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
+−240 VRa​ = 0.8 ΩIa​ = ?M+−E = 216 VωT

A DC motor on a 240 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 216 V. Find the armature current Ia.

V = 240 VE = 216 VRa = 0.8 Ω
Ia = A
2.Easy
+−120 VRa​ = 0.8 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 6 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 120 VIa = 6 ARa = 0.8 Ω
E = V
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 2.2 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.5 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.5 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)22.5 N·m1500 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 0.5 Ω must drive a load of 22.5 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 0.5 ΩT = 22.5 N·mn = 1500 rpm
V = V
6.Hard
+−110 VRst​ = ?Ra​ = 0.2 Ω80 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 110 VRa = 0.2 ΩImax = 80 A
Rst = Ω

Answer key

  1. 1. Ia = 30 A
  2. 2. E = 115.2 V
  3. 3. T = 7.003 N·m
  4. 4. n0 = 2101 rpm
  5. 5. V = 132.8 V
  6. 6. Rst = 1.175 Ω

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