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
+−110 VRa​ = 0.8 ΩIa​ = 10 AM+−EωT

A DC motor runs from 110 V with an armature current of 10 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 10 ARa = 0.8 Ω
Pdev = W
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = 15 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.8 Ω runs from 240 V and draws 15 A. Find its speed n in rpm.

V = 240 VRa = 0.8 ΩIa = 15 Ak = 0.6 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)3 N·mn0​nFL​

A DC motor (k = 0.75 V·s/rad, Ra = 2 Ω) runs from 220 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 0.75 V·s/radRa = 2 ΩIFL = 4 A
SR = %
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 3 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 3 kWn = 960 rpm
T = N·m
5.Hard
+−24 VRst​ = ?Ra​ = 0.5 Ω15 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 24 VRa = 0.5 ΩImax = 15 A
Rst = Ω
6.Hard
T (N·m)n (rpm)0.2 N·m600 rpm

A DC motor with k = 0.1 V·s/rad and Ra = 0.8 Ω must drive a load of 0.2 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.1 V·s/radRa = 0.8 ΩT = 0.2 N·mn = 600 rpm
V = V

Answer key

  1. 1. Pdev = 1020 W
  2. 2. n = 3629 rpm
  3. 3. SR = 3.774 %
  4. 4. T = 29.84 N·m
  5. 5. Rst = 1.1 Ω
  6. 6. V = 7.883 V

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