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
+−48 VRa​ = 0.25 ΩIa​ = ?M+−E = 44.25 VωT

A DC motor on a 48 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 44.25 V. Find the armature current Ia.

V = 48 VE = 44.25 VRa = 0.25 Ω
Ia = A
2.Easy
+−110 VRa​ = 1.5 ΩIa​ = ?M+−E = 102.5 VωT

A DC motor on a 110 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 102.5 V. Find the armature current Ia.

V = 110 VE = 102.5 VRa = 1.5 Ω
Ia = A
3.Medium
T (N·m)n (rpm)1.6 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 1.2 Ω) runs from 48 V and drives a load torque of 1.6 N·m. Find its speed n in rpm.

V = 48 Vk = 0.4 N·m/ARa = 1.2 ΩT = 1.6 N·m
n = rpm
4.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 5.5 kWn = 1000 rpm
T = N·m
5.Hard
+−120 VRst​ = ?Ra​ = 0.4 Ω20 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 120 VRa = 0.4 ΩImax = 20 A
Rst = Ω
6.Hard
+−240 VRa​ = 1 ΩIa​: 12 → 18 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 1 Ω) runs at 1500 rpm with an armature current of 12 A. The load changes and the current becomes 18 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 1 Ωn1 = 1500 rpmIa1 = 12 AIa2 = 18 A
n2 = rpm

Answer key

  1. 1. Ia = 15 A
  2. 2. Ia = 5 A
  3. 3. n = 1031 rpm
  4. 4. T = 52.52 N·m
  5. 5. Rst = 5.6 Ω
  6. 6. n2 = 1461 rpm

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