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​ = 0.5 ΩIa​ = ?M+−E = 46 VωT

A DC motor on a 48 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 46 V. Find the armature current Ia.

V = 48 VE = 46 VRa = 0.5 Ω
Ia = A
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = ?M+−E = 232 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 232 V. Find the armature current Ia.

V = 240 VE = 232 VRa = 0.8 Ω
Ia = A
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 1 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 1 kWn = 1800 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.75 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.75 V·s/rad
n0 = rpm
5.Hard
+−120 VRst​ = ?Ra​ = 1 Ω100 AM+−E = 0n = 0T

A 120 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 = 120 VRa = 1 ΩImax = 100 A
Rst = Ω
6.Hard
T (N·m)n (rpm)0.75 N·m1000 rpm

A DC motor with k = 0.15 V·s/rad and Ra = 0.8 Ω must drive a load of 0.75 N·m at 1000 rpm. Find the armature voltage V it needs.

k = 0.15 V·s/radRa = 0.8 ΩT = 0.75 N·mn = 1000 rpm
V = V

Answer key

  1. 1. Ia = 4 A
  2. 2. Ia = 10 A
  3. 3. T = 5.305 N·m
  4. 4. n0 = 1528 rpm
  5. 5. Rst = 0.2 Ω
  6. 6. V = 19.71 V

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