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​ = 0.25 ΩIa​ = 6 AM+−EωT

A DC motor runs from 48 V with an armature current of 6 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 6 ARa = 0.25 Ω
Pdev = W
2.Easy
+−220 VRa​ = 1.2 ΩIa​ = 25 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 25 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 220 VIa = 25 ARa = 1.2 Ω
E = V
3.Medium
T (N·m)n (rpm)40 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 0.2 Ω) runs from 220 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 1 V·s/radRa = 0.2 ΩIFL = 40 A
SR = %
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 7.5 kWn = 960 rpm
T = N·m
5.Hard
+−110 VRa​ = 1.5 ΩIa​: 5 → 2 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 1.5 Ω) runs at 1500 rpm with an armature current of 5 A. The load changes and the current becomes 2 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 1.5 Ωn1 = 1500 rpmIa1 = 5 AIa2 = 2 A
n2 = rpm
6.Hard
+−240 VD = ?Ra​ = 0.2 ΩM+−En = 600 rpmT = 40 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 0.2 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 40 N·m.

Vdc = 240 Vk = 1 V·s/radRa = 0.2 ΩT = 40 N·mn = 600 rpm
D = %

Answer key

  1. 1. Pdev = 279 W
  2. 2. E = 190 V
  3. 3. SR = 3.774 %
  4. 4. T = 74.6 N·m
  5. 5. n2 = 1566 rpm
  6. 6. D = 29.51 %

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