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​ = 1.2 ΩIa​ = 12 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 1.2 Ω runs from 220 V and draws 12 A. Find its speed n in rpm.

V = 220 VRa = 1.2 ΩIa = 12 Ak = 0.6 V·s/rad
n = rpm
2.Easy
+−240 VRa​M+−E = 228 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 228 V. Find the motor constant k in V·s/rad.

E = 228 Vn = 600 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 0.25 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.8 N·m/ARa = 0.25 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1 V·s/rad
n0 = rpm
5.Hard
+−24 VRa​ = 1.2 ΩM+−EωT

A small DC motor has an armature resistance of 1.2 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.2 Ω
Pmax = W
6.Hard
+−48 VRs​ = ?Ra​ = 1 ΩM+−En = 400 rpmT = 0.75 N·m

A DC motor (k = 0.15 V·s/rad, Ra = 1 Ω) on a fixed 48 V supply drives a load of 0.75 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 48 Vk = 0.15 V·s/radRa = 1 ΩT = 0.75 N·mn = 400 rpm
Rs = Ω

Answer key

  1. 1. n = 3272 rpm
  2. 2. k = 3.629 V·s/rad
  3. 3. Tstall = 352 N·m
  4. 4. n0 = 2292 rpm
  5. 5. Pmax = 120 W
  6. 6. Rs = 7.343 Ω

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