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
+−24 VRa​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 24 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 24 VRa = 0.4 Ω
Istart = A
2.Easy
+−120 VRa​ = 1.5 ΩIa​ = 5 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 5 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 120 VIa = 5 ARa = 1.5 Ω
E = V
3.Medium
T (N·m)n (rpm)0.3 N·mn0​nFL​

A DC motor (k = 0.15 V·s/rad, Ra = 2 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.15 V·s/radRa = 2 ΩIFL = 2 A
SR = %
4.Medium
T (N·m)n (rpm)48 N·mn0​nFL​

A DC motor (k = 1.2 V·s/rad, Ra = 0.8 Ω) runs from 240 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 1.2 V·s/radRa = 0.8 ΩIFL = 40 A
SR = %
5.Hard
+−220 VRs​ = ?Ra​ = 1 ΩM+−En = 800 rpmT = 12 N·m

A DC motor (k = 0.8 V·s/rad, Ra = 1 Ω) on a fixed 220 V supply drives a load of 12 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 220 Vk = 0.8 V·s/radRa = 1 ΩT = 12 N·mn = 800 rpm
Rs = Ω
6.Hard
+−240 VRa​ = 0.5 ΩIa​: 50 → 20 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.5 Ω) runs at 1440 rpm with an armature current of 50 A. The load changes and the current becomes 20 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.5 Ωn1 = 1440 rpmIa1 = 50 AIa2 = 20 A
n2 = rpm

Answer key

  1. 1. Istart = 60 A
  2. 2. E = 112.5 V
  3. 3. SR = 9.091 %
  4. 4. SR = 15.38 %
  5. 5. Rs = 9.199 Ω
  6. 6. n2 = 1540 rpm

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