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​ = 1 ΩIa​ = 20 AM+−EωT

A DC motor runs from 220 V with an armature current of 20 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 20 ARa = 1 Ω
Pdev = W
2.Easy
+−220 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.25 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.25 N·m/AIa = 15 A
T = N·m
3.Medium
T (N·m)n (rpm)6 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 1.5 Ω) runs from 220 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 1 V·s/radRa = 1.5 ΩIFL = 6 A
SR = %
4.Medium
T (N·m)n (rpm)15 N·mn0​n = ?

A DC motor (k = 0.6 N·m/A, Ra = 0.25 Ω) runs from 120 V and drives a load torque of 15 N·m. Find its speed n in rpm.

V = 120 Vk = 0.6 N·m/ARa = 0.25 ΩT = 15 N·m
n = rpm
5.Hard
+−48 VRa​ = 1.2 ΩM+−EωT

A small DC motor has an armature resistance of 1.2 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 1.2 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)12 N·m300 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 1 Ω must drive a load of 12 N·m at 300 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 1 ΩT = 12 N·mn = 300 rpm
V = V

Answer key

  1. 1. Pdev = 4000 W
  2. 2. T = 3.75 N·m
  3. 3. SR = 4.265 %
  4. 4. n = 1810 rpm
  5. 5. Pmax = 480 W
  6. 6. V = 47.7 V

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