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
+−240 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.5 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.5 N·m/AIa = 15 A
T = N·m
2.Easy
+−110 VRa​ = 1.2 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 12 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 110 VIa = 12 ARa = 1.2 Ω
E = V
3.Medium
T (N·m)n (rpm)0.6 N·mn0​nFL​

A DC motor (k = 0.3 V·s/rad, Ra = 1.2 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 = 0.3 V·s/radRa = 1.2 ΩIFL = 2 A
SR = %
4.Medium
+−110 VRa​ = 1 ΩIa​ = 5 AM+−EωT

A DC motor draws 5 A from a 110 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 26 W. Find its efficiency η.

V = 110 VIa = 5 ARa = 1 ΩProt = 26 W
η = %
5.Hard
T (N·m)n (rpm)0.6 N·m900 rpm

A DC motor with k = 0.12 V·s/rad and Ra = 0.5 Ω must drive a load of 0.6 N·m at 900 rpm. Find the armature voltage V it needs.

k = 0.12 V·s/radRa = 0.5 ΩT = 0.6 N·mn = 900 rpm
V = V
6.Hard
+−240 VRs​ = ?Ra​ = 1.2 ΩM+−En = 400 rpmT = 15 N·m

A DC motor (k = 1 V·s/rad, Ra = 1.2 Ω) on a fixed 240 V supply drives a load of 15 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 240 Vk = 1 V·s/radRa = 1.2 ΩT = 15 N·mn = 400 rpm
Rs = Ω

Answer key

  1. 1. T = 7.5 N·m
  2. 2. E = 95.6 V
  3. 3. SR = 5.263 %
  4. 4. η = 90.73 %
  5. 5. V = 13.81 V
  6. 6. Rs = 12.01 Ω

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