https://support.maxongroup.com/hc/en-us/articles/360004496254-maxon-Motors-as-Generators

That shows a ECM/BLDC motor fitted with a 3ph rectifier to drive a DC motor but would not work BLDC to BLDC.
You can however use two stepper motors as syncro's. Transmitter - receiver.
Max.

 

That shows a ECM/BLDC motor fitted with a 3ph rectifier to drive a DC motor but would not work BLDC to BLDC.
You can however use two stepper motors as syncro's. Transmitter - receiver.
Max.

BLDC generator to BLDC motor is Plain-Jane 3-phase power. The trapezoidal back EMF of your typical BLDC motor will be smoothed by motor inductance into a more sinusoidal shape
so you can use sinusoidal commutation to spin the motor with a encoder/resolver for position tracking.

We sum up the findings in the equation for the AC generator voltage:

where

• Uind,ampl the amplitude of the induced voltage per winding segment (V)
• kn the speed constant of the motor (rpm/V) (see chapter 1.1)
• f the AC frequency (Hz)
• n the motor speed (rpm)
• p the number of magnetic pole pairs.

Sure, it's not DC.

 

Keeping in mind that the generated power will be AC sinusoidal.
Also the commutation will be absent on ECM & BLDC motors.
Max.

If using two similar three coil motors, yes, the power would be AC, but that is actually what the direct connection needs to turn the second motor. I am, of course, talking about two motors with NO electronics, so that they can work like an alternator feeding a synchronous motor. That is the whole point I was trying to make.

 

Just remember the only cool projects, the ones people remember are the ones which do something. While lighting an LED is cool lighting a small filament type flashlight lamp is really cool. Especially when as more force is applied the lamp grows from dim to brilliant.

Ron

 

BLDC generator to BLDC motor is Plain-Jane 3-phase power. The trapezoidal back EMF of your typical BLDC motor will be smoothed by motor inductance into a more sinusoidal shape

Using a double beam 'scope, I have aligned my share of BLDC hall effect sensors over many decades, and the encoder equivalent on servo motors when implementing CNC retro fits, the typical BLDC motor generates 3 phase when back driven, but if you use either a 3ph or BLDC signal to drive one with another, commutation is needed.
BTW, the output of a back driven BLDC motor is 3phase output, the AC synchronous motors are just about identical, the difference is in the commutation.
Max.

 

If using two similar three coil motors, yes, the power would be AC, but that is actually what the direct connection needs to turn the second motor. I am, of course, talking about two motors with NO electronics, so that they can work like an alternator feeding a synchronous motor. That is the whole point I was trying to make.

I was refering to either a typical BLDC or Synchronous PM 3ph AC servo motor, both need commutation.
I have worked with these for 50yrs now so I have learned a little along the way.
Max.

 

Well, Yes, Max. The commutation for the 3 phases from the driven motor comes from the generator motor. That is why it takes two of them. Like I have stated already. Turn a crank on one and make the other one turn. Just really cool.

 

Using a double beam 'scope, I have aligned my share of BLDC hall effect sensors over many decades, and the encoder equivalent on servo motors when implementing CNC retro fits, the typical BLDC motor generates 3 phase when back driven, but if you use either a 3ph or BLDC signal to drive one with another, commutation is needed.
BTW, the output of a back driven BLDC motor is 3phase output, the AC synchronous motors are just about identical, the difference is in the commutation.
Max.

Sinusoidal Commutation comes from the 3-phase generator (BLDC motor used as a generator) in the form of rotating magnetic fields shifted by 120 degrees. I respect your experience but I also design hardware and write motor drive software that actually generates the needed wave-forms for AC servo and BLDC motors using controllers and H-bridges. Hall sensors, BEMF, FOC , Space Vector are all methods of efficient (conversion of the DC supply power into AC motor power signals), precise (control of torque, speed, position) operation of motors that are needed for most accurate applications but I can guarantee that back to back 3-phase works to rotate the motor just like it does with a 3-phase induction motor.

