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
+−220 VRa​M+−E = 190 Vn = 1200 rpmT

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 190 V. Find the motor constant k in V·s/rad.

E = 190 Vn = 1200 rpm
k = V·s/rad
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = 20 AM+−EωT

A DC motor runs from 110 V with an armature current of 20 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 20 ARa = 0.4 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.2 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.2 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)0.48 N·mn0​n = ?

A DC motor (k = 0.12 N·m/A, Ra = 0.8 Ω) runs from 24 V and drives a load torque of 0.48 N·m. Find its speed n in rpm.

V = 24 Vk = 0.12 N·m/ARa = 0.8 ΩT = 0.48 N·m
n = rpm
5.Hard
T (N·m)n (rpm)1.2 N·m400 rpm

A DC motor with k = 0.3 V·s/rad and Ra = 0.8 Ω must drive a load of 1.2 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radRa = 0.8 ΩT = 1.2 N·mn = 400 rpm
V = V
6.Hard
+−220 VRs​ = ?Ra​ = 1 ΩM+−En = 300 rpmT = 50 N·m

A DC motor (k = 2 V·s/rad, Ra = 1 Ω) on a fixed 220 V supply drives a load of 50 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 220 Vk = 2 V·s/radRa = 1 ΩT = 50 N·mn = 300 rpm
Rs = Ω

Answer key

  1. 1. k = 1.512 V·s/rad
  2. 2. Pdev = 2040 W
  3. 3. n0 = 1146 rpm
  4. 4. n = 1655 rpm
  5. 5. V = 15.77 V
  6. 6. Rs = 5.287 Ω

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