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
+−240 VRa​ = 0.8 ΩIa​ = 40 AM+−En = ?T

A DC motor with k = 0.75 V·s/rad and an armature resistance of 0.8 Ω runs from 240 V and draws 40 A. Find its speed n in rpm.

V = 240 VRa = 0.8 ΩIa = 40 Ak = 0.75 V·s/rad
n = rpm
2.Easy
+−120 VRa​ = 0.5 ΩIa​ = 25 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 25 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 120 VIa = 25 ARa = 0.5 Ω
E = V
3.Medium
T (N·m)n (rpm)6 N·mn0​n = ?

A DC motor (k = 0.6 N·m/A, Ra = 1 Ω) runs from 110 V and drives a load torque of 6 N·m. Find its speed n in rpm.

V = 110 Vk = 0.6 N·m/ARa = 1 ΩT = 6 N·m
n = rpm
4.Medium
T (N·m)n (rpm)12 N·mn0​n = ?

A DC motor (k = 0.8 N·m/A, Ra = 1 Ω) runs from 220 V and drives a load torque of 12 N·m. Find its speed n in rpm.

V = 220 Vk = 0.8 N·m/ARa = 1 ΩT = 12 N·m
n = rpm
5.Hard
+−120 VRs​ = ?Ra​ = 0.4 ΩM+−En = 600 rpmT = 7.5 N·m

A DC motor (k = 0.75 V·s/rad, Ra = 0.4 Ω) on a fixed 120 V supply drives a load of 7.5 N·m. Find the series resistance Rs that slows it down to 600 rpm.

V = 120 Vk = 0.75 V·s/radRa = 0.4 ΩT = 7.5 N·mn = 600 rpm
Rs = Ω
6.Hard
T (N·m)n (rpm)8 N·m600 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 0.8 Ω must drive a load of 8 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 0.8 ΩT = 8 N·mn = 600 rpm
V = V

Answer key

  1. 1. n = 2648 rpm
  2. 2. E = 107.5 V
  3. 3. n = 1592 rpm
  4. 4. n = 2447 rpm
  5. 5. Rs = 6.888 Ω
  6. 6. V = 58.27 V

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