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
+−120 VRa​ = 1 ΩIa​ = 4 AM+−EωT

A DC motor runs from 120 V with an armature current of 4 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 4 ARa = 1 Ω
Pdev = W
2.Easy
+−240 VRa​ = 1.2 ΩIa​ = ?M+−E = 216 VωT

A DC motor on a 240 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 216 V. Find the armature current Ia.

V = 240 VE = 216 VRa = 1.2 Ω
Ia = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 0.8 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 15 kWn = 3000 rpm
T = N·m
5.Hard
+−220 VD = ?Ra​ = 2 ΩM+−En = 600 rpmT = 8 N·m

A DC chopper feeds a motor (k = 0.8 V·s/rad, Ra = 2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 600 rpm with a load of 8 N·m.

Vdc = 220 Vk = 0.8 V·s/radRa = 2 ΩT = 8 N·mn = 600 rpm
D = %
6.Hard
+−220 VD = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 1.6 N·m

A DC chopper feeds a motor (k = 0.4 V·s/rad, Ra = 1.2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 1.6 N·m.

Vdc = 220 Vk = 0.4 V·s/radRa = 1.2 ΩT = 1.6 N·mn = 750 rpm
D = %

Answer key

  1. 1. Pdev = 464 W
  2. 2. Ia = 20 A
  3. 3. n0 = 2626 rpm
  4. 4. T = 47.75 N·m
  5. 5. D = 31.94 %
  6. 6. D = 16.46 %

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