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
+−220 VRa​Ia​ = 8 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 8 A.

k = 0.05 N·m/AIa = 8 A
T = N·m
2.Easy
+−48 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 48 V supply. Its normal full-load current is 4 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.8 Ω
Istart = A
3.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
4.Medium
T (N·m)n (rpm)3 N·mn0​nFL​

A DC motor (k = 0.75 V·s/rad, Ra = 0.8 Ω) runs from 48 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.75 V·s/radRa = 0.8 ΩIFL = 4 A
SR = %
5.Hard
T (N·m)n (rpm)36 N·m600 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 0.25 Ω must drive a load of 36 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 0.25 ΩT = 36 N·mn = 600 rpm
V = V
6.Hard
+−24 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.5 Ω
Pmax = W

Answer key

  1. 1. T = 0.4 N·m
  2. 2. Istart = 60 A
  3. 3. n0 = 1401 rpm
  4. 4. SR = 7.143 %
  5. 5. V = 82.9 V
  6. 6. Pmax = 96 W

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