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​ = 1.5 ΩIa​ = 6 AM+−EωT

A DC motor runs from 110 V with an armature current of 6 A. The armature resistance is 1.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 6 ARa = 1.5 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.5 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 0.5 V·s/rad and an armature resistance of 0.5 Ω runs from 120 V and draws 20 A. Find its speed n in rpm.

V = 120 VRa = 0.5 ΩIa = 20 Ak = 0.5 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 1500 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 7.5 kWn = 1500 rpm
T = N·m
4.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 0.5 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 0.5 kWn = 2900 rpm
T = N·m
5.Hard
+−48 VRs​ = ?Ra​ = 0.25 ΩM+−En = 400 rpmT = 4.8 N·m

A DC motor (k = 0.6 V·s/rad, Ra = 0.25 Ω) on a fixed 48 V supply drives a load of 4.8 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 48 Vk = 0.6 V·s/radRa = 0.25 ΩT = 4.8 N·mn = 400 rpm
Rs = Ω
6.Hard
+−110 VRst​ = ?Ra​ = 1 Ω15 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 110 VRa = 1 ΩImax = 15 A
Rst = Ω

Answer key

  1. 1. Pdev = 606 W
  2. 2. n = 2101 rpm
  3. 3. T = 47.75 N·m
  4. 4. T = 1.646 N·m
  5. 5. Rs = 2.608 Ω
  6. 6. Rst = 6.333 Ω

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