I want an electric motor back emf eddy current capable of providing a resisting force on the shaft whereby the resisting force is equivalent to moving a 100-kg mass acceleration from stationary to 1 metre in 1 second, moving against the electric motors back emf eddy current.

The application also requires controlling the back eddy current resistance to provide a range of resistance forces, for which I propose using a controller drive of some type. Therefore, I hope the application will provide from a nominal 10-kg resistance force using back emf eddy currents from stationary to 1 metre in second up to the required 100-kg mass resistance equivalent.

The application also requires the same electric motor provide forward assistive motive force, from a nominal 10-kg mass to a 100-kg mass from stationary to 1 metre in 1 second.

A desired function would be combining the eddy current reverse direction resisting movement followed by a forward direction assisting movement, with the same motor in a series of repetitious movements.

I don’t know anything about electric motors. What would I need for the application I envision?

Edit: **The only knowledge I have about eddy currents comes from watching a YouTube video demonstration of a rare earth magnet falling through a hollow non magnetised metal tube. In researching electric motors I discovered back EMF creates eddy currents, hence my combination of the two phrases. Can an electric motor do the same 'work' as a 100 kg magnet falling through a hollow non magnetic tube falling at the rate of 1 metre per second and displacing 1 metre only? That 1 metre distance would be the maximum distance required in the application. 1 Newton force accelerates 1 kg 1 metre in 1 second, I require 100 Newton's force to accelerate 100 kg 1 metre in 1 second. This approximates to a 1-2 HP motor moving the weight in a forward direction but what I want to know is if I can do this using back EMF / eddy currents simulating a 100 kg rare earth magnet falling through a non magnetic metal tube at the rate of 1 metre only in 1 second **

Second edit: The essence of my question is "Is there a formula whereby I can calculate the horespower rating of an electric motor in relation to an application in which I want to employ back EMF / eddy currents as a resistance, that is I want a mechanical force to reverse rotation of the electric motor shaft equivalent to the scenario I have described in the body of my question. Even if this is not a common scenario, this is the scenario I am trying to solve.

Maybe the first answer I'm seeking would be "yes this is possible" and then "here is the formula for working out the horsepower rating" and maybe even "yes such and such drive controller and AC induction motor are best equipment for said scenario".

third edit: I'm in the process of writing a patent for an exercise application. It is not a new idea to use electric motors as resistance means for exercise purposes, here's a reference to US5020794 Motor control for an exercise machine simulating a weight stack.

However in order that I not nullify my patent application by making public any description that reveals said patent, I cannot give too much information. Yes I could take a college course to study electrical engineering in the relevant area in which I want to apply this application, however time is something I don't have if I want urgency in making my patent application in a timely manner. I'm a one person team at this moment. I don't have cash on hand to contract an electrical engineer. If I can get my patent submitted and make a case for commercial feasibility funding then I will definitely require the services of an electrical engineer. It's not a requirement that I as the patentee explain in detail how to construct the mechanism in mind, the requirement of a patent is that a "person having ordinary skill in the art" being capable of constructing the mechanism, i.e. an electrical engineer (though the electrical component is not the patentable portion as patents exist which utilise electrical motors in fitness applications).

And so back to my requirements, I don't want to slow down a moving mass to zero velocity. I want to use the oppositional forces of eddy currents in this exercise application so that in practise an exercising person will apply a physical torque force to the shaft of an electric motor opposing the direction of the rotor, equivalent to accelerating a stationary mass of an equivalent 10-kg to 100-kg mass, up to a maximum of 1 metre only within a time frame of 1 second only, this being an approximation of a physical body moving a 10-kg to 100-kg plate weight stack 1 metre in 1 second. This approximation is what is generally performed in a gym by an exercising person. Yes there will be some variation but i'm rounding out the approximations as close enough.

I do not know if such an application is possible. I've found applications that use eddy currents to brake, in roller coasters, train braking and when power tools are powered down but none that use eddy currents in the manner I described. I only know from the youtube demonstrations of a magnet falling through a conducting material and being opposed by eddy currents that such an application should be possible.

