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​ = 0.25 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 1 V·s/rad and an armature resistance of 0.25 Ω runs from 120 V and draws 25 A. Find its speed n in rpm.

V = 120 VRa = 0.25 ΩIa = 25 Ak = 1 V·s/rad
n = rpm
2.Easy
+−24 VRa​ = 0.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 24 V supply. Its normal full-load current is 8 A. Find the armature current Istart at the moment of starting.

V = 24 VRa = 0.2 Ω
Istart = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.6 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 0.6 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.3 V·s/rad
n0 = rpm
5.Hard
+−120 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2.5 Ω
Pmax = W
6.Hard
+−240 VRst​ = ?Ra​ = 0.8 Ω25 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 240 VRa = 0.8 ΩImax = 25 A
Rst = Ω

Answer key

  1. 1. n = 1086 rpm
  2. 2. Istart = 120 A
  3. 3. n0 = 3820 rpm
  4. 4. n0 = 763.9 rpm
  5. 5. Pmax = 1440 W
  6. 6. Rst = 8.8 Ω

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