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​Ia​ = 2 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.08 N·m/A. Find the torque T it develops at an armature current of 2 A.

k = 0.08 N·m/AIa = 2 A
T = N·m
2.Easy
+−48 VRa​ = 0.8 ΩIa​ = ?M+−E = 41.6 VωT

A DC motor on a 48 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 41.6 V. Find the armature current Ia.

V = 48 VE = 41.6 VRa = 0.8 Ω
Ia = A
3.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 11 kWn = 2000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 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 V·s/rad
n0 = rpm
5.Hard
+−220 VRs​ = ?Ra​ = 1 ΩM+−En = 800 rpmT = 20 N·m

A DC motor (k = 2 V·s/rad, Ra = 1 Ω) on a fixed 220 V supply drives a load of 20 N·m. Find the series resistance Rs that slows it down to 800 rpm.

V = 220 Vk = 2 V·s/radRa = 1 ΩT = 20 N·mn = 800 rpm
Rs = Ω
6.Hard
+−24 VRa​ = 0.5 ΩM+−EωT

A small DC motor has an armature resistance of 0.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 0.5 Ω
Pmax = W

Answer key

  1. 1. T = 0.16 N·m
  2. 2. Ia = 8 A
  3. 3. T = 52.52 N·m
  4. 4. n0 = 2101 rpm
  5. 5. Rs = 4.245 Ω
  6. 6. Pmax = 288 W

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