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​ = 8 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 8 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 120 VIa = 8 ARa = 1.2 Ω
E = V
2.Easy
+−240 VRa​ = 1.2 ΩIa​ = 10 AM+−EωT

A DC motor runs from 240 V with an armature current of 10 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 10 ARa = 1.2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.4 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)22.5 N·mn0​n = ?

A DC motor (k = 1.5 N·m/A, Ra = 0.8 Ω) runs from 110 V and drives a load torque of 22.5 N·m. Find its speed n in rpm.

V = 110 Vk = 1.5 N·m/ARa = 0.8 ΩT = 22.5 N·m
n = rpm
5.Hard
+−240 VRst​ = ?Ra​ = 1.2 Ω30 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 240 VRa = 1.2 ΩImax = 30 A
Rst = Ω
6.Hard
+−240 VD = ?Ra​ = 1.5 ΩM+−En = 1500 rpmT = 3 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 1.5 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 1500 rpm with a load of 3 N·m.

Vdc = 240 Vk = 0.6 V·s/radRa = 1.5 ΩT = 3 N·mn = 1500 rpm
D = %

Answer key

  1. 1. E = 110.4 V
  2. 2. Pdev = 2280 W
  3. 3. n0 = 2626 rpm
  4. 4. n = 623.9 rpm
  5. 5. Rst = 6.8 Ω
  6. 6. D = 42.39 %

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