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
+−48 VRa​ = 1 ΩIa​ = ?M+−E = 46 VωT

A DC motor on a 48 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 46 V. Find the armature current Ia.

V = 48 VE = 46 VRa = 1 Ω
Ia = A
2.Easy
+−48 VRa​Ia​ = 25 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.6 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 0.6 N·m/AIa = 25 A
T = N·m
3.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
4.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 4 kWn = 1000 rpm
T = N·m
5.Hard
T (N·m)n (rpm)3.6 N·m300 rpm

A DC motor with k = 0.3 V·s/rad and Ra = 0.2 Ω must drive a load of 3.6 N·m at 300 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radRa = 0.2 ΩT = 3.6 N·mn = 300 rpm
V = V
6.Hard
+−240 VRs​ = ?Ra​ = 1.5 ΩM+−En = 300 rpmT = 6 N·m

A DC motor (k = 1.2 V·s/rad, Ra = 1.5 Ω) on a fixed 240 V supply drives a load of 6 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 240 Vk = 1.2 V·s/radRa = 1.5 ΩT = 6 N·mn = 300 rpm
Rs = Ω

Answer key

  1. 1. Ia = 2 A
  2. 2. T = 15 N·m
  3. 3. n0 = 1146 rpm
  4. 4. T = 38.2 N·m
  5. 5. V = 11.82 V
  6. 6. Rs = 38.96 Ω

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