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​ = 1 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 1.2 V·s/rad and an armature resistance of 1 Ω runs from 240 V and draws 20 A. Find its speed n in rpm.

V = 240 VRa = 1 ΩIa = 20 Ak = 1.2 V·s/rad
n = rpm
2.Easy
+−220 VRa​M+−E = 207.5 Vn = 2400 rpmT

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 207.5 V. Find the motor constant k in V·s/rad.

E = 207.5 Vn = 2400 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.4 N·m/A and Ra = 0.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.4 N·m/ARa = 0.2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.25 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.25 V·s/rad
n0 = rpm
5.Hard
+−240 VD = ?Ra​ = 1 ΩM+−En = 900 rpmT = 15 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 1 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 900 rpm with a load of 15 N·m.

Vdc = 240 Vk = 0.6 V·s/radRa = 1 ΩT = 15 N·mn = 900 rpm
D = %
6.Hard
+−220 VRst​ = ?Ra​ = 0.8 Ω30 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 220 VRa = 0.8 ΩImax = 30 A
Rst = Ω

Answer key

  1. 1. n = 1751 rpm
  2. 2. k = 0.8256 V·s/rad
  3. 3. Tstall = 96 N·m
  4. 4. n0 = 916.7 rpm
  5. 5. D = 33.98 %
  6. 6. Rst = 6.533 Ω

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