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
+−240 VRa​ = 2 ΩIa​ = 4 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 4 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 240 VIa = 4 ARa = 2 Ω
E = V
2.Easy
+−120 VRa​ = 0.25 ΩIa​ = 15 AM+−EωT

A DC motor runs from 120 V with an armature current of 15 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 15 ARa = 0.25 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.25 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.25 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)0.5 N·mn0​nFL​

A DC motor (k = 0.1 V·s/rad, Ra = 0.25 Ω) runs from 24 V. At full load it draws 5 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.1 V·s/radRa = 0.25 ΩIFL = 5 A
SR = %
5.Hard
+−110 VRa​ = 0.2 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 0.5 V·s/rad at full field, Ra = 0.2 Ω, V = 110 V) drives a constant load torque of 15 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 110 Vk = 0.5 V·s/radRa = 0.2 ΩT = 15 N·mΦ = 85 %
n = rpm
6.Hard
+−240 VD = ?Ra​ = 0.25 ΩM+−En = 300 rpmT = 60 N·m

A DC chopper feeds a motor (k = 1.5 V·s/rad, Ra = 0.25 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 300 rpm with a load of 60 N·m.

Vdc = 240 Vk = 1.5 V·s/radRa = 0.25 ΩT = 60 N·mn = 300 rpm
D = %

Answer key

  1. 1. E = 232 V
  2. 2. Pdev = 1744 W
  3. 3. n0 = 916.7 rpm
  4. 4. SR = 5.495 %
  5. 5. n = 2313 rpm
  6. 6. D = 23.8 %

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