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
+−220 VRa​Ia​ = 15 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 15 A.

k = 0.08 N·m/AIa = 15 A
T = N·m
2.Easy
+−240 VRa​ = 0.4 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.4 Ω runs from 240 V and draws 25 A. Find its speed n in rpm.

V = 240 VRa = 0.4 ΩIa = 25 Ak = 0.6 V·s/rad
n = rpm
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
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 15 kWn = 960 rpm
T = N·m
5.Hard
+−110 VRa​ = 0.8 ΩIa​: 15 → 22.5 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.8 Ω) runs at 1440 rpm with an armature current of 15 A. The load changes and the current becomes 22.5 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.8 Ωn1 = 1440 rpmIa1 = 15 AIa2 = 22.5 A
n2 = rpm
6.Hard
+−220 VRst​ = ?Ra​ = 0.2 Ω25 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 220 VRa = 0.2 ΩImax = 25 A
Rst = Ω

Answer key

  1. 1. T = 1.2 N·m
  2. 2. n = 3661 rpm
  3. 3. T = 49.39 N·m
  4. 4. T = 149.2 N·m
  5. 5. n2 = 1352 rpm
  6. 6. Rst = 8.6 Ω

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