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​Ia​ = 12 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.1 N·m/A. Find the torque T it develops at an armature current of 12 A.

k = 0.1 N·m/AIa = 12 A
T = N·m
2.Easy
+−24 VRa​ = 0.8 ΩIa​ = 4 AM+−EωT

A DC motor runs from 24 V with an armature current of 4 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 4 ARa = 0.8 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)9 N·mn0​n = ?

A DC motor (k = 1.5 N·m/A, Ra = 1.5 Ω) runs from 240 V and drives a load torque of 9 N·m. Find its speed n in rpm.

V = 240 Vk = 1.5 N·m/ARa = 1.5 ΩT = 9 N·m
n = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.12 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.12 V·s/rad
n0 = rpm
5.Hard
+−240 VRst​ = ?Ra​ = 0.25 Ω50 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 50 A.

V = 240 VRa = 0.25 ΩImax = 50 A
Rst = Ω
6.Hard
+−120 VRs​ = ?Ra​ = 0.8 ΩM+−En = 900 rpmT = 9 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 0.8 Ω) on a fixed 120 V supply drives a load of 9 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 120 Vk = 0.75 V·s/radRa = 0.8 ΩT = 9 N·mn = 900 rpm
Rs = Ω

Answer key

  1. 1. T = 1.2 N·m
  2. 2. Pdev = 83.2 W
  3. 3. n = 1471 rpm
  4. 4. n0 = 1910 rpm
  5. 5. Rst = 4.55 Ω
  6. 6. Rs = 3.31 Ω

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