There's more I want to achieve with this application but for the sake of the patent I cannot disclose too much. I apologise for the initial lack of disclosure.

  • \$\begingroup\$ You have some real EE terminology in your "electric motor back emf eddy current" but strung together like that doesn't make much sense. Draw some graphs of the torque versus time that you require and calculate the energy and power required to do that. Add the details into your question. \$\endgroup\$
    – Transistor
    Sep 1, 2017 at 15:47
  • \$\begingroup\$ (1) "1 Newton force moves 1 kg 1 metre in 1 second ..." Nope. \$ F = ma \$ so 1 N accelerates 1 kg at 1 m/s. After 1 s it will have reached 1 m/s, etc. (2) Most motors can act as generators. If you turn it a voltage is generated. If you power a motor from standstill it will draw a large current. As it speeds up the generator effect creates a "back EMF" to oppose the supply voltage and this reduces the run current. (3) Why don't you describe what the machine is supposed to do rather than trying to describe a solution with mixed up jargon. \$\endgroup\$
    – Transistor
    Sep 1, 2017 at 23:08
  • \$\begingroup\$ Sorry, my comment above should read \$ F=ma \$ so 1 N accelerates 1 kg at 1 m/s/s. \$\endgroup\$
    – Transistor
    Sep 1, 2017 at 23:38
  • \$\begingroup\$ "I require 100 Newton's force to move 100 kg 1 metre in 1 second." From above we can see that \$ a = \frac {F}{m} = \frac {100}{100} = 1 \; m/s^2 \$. At the end of 1 s it will be travelling at 1 m/s. This means that its average speed (since it started at zero velocity) is 0.5 m/s so in 1 s it will travel 0.5 m. You need to go back to your basic physics and get that right first. Start with \$ v = u + at \$ and go from there. Put the calculations into your question. \$\endgroup\$
    – Transistor
    Sep 1, 2017 at 23:42
  • \$\begingroup\$ Refer to the 3rd and 4th line of my 1st paragraph has accelerate, also 4th line 2nd paragraph has 'from stationary' just to show I understood what a newton force measures when writing my question. \$\endgroup\$ Sep 2, 2017 at 0:58

2 Answers 2


First figure out what force you require. You haven't mentioned lifting against gravity so we will assume horizontal motion on a frictionless surface.

  • You want to move 100 kg 1 m in 1 s. If we assume constant acceleration and deceleration that means we will have trapezoidal velocity profile.
  • The average velocity will therefore have to be 1 m/s and the peak 2 m/s.
  • To accelerate to 2 m/s in 0.5 s sets the acceleration rate at 4 m/s².
  • Now we know m and a we can calculate the required force: \$ F = ma = 100 \times 4 = 400 \; N \$.

To work out the torque required to achieve this you need to decide your mechanical radius. Using this you can calculate the required torque using \$ M = rF \$. So if using a 200 mm diameter drive pulley then r becomes 0.1 m and torque, \$ M = rF = 0.1 \times 400 = 40 \; Nm \$. Note that the motor needs to be able to provide 40 Nm throughout the speed range required.

Note that in all of this there is no mention of eddy currents, back-EMF.


You'd need to learn about electric motors : how to determine, for example, how current and torque are related, and how speed and voltage are related, and how both these relationships are connected to the motor's power. A book or college course should cover this.

Then work out what force and power you need (from simple Newtonian mechanics), and how that relates via your mechanical drive (gearing etc) to motor torque and speed.

That would be the starting point for looking for a suitable motor that can generate the forces you need.

You also need to consider that the motor must act as a generator, and that at any given speed it will generate a specific voltage. Can that voltage drive the current you need for the required torque, through your choice of braking resistance? This will help you choose the appropriate braking resistance. If you cannot achieve the required braking force this way you may need to apply active braking from a power source, or choose a better motor.

But the first question should be : isn't there an easier way to accomplish your braking needs?

  • \$\begingroup\$ The application in question is research for using an electric motor in an exercise machine where the scenarios I described would be applied. \$\endgroup\$ Sep 1, 2017 at 14:36

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