0.6 N·m

A DC motor (k = 0.15 V·s/rad, Ra = 0.5 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 400 rpm.

V = 24 Vk = 0.15 V·s/radRa = 0.5 ΩT = 0.6 N·mn = 400 rpm
Rs = Ω
6.Hard
+−240 VRs​ = ?Ra​ = 0.2 ΩM+−En = 800 rpmT = 48 N·m

A DC motor (k = 1.2 V·s/rad, Ra = 0.2 Ω) on a fixed 240 V supply drives a load of 48 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 240 Vk = 1.2 V·s/radRa = 0.2 ΩT = 48 N·mn = 800 rpm
Rs = Ω

Answer key

  1. 1. Pdev = 2080 W
  2. 2. Pdev = 1368 W
  3. 3. SR = 7.143 %
  4. 4. SR = 6.667 %
  5. 5. Rs = 3.929 Ω
  6. 6. Rs = 3.287 Ω

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