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​Ia​ = 10 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 2 N·m/A. Find the torque T it develops at an armature current of 10 A.

k = 2 N·m/AIa = 10 A
T = N·m
2.Easy
+−220 VRa​ = 0.25 ΩIa​ = 50 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 50 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 220 VIa = 50 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.6 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 0.6 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 7.5 kWn = 960 rpm
T = N·m
5.Hard
+−240 VRs​ = ?Ra​ = 1.5 ΩM+−En = 1500 rpmT = 4.8 N·m

A DC motor (k = 0.6 V·s/rad, Ra = 1.5 Ω) on a fixed 240 V supply drives a load of 4.8 N·m. Find the series resistance Rs that slows it down to 1500 rpm.

V = 240 Vk = 0.6 V·s/radRa = 1.5 ΩT = 4.8 N·mn = 1500 rpm
Rs = Ω
6.Hard
+−110 VRs​ = ?Ra​ = 1.2 ΩM+−En = 400 rpmT = 1.5 N·m

A DC motor (k = 0.3 V·s/rad, Ra = 1.2 Ω) on a fixed 110 V supply drives a load of 1.5 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 110 Vk = 0.3 V·s/radRa = 1.2 ΩT = 1.5 N·mn = 400 rpm
Rs = Ω

Answer key

  1. 1. T = 20 N·m
  2. 2. E = 207.5 V
  3. 3. n0 = 3820 rpm
  4. 4. T = 74.6 N·m
  5. 5. Rs = 16.72 Ω
  6. 6. Rs = 18.29 Ω

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