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​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 240 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 2 Ω
Istart = A
2.Easy
+−220 VRa​Ia​ = 5 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.05 N·m/A. Find the torque T it develops at an armature current of 5 A.

k = 0.05 N·m/AIa = 5 A
T = N·m
3.Medium
+−110 VRa​ = 0.5 ΩIa​ = 10 AM+−EωT

A DC motor draws 10 A from a 110 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 84 W. Find its efficiency η.

V = 110 VIa = 10 ARa = 0.5 ΩProt = 84 W
η = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.2 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.2 V·s/rad
n0 = rpm
5.Hard
+−110 VRa​ = 1.5 ΩM+−En = ?T = 3.6 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 1.5 Ω, V = 110 V) drives a constant load torque of 3.6 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 110 Vk = 0.6 V·s/radRa = 1.5 ΩT = 3.6 N·mΦ = 85 %
n = rpm
6.Hard
+−110 VRs​ = ?Ra​ = 1 ΩM+−En = 300 rpmT = 7.5 N·m

A DC motor (k = 1.5 V·s/rad, Ra = 1 Ω) on a fixed 110 V supply drives a load of 7.5 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 110 Vk = 1.5 V·s/radRa = 1 ΩT = 7.5 N·mn = 300 rpm
Rs = Ω

Answer key

  1. 1. Istart = 120 A
  2. 2. T = 0.25 N·m
  3. 3. η = 87.82 %
  4. 4. n0 = 1146 rpm
  5. 5. n = 1861 rpm
  6. 6. Rs = 11.58 Ω

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