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​ = 0.5 ΩIa​ = 2 AM+−EωT

A DC motor runs from 24 V with an armature current of 2 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 2 ARa = 0.5 Ω
Pdev = W
2.Easy
+−220 VRa​ = 1.2 ΩIa​ = 20 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 20 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 220 VIa = 20 ARa = 1.2 Ω
E = V
3.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 15 kWn = 2900 rpm
T = N·m
4.Medium
T (N·m)n (rpm)0.6 N·mn0​n = ?

A DC motor (k = 0.12 N·m/A, Ra = 0.4 Ω) runs from 24 V and drives a load torque of 0.6 N·m. Find its speed n in rpm.

V = 24 Vk = 0.12 N·m/ARa = 0.4 ΩT = 0.6 N·m
n = rpm
5.Hard
+−110 VRst​ = ?Ra​ = 0.2 Ω30 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 110 VRa = 0.2 ΩImax = 30 A
Rst = Ω
6.Hard
+−48 VD = ?Ra​ = 0.8 ΩM+−En = 750 rpmT = 0.2 N·m

A DC chopper feeds a motor (k = 0.1 V·s/rad, Ra = 0.8 Ω) from a 48 V DC bus. The average armature voltage is D·48 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 0.2 N·m.

Vdc = 48 Vk = 0.1 V·s/radRa = 0.8 ΩT = 0.2 N·mn = 750 rpm
D = %

Answer key

  1. 1. Pdev = 46 W
  2. 2. E = 196 V
  3. 3. T = 49.39 N·m
  4. 4. n = 1751 rpm
  5. 5. Rst = 3.467 Ω
  6. 6. D = 19.7 %

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