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​ = 1.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 1.2 Ω
Istart = A
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.4 Ω runs from 110 V and draws 25 A. Find its speed n in rpm.

V = 110 VRa = 0.4 ΩIa = 25 Ak = 0.8 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)15 N·mn0​nFL​

A DC motor (k = 0.6 V·s/rad, Ra = 0.25 Ω) runs from 120 V. At full load it draws 25 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 120 Vk = 0.6 V·s/radRa = 0.25 ΩIFL = 25 A
SR = %
4.Medium
T (N·m)n (rpm)1 N·mn0​nFL​

A DC motor (k = 0.25 V·s/rad, Ra = 1.5 Ω) runs from 48 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.25 V·s/radRa = 1.5 ΩIFL = 4 A
SR = %
5.Hard
+−110 VD = ?Ra​ = 2 ΩM+−En = 750 rpmT = 2 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 2 Ω) from a 110 V DC bus. The average armature voltage is D·110 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 2 N·m.

Vdc = 110 Vk = 1 V·s/radRa = 2 ΩT = 2 N·mn = 750 rpm
D = %
6.Hard
+−220 VRa​ = 0.4 ΩIa​: 30 → 12 AM+−En = ?T

A separately excited DC motor on 220 V (Ra = 0.4 Ω) runs at 1000 rpm with an armature current of 30 A. The load changes and the current becomes 12 A, with the field unchanged. Find the new speed n2.

V = 220 VRa = 0.4 Ωn1 = 1000 rpmIa1 = 30 AIa2 = 12 A
n2 = rpm

Answer key

  1. 1. Istart = 100 A
  2. 2. n = 1194 rpm
  3. 3. SR = 5.495 %
  4. 4. SR = 14.29 %
  5. 5. D = 75.04 %
  6. 6. n2 = 1035 rpm

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