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​ = 1.5 ΩIa​ = 10 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 10 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 220 VIa = 10 ARa = 1.5 Ω
E = V
2.Easy
+−110 VRa​ = 1 ΩIa​ = 4 AM+−EωT

A DC motor runs from 110 V with an armature current of 4 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 4 ARa = 1 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.1 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.1 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1 V·s/rad
n0 = rpm
5.Hard
+−220 VRa​ = 1.5 ΩM+−En = ?T = 4.8 N·m

A separately excited DC motor (k = 0.8 V·s/rad at full field, Ra = 1.5 Ω, V = 220 V) drives a constant load torque of 4.8 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 220 Vk = 0.8 V·s/radRa = 1.5 ΩT = 4.8 N·mΦ = 90 %
n = rpm
6.Hard
+−120 VRa​ = 1.5 ΩM+−En = ?T = 2 N·m

A separately excited DC motor (k = 0.5 V·s/rad at full field, Ra = 1.5 Ω, V = 120 V) drives a constant load torque of 2 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 120 Vk = 0.5 V·s/radRa = 1.5 ΩT = 2 N·mΦ = 80 %
n = rpm

Answer key

  1. 1. E = 205 V
  2. 2. Pdev = 424 W
  3. 3. n0 = 2292 rpm
  4. 4. n0 = 2292 rpm
  5. 5. n = 2785 rpm
  6. 6. n = 2686 rpm

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