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​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 110 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.8 Ω
Istart = A
2.Easy
+−110 VRa​M+−E = 103.6 Vn = 1200 rpmT

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 103.6 V. Find the motor constant k in V·s/rad.

E = 103.6 Vn = 1200 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.2 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 1.2 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.25 N·m/A and Ra = 0.2 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.25 N·m/ARa = 0.2 Ω
Tstall = N·m
5.Hard
+−220 VD = ?Ra​ = 0.5 ΩM+−En = 600 rpmT = 80 N·m

A DC chopper feeds a motor (k = 2 V·s/rad, Ra = 0.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 80 N·m.

Vdc = 220 Vk = 2 V·s/radRa = 0.5 ΩT = 80 N·mn = 600 rpm
D = %
6.Hard
+−110 VRst​ = ?Ra​ = 0.2 Ω60 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 60 A.

V = 110 VRa = 0.2 ΩImax = 60 A
Rst = Ω

Answer key

  1. 1. Istart = 137.5 A
  2. 2. k = 0.8244 V·s/rad
  3. 3. n0 = 1751 rpm
  4. 4. Tstall = 30 N·m
  5. 5. D = 66.21 %
  6. 6. Rst = 1.633 Ω

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