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​ = ?M+−E = 116 VωT

A DC motor on a 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 116 V. Find the armature current Ia.

V = 120 VE = 116 VRa = 0.2 Ω
Ia = A
2.Easy
+−110 VRa​Ia​ = 25 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 25 A.

k = 0.08 N·m/AIa = 25 A
T = N·m
3.Medium
T (N·m)n (rpm)12.5 N·mn0​n = ?

A DC motor (k = 0.5 N·m/A, Ra = 0.25 Ω) runs from 110 V and drives a load torque of 12.5 N·m. Find its speed n in rpm.

V = 110 Vk = 0.5 N·m/ARa = 0.25 ΩT = 12.5 N·m
n = rpm
4.Medium
T (N·m)n (rpm)15 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 1.2 Ω) runs from 240 V and drives a load torque of 15 N·m. Find its speed n in rpm.

V = 240 Vk = 0.75 N·m/ARa = 1.2 ΩT = 15 N·m
n = rpm
5.Hard
+−120 VRst​ = ?Ra​ = 0.2 Ω20 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 120 VRa = 0.2 ΩImax = 20 A
Rst = Ω
6.Hard
+−120 VD = ?Ra​ = 1.2 ΩM+−En = 1200 rpmT = 0.24 N·m

A DC chopper feeds a motor (k = 0.12 V·s/rad, Ra = 1.2 Ω) from a 120 V DC bus. The average armature voltage is D·120 V. Find the duty cycle D that runs the motor at 1200 rpm with a load of 0.24 N·m.

Vdc = 120 Vk = 0.12 V·s/radRa = 1.2 ΩT = 0.24 N·mn = 1200 rpm
D = %

Answer key

  1. 1. Ia = 20 A
  2. 2. T = 2 N·m
  3. 3. n = 1981 rpm
  4. 4. n = 2750 rpm
  5. 5. Rst = 5.8 Ω
  6. 6. D = 14.57 %

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