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 = 46 Vn = 1200 rpmT

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 46 V. Find the motor constant k in V·s/rad.

E = 46 Vn = 1200 rpm
k = V·s/rad
2.Easy
+−120 VRa​ = 0.25 ΩIa​ = 20 AM+−EωT

A DC motor runs from 120 V with an armature current of 20 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 20 ARa = 0.25 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.1 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.1 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.5 N·m/A and Ra = 1 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 1.5 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−110 VRs​ = ?Ra​ = 0.25 ΩM+−En = 600 rpmT = 25 N·m

A DC motor (k = 1 V·s/rad, Ra = 0.25 Ω) on a fixed 110 V supply drives a load of 25 N·m. Find the series resistance Rs that slows it down to 600 rpm.

V = 110 Vk = 1 V·s/radRa = 0.25 ΩT = 25 N·mn = 600 rpm
Rs = Ω
6.Hard
T (N·m)n (rpm)1.8 N·m1200 rpm

A DC motor with k = 0.3 V·s/rad and Ra = 0.25 Ω must drive a load of 1.8 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radRa = 0.25 ΩT = 1.8 N·mn = 1200 rpm
V = V

Answer key

  1. 1. k = 0.3661 V·s/rad
  2. 2. Pdev = 2300 W
  3. 3. n0 = 2292 rpm
  4. 4. Tstall = 330 N·m
  5. 5. Rs = 1.637 Ω
  6. 6. V = 39.2 V

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