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
+−240 VRa​ = 0.2 ΩIa​ = ?M+−E = 230 VωT

A DC motor on a 240 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 230 V. Find the armature current Ia.

V = 240 VE = 230 VRa = 0.2 Ω
Ia = A
2.Easy
+−24 VRa​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 24 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 24 VRa = 0.4 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.25 N·m/A and Ra = 0.8 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.25 N·m/ARa = 0.8 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.25 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.25 V·s/rad
n0 = rpm
5.Hard
+−110 VRa​ = 1.2 ΩIa​: 10 → 15 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 1.2 Ω) runs at 1500 rpm with an armature current of 10 A. The load changes and the current becomes 15 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 1.2 Ωn1 = 1500 rpmIa1 = 10 AIa2 = 15 A
n2 = rpm
6.Hard
+−220 VRst​ = ?Ra​ = 0.8 Ω50 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 50 A.

V = 220 VRa = 0.8 ΩImax = 50 A
Rst = Ω

Answer key

  1. 1. Ia = 50 A
  2. 2. Istart = 60 A
  3. 3. Tstall = 15 N·m
  4. 4. n0 = 916.7 rpm
  5. 5. n2 = 1408 rpm
  6. 6. Rst = 3.6 Ω

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