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​ = 15 AM+−En = ?T

A DC motor with k = 0.4 V·s/rad and an armature resistance of 0.2 Ω runs from 48 V and draws 15 A. Find its speed n in rpm.

V = 48 VRa = 0.2 ΩIa = 15 Ak = 0.4 V·s/rad
n = rpm
2.Easy
+−240 VRa​ = 1 ΩIa​ = ?M+−E = 215 VωT

A DC motor on a 240 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 215 V. Find the armature current Ia.

V = 240 VE = 215 VRa = 1 Ω
Ia = A
3.Medium
T (N·m)n (rpm)6 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 0.8 Ω) runs from 110 V and drives a load torque of 6 N·m. Find its speed n in rpm.

V = 110 Vk = 0.75 N·m/ARa = 0.8 ΩT = 6 N·m
n = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.4 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)9 N·m600 rpm

A DC motor with k = 0.6 V·s/rad and Ra = 0.25 Ω must drive a load of 9 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radRa = 0.25 ΩT = 9 N·mn = 600 rpm
V = V
6.Hard
+−240 VRa​ = 0.2 ΩIa​: 40 → 80 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.2 Ω) runs at 1800 rpm with an armature current of 40 A. The load changes and the current becomes 80 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.2 Ωn1 = 1800 rpmIa1 = 40 AIa2 = 80 A
n2 = rpm

Answer key

  1. 1. n = 1074 rpm
  2. 2. Ia = 25 A
  3. 3. n = 1319 rpm
  4. 4. n0 = 1146 rpm
  5. 5. V = 41.45 V
  6. 6. n2 = 1738 rpm

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