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​ = 1.2 ΩIa​ = 12 AM+−En = ?T

A DC motor with k = 1.2 V·s/rad and an armature resistance of 1.2 Ω runs from 240 V and draws 12 A. Find its speed n in rpm.

V = 240 VRa = 1.2 ΩIa = 12 Ak = 1.2 V·s/rad
n = rpm
2.Easy
+−24 VRa​Ia​ = 4 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.12 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.12 N·m/AIa = 4 A
T = N·m
3.Medium
T (N·m)n (rpm)3.2 N·mn0​nFL​

A DC motor (k = 0.8 V·s/rad, Ra = 1.2 Ω) runs from 120 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 120 Vk = 0.8 V·s/radRa = 1.2 ΩIFL = 4 A
SR = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.6 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 0.6 V·s/rad
n0 = rpm
5.Hard
+−240 VRst​ = ?Ra​ = 1 Ω15 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 240 VRa = 1 ΩImax = 15 A
Rst = Ω
6.Hard
+−48 VRa​ = 1 ΩIa​: 2 → 3 AM+−En = ?T

A separately excited DC motor on 48 V (Ra = 1 Ω) runs at 1440 rpm with an armature current of 2 A. The load changes and the current becomes 3 A, with the field unchanged. Find the new speed n2.

V = 48 VRa = 1 Ωn1 = 1440 rpmIa1 = 2 AIa2 = 3 A
n2 = rpm

Answer key

  1. 1. n = 1795 rpm
  2. 2. T = 0.48 N·m
  3. 3. SR = 4.167 %
  4. 4. n0 = 3501 rpm
  5. 5. Rst = 15 Ω
  6. 6. n2 = 1409 rpm

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