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
+−120 VRa​Ia​ = 6 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 1.2 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 1.2 N·m/AIa = 6 A
T = N·m
2.Easy
+−110 VRa​ = 0.25 ΩIa​ = 15 AM+−EωT

A DC motor runs from 110 V with an armature current of 15 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 15 ARa = 0.25 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.6 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.6 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.4 N·m/A and Ra = 1.5 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.4 N·m/ARa = 1.5 Ω
Tstall = N·m
5.Hard
+−240 VD = ?Ra​ = 0.25 ΩM+−En = 400 rpmT = 48 N·m

A DC chopper feeds a motor (k = 1.2 V·s/rad, Ra = 0.25 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 400 rpm with a load of 48 N·m.

Vdc = 240 Vk = 1.2 V·s/radRa = 0.25 ΩT = 48 N·mn = 400 rpm
D = %
6.Hard
+−24 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 2.5 Ω
Pmax = W

Answer key

  1. 1. T = 7.2 N·m
  2. 2. Pdev = 1594 W
  3. 3. n0 = 1910 rpm
  4. 4. Tstall = 12.8 N·m
  5. 5. D = 25.11 %
  6. 6. Pmax = 57.6 W

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