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​ = 0.2 ΩIa​ = ?M+−E = 44 VωT

A DC motor on a 48 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 44 V. Find the armature current Ia.

V = 48 VE = 44 VRa = 0.2 Ω
Ia = A
2.Easy
+−220 VRa​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 220 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 0.25 Ω
Istart = A
3.Medium
T (N·m)n (rpm)2.4 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 0.25 Ω) runs from 48 V and drives a load torque of 2.4 N·m. Find its speed n in rpm.

V = 48 Vk = 0.4 N·m/ARa = 0.25 ΩT = 2.4 N·m
n = rpm
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 0.75 kWn = 960 rpm
T = N·m
5.Hard
+−110 VRa​ = 0.4 ΩIa​: 20 → 40 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.4 Ω) runs at 1200 rpm with an armature current of 20 A. The load changes and the current becomes 40 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.4 Ωn1 = 1200 rpmIa1 = 20 AIa2 = 40 A
n2 = rpm
6.Hard
+−120 VRa​ = 0.2 ΩIa​: 50 → 20 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.2 Ω) runs at 1500 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 = 120 VRa = 0.2 Ωn1 = 1500 rpmIa1 = 50 AIa2 = 20 A
n2 = rpm

Answer key

  1. 1. Ia = 20 A
  2. 2. Istart = 880 A
  3. 3. n = 1110 rpm
  4. 4. T = 7.46 N·m
  5. 5. n2 = 1106 rpm
  6. 6. n2 = 1582 rpm

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