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​ = 20 AM+−EωT

A DC motor runs from 220 V with an armature current of 20 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 20 ARa = 0.5 Ω
Pdev = W
2.Easy
+−220 VRa​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 2 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.5 N·m/A and Ra = 1.2 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 1.5 N·m/ARa = 1.2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 2 V·s/rad
n0 = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 1 ΩM+−En = 300 rpmT = 4.5 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 1 Ω) on a fixed 120 V supply drives a load of 4.5 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 120 Vk = 0.75 V·s/radRa = 1 ΩT = 4.5 N·mn = 300 rpm
Rs = Ω
6.Hard
+−110 VRa​ = 0.4 ΩIa​: 25 → 12.5 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.4 Ω) runs at 1200 rpm with an armature current of 25 A. The load changes and the current becomes 12.5 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.4 Ωn1 = 1200 rpmIa1 = 25 AIa2 = 12.5 A
n2 = rpm

Answer key

  1. 1. Pdev = 4200 W
  2. 2. Istart = 110 A
  3. 3. Tstall = 137.5 N·m
  4. 4. n0 = 1146 rpm
  5. 5. Rs = 15.07 Ω
  6. 6. n2 = 1260 rpm

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