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
+−24 VRa​ = 1.2 ΩIa​ = ?M+−E = 21.6 VωT

A DC motor on a 24 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 21.6 V. Find the armature current Ia.

V = 24 VE = 21.6 VRa = 1.2 Ω
Ia = A
2.Easy
+−240 VRa​Ia​ = 6 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.25 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 0.25 N·m/AIa = 6 A
T = N·m
3.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 0.75 kWn = 2900 rpm
T = N·m
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 0.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 0.5 kWn = 1450 rpm
T = N·m
5.Hard
+−48 VRs​ = ?Ra​ = 1.2 ΩM+−En = 400 rpmT = 2.4 N·m

A DC motor (k = 0.6 V·s/rad, Ra = 1.2 Ω) on a fixed 48 V supply drives a load of 2.4 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 48 Vk = 0.6 V·s/radRa = 1.2 ΩT = 2.4 N·mn = 400 rpm
Rs = Ω
6.Hard
+−48 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 2 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 2 Ω
Pmax = W

Answer key

  1. 1. Ia = 2 A
  2. 2. T = 1.5 N·m
  3. 3. T = 2.47 N·m
  4. 4. T = 3.293 N·m
  5. 5. Rs = 4.517 Ω
  6. 6. Pmax = 288 W

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