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​ = 0.5 ΩIa​ = 50 AM+−EωT

A DC motor runs from 220 V with an armature current of 50 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 50 ARa = 0.5 Ω
Pdev = W
2.Easy
+−240 VRa​M+−E = 216 Vn = 900 rpmT

A DC motor turns at 900 rpm, and its back-EMF at that speed is 216 V. Find the motor constant k in V·s/rad.

E = 216 Vn = 900 rpm
k = V·s/rad
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 1.5 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.5 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1.5 V·s/rad
n0 = rpm
5.Hard
+−24 VRa​ = 1.2 ΩM+−EωT

A small DC motor has an armature resistance of 1.2 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.2 Ω
Pmax = W
6.Hard
+−24 VRs​ = ?Ra​ = 0.2 ΩM+−En = 600 rpmT = 2.5 N·m

A DC motor (k = 0.25 V·s/rad, Ra = 0.2 Ω) on a fixed 24 V supply drives a load of 2.5 N·m. Find the series resistance Rs that slows it down to 600 rpm.

V = 24 Vk = 0.25 V·s/radRa = 0.2 ΩT = 2.5 N·mn = 600 rpm
Rs = Ω

Answer key

  1. 1. Pdev = 9750 W
  2. 2. k = 2.292 V·s/rad
  3. 3. T = 4.775 N·m
  4. 4. n0 = 1528 rpm
  5. 5. Pmax = 120 W
  6. 6. Rs = 0.6292 Ω

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