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
+−48 VRa​Ia​ = 25 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.4 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 0.4 N·m/AIa = 25 A
T = N·m
2.Easy
+−240 VRa​Ia​ = 6 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 0.75 N·m/AIa = 6 A
T = N·m
3.Medium
+−220 VRa​ = 0.8 ΩIa​ = 12 AM+−EωT

A DC motor draws 12 A from a 220 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 101 W. Find its efficiency η.

V = 220 VIa = 12 ARa = 0.8 ΩProt = 101 W
η = %
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 15 kWn = 960 rpm
T = N·m
5.Hard
+−120 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2.5 Ω
Pmax = W
6.Hard
+−120 VD = ?Ra​ = 0.8 ΩM+−En = 1000 rpmT = 4.5 N·m

A DC chopper feeds a motor (k = 0.3 V·s/rad, Ra = 0.8 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 4.5 N·m.

Vdc = 120 Vk = 0.3 V·s/radRa = 0.8 ΩT = 4.5 N·mn = 1000 rpm
D = %

Answer key

  1. 1. T = 10 N·m
  2. 2. T = 4.5 N·m
  3. 3. η = 91.81 %
  4. 4. T = 149.2 N·m
  5. 5. Pmax = 1440 W
  6. 6. D = 36.18 %

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