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​ = 1.5 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 8 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 120 VIa = 8 ARa = 1.5 Ω
E = V
2.Easy
+−48 VRa​ = 0.4 ΩIa​ = 6 AM+−EωT

A DC motor runs from 48 V with an armature current of 6 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 6 ARa = 0.4 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.5 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 1.5 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 1500 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 2.2 kWn = 1500 rpm
T = N·m
5.Hard
+−240 VRa​ = 0.25 ΩM+−En = ?T = 18 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 0.25 Ω, V = 240 V) drives a constant load torque of 18 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 240 Vk = 0.6 V·s/radRa = 0.25 ΩT = 18 N·mΦ = 80 %
n = rpm
6.Hard
+−110 VRs​ = ?Ra​ = 0.2 ΩM+−En = 800 rpmT = 18.75 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 0.2 Ω) on a fixed 110 V supply drives a load of 18.75 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 110 Vk = 0.75 V·s/radRa = 0.2 ΩT = 18.75 N·mn = 800 rpm
Rs = Ω

Answer key

  1. 1. E = 108 V
  2. 2. Pdev = 273.6 W
  3. 3. n0 = 763.9 rpm
  4. 4. T = 14.01 N·m
  5. 5. n = 4588 rpm
  6. 6. Rs = 1.687 Ω

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