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
+−24 VRa​ = 0.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.2 Ω is switched directly onto a 24 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 24 VRa = 0.2 Ω
Istart = A
2.Easy
+−240 VRa​M+−E = 232.5 Vn = 1000 rpmT

A DC motor turns at 1000 rpm, and its back-EMF at that speed is 232.5 V. Find the motor constant k in V·s/rad.

E = 232.5 Vn = 1000 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.8 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)12 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 0.2 Ω) runs from 110 V and drives a load torque of 12 N·m. Find its speed n in rpm.

V = 110 Vk = 0.3 N·m/ARa = 0.2 ΩT = 12 N·m
n = rpm
5.Hard
T (N·m)n (rpm)48 N·m300 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 0.2 Ω must drive a load of 48 N·m at 300 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 0.2 ΩT = 48 N·mn = 300 rpm
V = V
6.Hard
+−24 VRa​ = 0.25 ΩIa​: 4 → 1.6 AM+−En = ?T

A separately excited DC motor on 24 V (Ra = 0.25 Ω) runs at 900 rpm with an armature current of 4 A. The load changes and the current becomes 1.6 A, with the field unchanged. Find the new speed n2.

V = 24 VRa = 0.25 Ωn1 = 900 rpmIa1 = 4 AIa2 = 1.6 A
n2 = rpm

Answer key

  1. 1. Istart = 120 A
  2. 2. k = 2.22 V·s/rad
  3. 3. n0 = 1432 rpm
  4. 4. n = 3247 rpm
  5. 5. V = 45.7 V
  6. 6. n2 = 923.5 rpm

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