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​M+−E = 102.8 Vn = 2400 rpmT

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 102.8 V. Find the motor constant k in V·s/rad.

E = 102.8 Vn = 2400 rpm
k = V·s/rad
2.Easy
+−110 VRa​ = 1 ΩIa​ = 8 AM+−EωT

A DC motor runs from 110 V with an armature current of 8 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 8 ARa = 1 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.15 N·m/A and Ra = 0.5 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.15 N·m/ARa = 0.5 Ω
Tstall = N·m
4.Medium
+−220 VRa​ = 2 ΩIa​ = 12 AM+−EωT

A DC motor draws 12 A from a 220 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 188 W. Find its efficiency η.

V = 220 VIa = 12 ARa = 2 ΩProt = 188 W
η = %
5.Hard
+−48 VD = ?Ra​ = 1 ΩM+−En = 600 rpmT = 2 N·m

A DC chopper feeds a motor (k = 0.4 V·s/rad, Ra = 1 Ω) from a 48 V DC bus. The average armature voltage is D·48 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 2 N·m.

Vdc = 48 Vk = 0.4 V·s/radRa = 1 ΩT = 2 N·mn = 600 rpm
D = %
6.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

Answer key

  1. 1. k = 0.409 V·s/rad
  2. 2. Pdev = 816 W
  3. 3. Tstall = 7.2 N·m
  4. 4. η = 81.97 %
  5. 5. D = 62.78 %
  6. 6. Pmax = 480 W

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