Here I'm using software to generate a regular 3-phase signal (just like a generator would) for the motor. There is no external commutation from Hall sensors, encoders, resolver, back-emf or loop controls. The motor spins because the 120 degree shifted rotating magnetic fields generate a torque in X direction.

 

For a film maker the power of fake video is the result of teaching the glory of Hollywood technical productions.
So the role of science has been replaced in schools for film documentary and science fiction rewarded by twisted ideology.
Many museum videos on science have been removed. An example is the museum videos on canal rays, there are over 20 missing videos that are no longer found. When Cambodia came under a different regime there was a genocide of intellectuals and the masses were forced to work agriculture. On that same note Africa did ethnic cleansing using paid agitators to start resentment protests using tribalistic legacy. Fake videos and dumbing down the large plantations, followed by genocide to those not laboring for the regime and whose ethnicity is not following the regime.
Good science videos are harder to distinguish from fakes. Colleges teach pseudoscience. The dark arts of metaphysics are given credulity.
Which basic demonstration shows induced EMF and Back EMF. Stop over thinking the education trying to be tricky.

 

To the OP .... A demonstration I used to do for science class was with a copper tube slightly larger than a stack of rare earth magnets. The magnets in my case were about the diameter of a pennie. I also used pennies glued together to approximate the weight of the magnets.

The Demonstration Materials: Two copper tubes (or sheet - see Note below) , Magnets, and Pennies

Demonstration:
First drop the Magnet and Pennies from the same height to show that both objects will land on the ground at the same time
Next drop the Magnet and Pennies through the copper tubes at the same time and have the students guess which one will
win the race. Even make a smaller version of the stacked pennies as well as a larger version.

The magnets will always lose because as you drop the magnet(s) it will induce a charge in the copper tube (big coil) in a way that produces a counter magnetic field in opposition to the falling magnet(s). This creates resistance that will slow down the falling magnets.

Note: You can also do this experiment on a sheet of copper or aluminum at an incline and have the magnets and pennies slide down the sheet.

 

Many folks lack a dual beam scope, and I would never ever send one of mine to a school for a demonstration.
AND CERTAINLY the internet and u-toob are not to be always believed. And with a good computer and the right software you can produce about any images that could be imagined, and quite a few that can not be imagined. In many instances "Road Runer and the Wiley Coyote" are the closest to reality that you will see.

 

To the OP .... A demonstration I used to do for science class was with a copper tube slightly larger than a stack of rare earth magnets. The magnets in my case were about the diameter of a pennie. I also used pennies glued together to approximate the weight of the magnets.

The Demonstration Materials: Two copper tubes (or sheet - see Note below) , Magnets, and Pennies

Demonstration:
First drop the Magnet and Pennies from the same height to show that both objects will land on the ground at the same time
Next drop the Magnet and Pennies through the copper tubes at the same time and have the students guess which one will
win the race. Even make a smaller version of the stacked pennies as well as a larger version.

The magnets will always lose because as you drop the magnet(s) it will induce a charge in the copper tube (big coil) in a way that produces a counter magnetic field in opposition to the falling magnet(s). This creates resistance that will slow down the falling magnets.

Note: You can also do this experiment on a sheet of copper or aluminum at an incline and have the magnets and pennies slide down the sheet.

I made a similar demo of a clear plastic tube, copper tube and strong magnet for my kids science class years ago. They were pretty amazed at the fall time difference.

 

This creates resistance that will slow down the falling magnets.

Resistance yes, but not against the falling magnet, but resistance in the copper tube.
The operative phrase here is conservation of energy.

The falling object has initial potential energy = \(mgh\)
The potential energy is converted into kinetic energy = ½\(mv^2 \)

As the magnet falls, current is induced into the copper tube. This current is converted into heat energy.

The astute student will ask "Which direction is the induced current flowing?"
The induced current produces a magnetic flux. In one direction, the flux would accelerate the magnet. In the opposite direction, the flux would oppose the motion of the magnet.

Since energy has to be conserved the magnet falls at a lower speed.

