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​M+−E = 105 Vn = 2400 rpmT

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 105 V. Find the motor constant k in V·s/rad.

E = 105 Vn = 2400 rpm
k = V·s/rad
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = 30 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 30 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 110 VIa = 30 ARa = 0.4 Ω
E = V
3.Medium
T (N·m)n (rpm)1.2 N·mn0​nFL​

A DC motor (k = 0.6 V·s/rad, Ra = 1.5 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.6 V·s/radRa = 1.5 ΩIFL = 2 A
SR = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 0.8 V·s/rad
n0 = rpm
5.Hard
+−48 VRa​ = 0.5 ΩIa​: 12 → 6 AM+−En = ?T

A separately excited DC motor on 48 V (Ra = 0.5 Ω) runs at 1440 rpm with an armature current of 12 A. The load changes and the current becomes 6 A, with the field unchanged. Find the new speed n2.

V = 48 VRa = 0.5 Ωn1 = 1440 rpmIa1 = 12 AIa2 = 6 A
n2 = rpm
6.Hard
+−24 VRst​ = ?Ra​ = 0.5 Ω10 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 10 A.

V = 24 VRa = 0.5 ΩImax = 10 A
Rst = Ω

Answer key

  1. 1. k = 0.4178 V·s/rad
  2. 2. E = 98 V
  3. 3. SR = 6.667 %
  4. 4. n0 = 2865 rpm
  5. 5. n2 = 1543 rpm
  6. 6. Rst = 1.9 Ω

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