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​ = 0.2 ΩIa​ = 40 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 40 A. Its armature resistance is 0.2 Ω. Find the back-EMF E.

V = 120 VIa = 40 ARa = 0.2 Ω
E = V
2.Easy
+−220 VRa​ = 0.5 ΩIa​ = 15 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 15 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 220 VIa = 15 ARa = 0.5 Ω
E = V
3.Medium
T (N·m)n (rpm)1.2 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 0.5 Ω) runs from 48 V and drives a load torque of 1.2 N·m. Find its speed n in rpm.

V = 48 Vk = 0.3 N·m/ARa = 0.5 ΩT = 1.2 N·m
n = rpm
4.Medium
T (N·m)n (rpm)3.75 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 2 Ω) runs from 220 V and drives a load torque of 3.75 N·m. Find its speed n in rpm.

V = 220 Vk = 0.75 N·m/ARa = 2 ΩT = 3.75 N·m
n = rpm
5.Hard
+−220 VD = ?Ra​ = 0.4 ΩM+−En = 300 rpmT = 25 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 0.4 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 300 rpm with a load of 25 N·m.

Vdc = 220 Vk = 1 V·s/radRa = 0.4 ΩT = 25 N·mn = 300 rpm
D = %
6.Hard
+−24 VRst​ = ?Ra​ = 1 Ω20 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 24 VRa = 1 ΩImax = 20 A
Rst = Ω

Answer key

  1. 1. E = 112 V
  2. 2. E = 212.5 V
  3. 3. n = 1464 rpm
  4. 4. n = 2674 rpm
  5. 5. D = 18.83 %
  6. 6. Rst = 0.2 Ω

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