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​ = 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
2.Easy
+−240 VRa​ = 1 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 8 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 240 VIa = 8 ARa = 1 Ω
E = V
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.4 N·m/A and Ra = 0.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.4 N·m/ARa = 0.2 Ω
Tstall = N·m
4.Medium
+−120 VRa​ = 1 ΩIa​ = 8 AM+−EωT

A DC motor draws 8 A from a 120 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 36 W. Find its efficiency η.

V = 120 VIa = 8 ARa = 1 ΩProt = 36 W
η = %
5.Hard
T (N·m)n (rpm)3.75 N·m1200 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 2 Ω must drive a load of 3.75 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 2 ΩT = 3.75 N·mn = 1200 rpm
V = V
6.Hard
T (N·m)n (rpm)6 N·m600 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 1.5 Ω must drive a load of 6 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 1.5 ΩT = 6 N·mn = 600 rpm
V = V

Answer key

  1. 1. E = 205 V
  2. 2. E = 232 V
  3. 3. Tstall = 96 N·m
  4. 4. η = 89.58 %
  5. 5. V = 104.2 V
  6. 6. V = 82.9 V

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