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
+−110 VRa​ = 0.25 ΩIa​ = 20 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 20 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 110 VIa = 20 ARa = 0.25 Ω
E = V
2.Easy
+−220 VRa​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 4 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 2 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)n0​ = ?

A DC motor with k = 0.75 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.75 V·s/rad
n0 = rpm
5.Hard
+−220 VD = ?Ra​ = 1.5 ΩM+−En = 300 rpmT = 6 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 1.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 6 N·m.

Vdc = 220 Vk = 1 V·s/radRa = 1.5 ΩT = 6 N·mn = 300 rpm
D = %
6.Hard
T (N·m)n (rpm)25 N·m300 rpm

A DC motor with k = 1 V·s/rad and Ra = 0.4 Ω must drive a load of 25 N·m at 300 rpm. Find the armature voltage V it needs.

k = 1 V·s/radRa = 0.4 ΩT = 25 N·mn = 300 rpm
V = V

Answer key

  1. 1. E = 105 V
  2. 2. Istart = 110 A
  3. 3. T = 39.79 N·m
  4. 4. n0 = 1401 rpm
  5. 5. D = 18.37 %
  6. 6. V = 41.42 V

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