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
+−24 VRa​ = 1.2 ΩIa​ = 2 AM+−EωT

A DC motor runs from 24 V with an armature current of 2 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 2 ARa = 1.2 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.4 ΩIa​ = 12 AM+−En = ?T

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.4 Ω runs from 120 V and draws 12 A. Find its speed n in rpm.

V = 120 VRa = 0.4 ΩIa = 12 Ak = 0.8 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)12 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 0.2 Ω) runs from 48 V and drives a load torque of 12 N·m. Find its speed n in rpm.

V = 48 Vk = 0.4 N·m/ARa = 0.2 ΩT = 12 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 0.5 Ω is connected to 120 V. Find its stall torque Tstall, the torque at zero speed.

V = 120 Vk = 0.75 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−220 VRs​ = ?Ra​ = 0.4 ΩM+−En = 900 rpmT = 30 N·m

A DC motor (k = 1.2 V·s/rad, Ra = 0.4 Ω) on a fixed 220 V supply drives a load of 30 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 220 Vk = 1.2 V·s/radRa = 0.4 ΩT = 30 N·mn = 900 rpm
Rs = Ω
6.Hard
+−120 VRst​ = ?Ra​ = 1.2 Ω40 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance Rst that limits the starting current to 40 A.

V = 120 VRa = 1.2 ΩImax = 40 A
Rst = Ω

Answer key

  1. 1. Pdev = 43.2 W
  2. 2. n = 1375 rpm
  3. 3. n = 1003 rpm
  4. 4. Tstall = 180 N·m
  5. 5. Rs = 3.876 Ω
  6. 6. Rst = 1.8 Ω

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