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​ = 1.5 ΩIa​ = ?M+−E = 190 VωT

A DC motor on a 220 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 190 V. Find the armature current Ia.

V = 220 VE = 190 VRa = 1.5 Ω
Ia = A
2.Easy
+−220 VRa​ = 0.25 ΩIa​ = ?M+−E = 207.5 VωT

A DC motor on a 220 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 207.5 V. Find the armature current Ia.

V = 220 VE = 207.5 VRa = 0.25 Ω
Ia = A
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 7.5 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)3.75 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 1.5 Ω) runs from 110 V and drives a load torque of 3.75 N·m. Find its speed n in rpm.

V = 110 Vk = 0.75 N·m/ARa = 1.5 ΩT = 3.75 N·m
n = rpm
5.Hard
+−240 VRa​ = 0.2 ΩIa​: 50 → 20 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.2 Ω) runs at 1200 rpm with an armature current of 50 A. The load changes and the current becomes 20 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.2 Ωn1 = 1200 rpmIa1 = 50 AIa2 = 20 A
n2 = rpm
6.Hard
+−120 VRa​ = 2 ΩIa​: 4 → 2.4 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 2 Ω) runs at 1440 rpm with an armature current of 4 A. The load changes and the current becomes 2.4 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 2 Ωn1 = 1440 rpmIa1 = 4 AIa2 = 2.4 A
n2 = rpm

Answer key

  1. 1. Ia = 20 A
  2. 2. Ia = 50 A
  3. 3. T = 23.87 N·m
  4. 4. n = 1305 rpm
  5. 5. n2 = 1231 rpm
  6. 6. n2 = 1481 rpm

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