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​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 110 V supply. Its normal full-load current is 25 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.4 Ω
Istart = A
2.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 4 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 2 Ω
Istart = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.15 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.15 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.4 V·s/rad
n0 = rpm
5.Hard
+−120 VRa​ = 1.2 ΩM+−EωT

A small DC motor has an armature resistance of 1.2 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 1.2 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)3.6 N·m400 rpm

A DC motor with k = 0.6 V·s/rad and Ra = 0.8 Ω must drive a load of 3.6 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radRa = 0.8 ΩT = 3.6 N·mn = 400 rpm
V = V

Answer key

  1. 1. Istart = 275 A
  2. 2. Istart = 120 A
  3. 3. n0 = 1528 rpm
  4. 4. n0 = 2865 rpm
  5. 5. Pmax = 3000 W
  6. 6. V = 29.93 V

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