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
+−220 VRa​ = 0.8 ΩIa​ = ?M+−E = 208 VωT

A DC motor on a 220 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 208 V. Find the armature current Ia.

V = 220 VE = 208 VRa = 0.8 Ω
Ia = A
2.Easy
+−120 VRa​ = 0.5 ΩIa​ = 12 AM+−EωT

A DC motor runs from 120 V with an armature current of 12 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 12 ARa = 0.5 Ω
Pdev = W
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)0.4 N·mn0​nFL​

A DC motor (k = 0.2 V·s/rad, Ra = 0.4 Ω) runs from 24 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.2 V·s/radRa = 0.4 ΩIFL = 2 A
SR = %
5.Hard
+−240 VRa​ = 0.4 ΩIa​: 50 → 25 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.4 Ω) runs at 1440 rpm with an armature current of 50 A. The load changes and the current becomes 25 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.4 Ωn1 = 1440 rpmIa1 = 50 AIa2 = 25 A
n2 = rpm
6.Hard
+−110 VRa​ = 0.25 ΩIa​: 40 → 10 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.25 Ω) runs at 1500 rpm with an armature current of 40 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.25 Ωn1 = 1500 rpmIa1 = 40 AIa2 = 10 A
n2 = rpm

Answer key

  1. 1. Ia = 15 A
  2. 2. Pdev = 1368 W
  3. 3. T = 24.7 N·m
  4. 4. SR = 3.448 %
  5. 5. n2 = 1505 rpm
  6. 6. n2 = 1613 rpm

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