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​M+−E = 45 Vn = 1800 rpmT

A DC motor turns at 1800 rpm, and its back-EMF at that speed is 45 V. Find the motor constant k in V·s/rad.

E = 45 Vn = 1800 rpm
k = V·s/rad
2.Easy
+−24 VRa​ = 0.5 ΩIa​ = ?M+−E = 21.5 VωT

A DC motor on a 24 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 21.5 V. Find the armature current Ia.

V = 24 VE = 21.5 VRa = 0.5 Ω
Ia = A
3.Medium
+−VRa​M+−En = 750 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 2.2 kWn = 750 rpm
T = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)3 N·m500 rpm

A DC motor with k = 0.5 V·s/rad and Ra = 0.4 Ω must drive a load of 3 N·m at 500 rpm. Find the armature voltage V it needs.

k = 0.5 V·s/radRa = 0.4 ΩT = 3 N·mn = 500 rpm
V = V
6.Hard
+−220 VRa​ = 1.5 ΩIa​: 20 → 5 AM+−En = ?T

A separately excited DC motor on 220 V (Ra = 1.5 Ω) runs at 1200 rpm with an armature current of 20 A. The load changes and the current becomes 5 A, with the field unchanged. Find the new speed n2.

V = 220 VRa = 1.5 Ωn1 = 1200 rpmIa1 = 20 AIa2 = 5 A
n2 = rpm

Answer key

  1. 1. k = 0.2387 V·s/rad
  2. 2. Ia = 5 A
  3. 3. T = 28.01 N·m
  4. 4. n0 = 2292 rpm
  5. 5. V = 28.58 V
  6. 6. n2 = 1342 rpm

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