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​ = 2 AM+−E = ?ωT

A DC motor runs from a 24 V supply and draws an armature current of 2 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 24 VIa = 2 ARa = 0.4 Ω
E = V
2.Easy
+−48 VRa​ = 0.4 ΩIa​ = 8 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.4 Ω runs from 48 V and draws 8 A. Find its speed n in rpm.

V = 48 VRa = 0.4 ΩIa = 8 Ak = 0.6 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)12 N·mn0​nFL​

A DC motor (k = 1.2 V·s/rad, Ra = 1 Ω) runs from 120 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 120 Vk = 1.2 V·s/radRa = 1 ΩIFL = 10 A
SR = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.2 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 1.2 V·s/rad
n0 = rpm
5.Hard
+−120 VRst​ = ?Ra​ = 1.2 Ω10 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 120 VRa = 1.2 ΩImax = 10 A
Rst = Ω
6.Hard
+−120 VRa​ = 0.8 ΩM+−EωT

A small DC motor has an armature resistance of 0.8 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 0.8 Ω
Pmax = W

Answer key

  1. 1. E = 23.2 V
  2. 2. n = 713 rpm
  3. 3. SR = 9.091 %
  4. 4. n0 = 954.9 rpm
  5. 5. Rst = 10.8 Ω
  6. 6. Pmax = 4500 W

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