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​Ia​ = 4 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.1 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.1 N·m/AIa = 4 A
T = N·m
2.Easy
+−48 VRa​ = 0.5 ΩIa​ = ?M+−E = 45.5 VωT

A DC motor on a 48 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 45.5 V. Find the armature current Ia.

V = 48 VE = 45.5 VRa = 0.5 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.25 N·m/A and Ra = 1.5 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.25 N·m/ARa = 1.5 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.5 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.5 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)6 N·m1200 rpm

A DC motor with k = 1 V·s/rad and Ra = 2 Ω must drive a load of 6 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 1 V·s/radRa = 2 ΩT = 6 N·mn = 1200 rpm
V = V
6.Hard
+−220 VRst​ = ?Ra​ = 0.2 Ω100 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 220 VRa = 0.2 ΩImax = 100 A
Rst = Ω

Answer key

  1. 1. T = 0.4 N·m
  2. 2. Ia = 5 A
  3. 3. Tstall = 8 N·m
  4. 4. n0 = 2101 rpm
  5. 5. V = 137.7 V
  6. 6. Rst = 2 Ω

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