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
+−120 VRa​ = 1.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 12 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 1.2 Ω
Istart = A
2.Easy
+−24 VRa​ = 0.2 ΩIa​ = 12 AM+−EωT

A DC motor runs from 24 V with an armature current of 12 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 12 ARa = 0.2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.5 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.5 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 750 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 4 kWn = 750 rpm
T = N·m
5.Hard
+−24 VRst​ = ?Ra​ = 0.2 Ω15 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 24 VRa = 0.2 ΩImax = 15 A
Rst = Ω
6.Hard
+−110 VRst​ = ?Ra​ = 1 Ω60 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 60 A.

V = 110 VRa = 1 ΩImax = 60 A
Rst = Ω

Answer key

  1. 1. Istart = 100 A
  2. 2. Pdev = 259.2 W
  3. 3. n0 = 2292 rpm
  4. 4. T = 50.93 N·m
  5. 5. Rst = 1.4 Ω
  6. 6. Rst = 0.8333 Ω

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