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​ = 0.4 ΩIa​ = 50 AM+−EωT

A DC motor runs from 220 V with an armature current of 50 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 50 ARa = 0.4 Ω
Pdev = W
2.Easy
+−220 VRa​ = 1 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1 Ω is switched directly onto a 220 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 1 Ω
Istart = A
3.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 4 kWn = 960 rpm
T = N·m
4.Medium
T (N·m)n (rpm)0.24 N·mn0​n = ?

A DC motor (k = 0.12 N·m/A, Ra = 1 Ω) runs from 24 V and drives a load torque of 0.24 N·m. Find its speed n in rpm.

V = 24 Vk = 0.12 N·m/ARa = 1 ΩT = 0.24 N·m
n = rpm
5.Hard
+−240 VRa​ = 1.2 ΩIa​: 12 → 7.2 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 1.2 Ω) runs at 900 rpm with an armature current of 12 A. The load changes and the current becomes 7.2 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 1.2 Ωn1 = 900 rpmIa1 = 12 AIa2 = 7.2 A
n2 = rpm
6.Hard
+−110 VRa​ = 0.2 ΩIa​: 40 → 80 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.2 Ω) runs at 1800 rpm with an armature current of 40 A. The load changes and the current becomes 80 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.2 Ωn1 = 1800 rpmIa1 = 40 AIa2 = 80 A
n2 = rpm

Answer key

  1. 1. Pdev = 10000 W
  2. 2. Istart = 220 A
  3. 3. T = 39.79 N·m
  4. 4. n = 1751 rpm
  5. 5. n2 = 923 rpm
  6. 6. n2 = 1659 rpm

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