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.

New worksheetPractise online with hints

For teachers: free to print, copy and use in class. Every new worksheet has new values and its own answer key. A link to ExamProblems on your course page is appreciated.

Name: ____________________   Date: ____________

1.Easy
+−120 VRa​M+−E = 110.4 Vn = 750 rpmT

A DC motor turns at 750 rpm, and its back-EMF at that speed is 110.4 V. Find the motor constant k in V·s/rad.

E = 110.4 Vn = 750 rpm
k = V·s/rad
2.Easy
+−24 VRa​ = 0.4 ΩIa​ = 2 AM+−En = ?T

A DC motor with k = 0.15 V·s/rad and an armature resistance of 0.4 Ω runs from 24 V and draws 2 A. Find its speed n in rpm.

V = 24 VRa = 0.4 ΩIa = 2 Ak = 0.15 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)1 N·mn0​n = ?

A DC motor (k = 0.25 N·m/A, Ra = 0.4 Ω) runs from 48 V and drives a load torque of 1 N·m. Find its speed n in rpm.

V = 48 Vk = 0.25 N·m/ARa = 0.4 ΩT = 1 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 2 N·m/A and Ra = 0.5 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 2 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−24 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 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 = 2 Ω
Pmax = W
6.Hard
+−240 VRs​ = ?Ra​ = 1 ΩM+−En = 750 rpmT = 12 N·m

A DC motor (k = 0.8 V·s/rad, Ra = 1 Ω) on a fixed 240 V supply drives a load of 12 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 240 Vk = 0.8 V·s/radRa = 1 ΩT = 12 N·mn = 750 rpm
Rs = Ω

Answer key

  1. 1. k = 1.406 V·s/rad
  2. 2. n = 1477 rpm
  3. 3. n = 1772 rpm
  4. 4. Tstall = 960 N·m
  5. 5. Pmax = 72 W
  6. 6. Rs = 10.81 Ω

Full step-by-step solutions: practise this topic on examproblems.com.