 

From there you go to:

 

it does need to turn fairly fast

In the video in post #9 I demonstrated and explained how the gear reduction (actually gear induction) caused the motor to spin significantly higher RPM's of the motor. The gear ratio of that device in the video is a total of [edit] 1 to 36 [end edit], meaning the motor spun 36 times faster than the thumb wheel. And all that to simply light up one LED or the other, depending on which way it was spun. It took that much gear induction just to light up a simple LED. Spinning one motor from another (similar) is going to be very very difficult. An experimenter might have to hook up a lanyard to the shaft of one motor and pull the hell out of it to get the second motor to show visible signs of movement.

Dropping a magnet down the length of a copper pipe will demonstrate that something mysterious is going on in the mind of a youth. WE know it's creating eddy currents that resist the movement of the magnet. But a child's mind might not be able to grasp that concept. Shaking a magnet within a coil of wire can produce enough energy to light a simple LED. But if you look at those shake light flashlights, they take a lot of shaking and don't produce much light. The amount of energy you can get out of a device is going to be no better than equal to the energy you put into it. And due to imperfections in wires and connections as well as any other resistance, such as wind or friction, you're not going to get all that energy to transfer into electrical energy. Some of it will be lost as heat and some may be lost in air movement. It has also been shown how an olympian bicyclist can pump a generator long enough and strong enough to toast a piece of bread. Just search YouTube for Olympian Bicyclist and toast.

There are plenty of ways to demonstrate the presence of electricity being generated from moving a magnet in a coil. The more turns there are to the coil the more current it will generate. The faster the magnet moves the higher the voltage. Then whatever device you're using to demonstrate the result, such as an analog meter (I believe Radio Shack may still offer such). But if not - the suggestion of using a compass and a coil of wire at one side will move the pointer either AT or AWAY from the coil. But the coil needs to be on one side or the other, not wrapped around. But even wrapped around you will likely see the needle want to point up towards the glass or down towards the base. But the more turns the stronger the deflection. Getting an LED to light will be difficult. You'd probably need a large coil and a strong magnet. And I can't begin to suggest how big it may need to be. But it's all doable.

 

Finished searching RadioShack's website. They don't appear to offer such meters any longer. However, on Amazon, I found a number of panel meters by searching "analog meter panel" and got a whole bunch. Here's a link to just one of those meters.

 

Many folks lack a dual beam scope, and I would never ever send one of mine to a school for a demonstration.

For the vast majority of tests and typical experiments, and if on a budget, there is the double beam storage 'scope from such as Here .
I find it invaluable for portable off site trouble shooting using a lap-top.
Another advantage is the storage feature.
I wouldn't be without mine.
Max.

 

If you wish to experiment with a magnet and a coil of wire, this is the type of meter you should use.
It measures up to 50μA and has a center-zero scale so that you can read the current going in both positive and negative directions.



https://www.ebay.com/itm/Simpson-Wi...347044?hash=item36662b4a24:g:3eQAAOSwuCdfA-s-

 

The demonstration with the two motors was NOT done with those very poor generators that come with the $2 LED lights. Those are the very cheapest pieces of junk ever produced. The most recent demonstration I saw used two motors from an automotive radiator fan system. With the motors connected the person could spin one fan blade with their finger and make the other one turn.The automotive cooling fan motors were from the discarded fan assembly of a neighbors wrecked car being repaired in his driveway. One fan was missing a blade and the mounting panel was rather bent.
The first demonstration was almost more impressive. Some sort of industrial surplus motors about 4 inches long and an inch and a half square, with shafts about 1/4 inch diameter, connected. When my friend twisted the one shaft with his hand the other motor turned. That is the difference between real parts and consumer JUNK. Of course the motors did have ball bearings and they did weigh a bit more and probably were fairly expensive when they were new.

 

The demonstration with the two motors was NOT done with those very poor generators that come with the $2 LED lights.

That sounds like the small cheap 3phase generator off ebay.
I successfully used one on a wind vane for my son who want it to illuminate his and make blades visible after dark.
Kept the rotor stationary and rotated the housing, fed 3 sets of lights off each of the 3phases.
Max.