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
+−48 VRa​ = 0.4 ΩIa​ = ?M+−E = 44 VωT

A DC motor on a 48 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 44 V. Find the armature current Ia.

V = 48 VE = 44 VRa = 0.4 Ω
Ia = A
2.Easy
+−110 VRa​ = 1.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 110 V supply. Its normal full-load current is 4 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 1.2 Ω
Istart = A
3.Medium
+−VRa​M+−En = 1500 rpmT = ?

A DC motor delivers 3 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 3 kWn = 1500 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 2 V·s/rad
n0 = rpm
5.Hard
+−110 VRst​ = ?Ra​ = 1 Ω100 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 100 A.

V = 110 VRa = 1 ΩImax = 100 A
Rst = Ω
6.Hard
+−240 VD = ?Ra​ = 2 ΩM+−En = 1200 rpmT = 3 N·m

A DC chopper feeds a motor (k = 0.75 V·s/rad, Ra = 2 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 1200 rpm with a load of 3 N·m.

Vdc = 240 Vk = 0.75 V·s/radRa = 2 ΩT = 3 N·mn = 1200 rpm
D = %

Answer key

  1. 1. Ia = 10 A
  2. 2. Istart = 91.67 A
  3. 3. T = 19.1 N·m
  4. 4. n0 = 1146 rpm
  5. 5. Rst = 0.1 Ω
  6. 6. D = 42.6 %

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