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
+−110 VRa​ = 0.5 ΩIa​ = 20 AM+−EωT

A DC motor runs from 110 V with an armature current of 20 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 20 ARa = 0.5 Ω
Pdev = W
2.Easy
+−240 VRa​ = 1 ΩIa​ = 25 AM+−EωT

A DC motor runs from 240 V with an armature current of 25 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 25 ARa = 1 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 2 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 2 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)0.8 N·mn0​n = ?

A DC motor (k = 0.2 N·m/A, Ra = 0.5 Ω) runs from 24 V and drives a load torque of 0.8 N·m. Find its speed n in rpm.

V = 24 Vk = 0.2 N·m/ARa = 0.5 ΩT = 0.8 N·m
n = rpm
5.Hard
+−24 VRst​ = ?Ra​ = 0.8 Ω15 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 24 VRa = 0.8 ΩImax = 15 A
Rst = Ω
6.Hard
+−220 VRa​ = 0.8 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 1.5 V·s/rad at full field, Ra = 0.8 Ω, V = 220 V) drives a constant load torque of 15 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 220 Vk = 1.5 V·s/radRa = 0.8 ΩT = 15 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Pdev = 2000 W
  2. 2. Pdev = 5375 W
  3. 3. n0 = 1050 rpm
  4. 4. n = 1050 rpm
  5. 5. Rst = 0.8 Ω
  6. 6. n = 1493 rpm

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