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 = 0n = 0T

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 48 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.5 Ω
Istart = A
2.Easy
+−220 VRa​ = 0.5 ΩIa​ = 30 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 30 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 220 VIa = 30 ARa = 0.5 Ω
E = V
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 11 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 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 0.75 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)15 N·m500 rpm

A DC motor with k = 1 V·s/rad and Ra = 1 Ω must drive a load of 15 N·m at 500 rpm. Find the armature voltage V it needs.

k = 1 V·s/radRa = 1 ΩT = 15 N·mn = 500 rpm
V = V
6.Hard
+−120 VRa​ = 0.25 ΩM+−En = ?T = 16 N·m

A separately excited DC motor (k = 0.8 V·s/rad at full field, Ra = 0.25 Ω, V = 120 V) drives a constant load torque of 16 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.8 V·s/radRa = 0.25 ΩT = 16 N·mΦ = 75 %
n = rpm

Answer key

  1. 1. Istart = 96 A
  2. 2. E = 205 V
  3. 3. T = 58.36 N·m
  4. 4. n0 = 2801 rpm
  5. 5. V = 67.36 V
  6. 6. n = 1804 rpm

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