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​ = 0.2 ΩIa​ = ?M+−E = 114 VωT

A DC motor on a 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 114 V. Find the armature current Ia.

V = 120 VE = 114 VRa = 0.2 Ω
Ia = A
2.Easy
+−240 VRa​ = 2 ΩIa​ = 10 AM+−EωT

A DC motor runs from 240 V with an armature current of 10 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 10 ARa = 2 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 15 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.8 V·s/rad
n0 = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 0.5 ΩM+−En = 600 rpmT = 16 N·m

A DC motor (k = 0.8 V·s/rad, Ra = 0.5 Ω) on a fixed 120 V supply drives a load of 16 N·m. Find the series resistance Rs that slows it down to 600 rpm.

V = 120 Vk = 0.8 V·s/radRa = 0.5 ΩT = 16 N·mn = 600 rpm
Rs = Ω
6.Hard
+−24 VRs​ = ?Ra​ = 1 ΩM+−En = 300 rpmT = 0.6 N·m

A DC motor (k = 0.3 V·s/rad, Ra = 1 Ω) on a fixed 24 V supply drives a load of 0.6 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 24 Vk = 0.3 V·s/radRa = 1 ΩT = 0.6 N·mn = 300 rpm
Rs = Ω

Answer key

  1. 1. Ia = 30 A
  2. 2. Pdev = 2200 W
  3. 3. T = 47.75 N·m
  4. 4. n0 = 1432 rpm
  5. 5. Rs = 2.987 Ω
  6. 6. Rs = 6.288 Ω

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