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
+−48 VRa​ = 1 ΩIa​ = 4 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 4 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 48 VIa = 4 ARa = 1 Ω
E = V
2.Easy
+−220 VRa​ = 2 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 6 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 220 VIa = 6 ARa = 2 Ω
E = V
3.Medium
T (N·m)n (rpm)3 N·mn0​n = ?

A DC motor (k = 1.5 N·m/A, Ra = 2 Ω) runs from 110 V and drives a load torque of 3 N·m. Find its speed n in rpm.

V = 110 Vk = 1.5 N·m/ARa = 2 ΩT = 3 N·m
n = rpm
4.Medium
T (N·m)n (rpm)2.5 N·mn0​nFL​

A DC motor (k = 0.25 V·s/rad, Ra = 0.25 Ω) runs from 24 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.25 V·s/radRa = 0.25 ΩIFL = 10 A
SR = %
5.Hard
T (N·m)n (rpm)12 N·m1500 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 1 Ω must drive a load of 12 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 1 ΩT = 12 N·mn = 1500 rpm
V = V
6.Hard
+−110 VRst​ = ?Ra​ = 1 Ω100 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 110 VRa = 1 ΩImax = 100 A
Rst = Ω

Answer key

  1. 1. E = 44 V
  2. 2. E = 208 V
  3. 3. n = 674.8 rpm
  4. 4. SR = 11.63 %
  5. 5. V = 140.7 V
  6. 6. Rst = 0.1 Ω

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