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​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 240 V supply. Its normal full-load current is 40 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.4 Ω
Istart = A
2.Easy
+−48 VRa​ = 0.25 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 12 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 48 VIa = 12 ARa = 0.25 Ω
E = V
3.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 7.5 kWn = 2900 rpm
T = N·m
4.Medium
T (N·m)n (rpm)24 N·mn0​nFL​

A DC motor (k = 1.2 V·s/rad, Ra = 1.5 Ω) runs from 220 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 1.2 V·s/radRa = 1.5 ΩIFL = 20 A
SR = %
5.Hard
+−240 VRst​ = ?Ra​ = 0.2 Ω100 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 240 VRa = 0.2 ΩImax = 100 A
Rst = Ω
6.Hard
T (N·m)n (rpm)0.48 N·m800 rpm

A DC motor with k = 0.08 V·s/rad and Ra = 0.2 Ω must drive a load of 0.48 N·m at 800 rpm. Find the armature voltage V it needs.

k = 0.08 V·s/radRa = 0.2 ΩT = 0.48 N·mn = 800 rpm
V = V

Answer key

  1. 1. Istart = 600 A
  2. 2. E = 45 V
  3. 3. T = 24.7 N·m
  4. 4. SR = 15.79 %
  5. 5. Rst = 2.2 Ω
  6. 6. V = 7.902 V

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