Dynamic Braking

[murcott]

Dear All,

I am building a Dynamic Braking system. This is where the magnetic forces generated in a motor are used to act as a brake, simply by closing the circuit.

http://en.wikipedia.org/wiki/Dynamic_braking

I have built a three phase alternator and attached a pulley system to it so that I can get the RPM up to about 1000 by manually turning a crank handle. The alternator works nicely as a brake when the current is left as AC. I simply connect the three sets of windings in the stator, and the rotor comes to an abrupt stop.

However, when I want to convert the current to DC , I have issue. I have built a three phase full wave bridge rectifier. The diodes are shottky and are rated to 10A. My alternator produces no where 10A, so they should be fine. When it comes to closing the circuit , the braking effect is very low compared to when left as AC. About half the resistive force.
I am a Mechanical engineer having a bash at the electrical stuff, all very interesting stuff, but I am a real rookie.
Can anyone explain why the braking effect is lost? The gauge of wire I use is consistent across the whole circuit, and my diodes should be fine..... I think am limiting the amount of current coming through. Could it be my rectifier has too much resistance?

Your thoughts would be very very welcome.
Cheers,

Murcott

 

[itsmoked]

You aren't clear on your initial AC test. Are you saying you just short all the leads directly together?

If that's true then you've greatly changed the loading by now interposing a rectifier bank and what else? Are you somehow using the DC?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MikeHalloran]

You now have two discussions going on about the exact same subject.

Please Red Flag one of them and ask that it be removed.

Please do not double post again. Thank you.



Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

For Dynamic Braking to work in an AC machine, you create a generator, then the spinning load acts as the "prime mover" of that generator, you take the regenerated electrical energy from that and dump it into a load bank to get rid of it. When using your alternator and shorting the leads together, it keeps the alternator acting as a generator because the leads are connected, so it maintains self-excitement. But in that instance, the alternator itself is acting as the load bank, absorbing that kinetic energy in the spinning load as heat in the windings; not good.

Not sure of your goal behind rectifying the output to DC, but you may be forgetting the part about keeping the alternator windings excited. There will be some residual magnetism in the core to keep some excitation available, but it will not be much, which is likely what you experienced. If your goal was to rectify to DC so that you can fire the energy off into an external load bank, such as resistors, then you must find a way to keep the field excitation alive in the process.

"Will work for (the memory of) salami"

 

[waross]

I have built a three phase alternator and attached a pulley system to it so that I can get the RPM up to about 1000 by manually turning a crank handle.

I'm thinking something like a bicycle generator with permanent magnets. DIY hobby stuff. No idea what the voltage may be. The voltage may be so low that the drop across the diodes has become significant.
Remember the old school crank meggers? We could anticipate a very low reading by the extra resistance on the crank handle. This may be on the same scale.
BTW; Had a discussion about old meggers the other day. The other fellow thought that the electronic meggers with an analogue meter instead of a digital display were old school. He had never cranked a megger. It made me feel old.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[murcott]

An interesting discussion starting here , thanks.

Mike - Yes, I just crudely short the leads together.

To answer all, I am intent on making a regenerative brake for a bicylce. I am keeping it as a Dynamic brake for the time being to keep it simple. My aim is to use the DC current to charge a battery of some sort.

Jraef - your explanation makes things a little clearer. Thanks. It is my intention to use a battery in the DC circuit as a load bank. The AC test is puerly to make sure the alternator works. Without. It's seems perfectly feasible to replicate a technology widely used on trains , for example. Does such technology need to give additional excitation to the alternator?

Bill, you guessed right about the bicycle. I'm getting jsut 4v out of dc circuit. Mike suyggested the impedence in AC circuit might be higher than i expected. My diodes are rated to 10A. I can get some 100A ones quite easily. I'll also need to get a decent multimeter, as my current one can't measure AC .

 

[LionelHutz]

It's nice we get to guess what type of motor you're using. Is this a 3-phase induction motor? If yes, then it won't work as a generator in the way it appears you're trying to use it.

Overall, your simple idea has an even bigger problem. Say you get the motor excited (by changing the rotor to permanent magnets by example). This new 3-phase "wild" generator with a rectifier connected to a battery will not provide a constant load as the speed changes. It will require reaching a certain speed before it even begins to generate any load or current flow to the battery. Then, the load will rapidly increase above that speed. It'll be rather useless for braking.

So, you could change the motor to a field wound rotor. Then you have to vary the DC current supplied to the rotor to adjust the generator output voltage with the intent of providing varying braking levels. But, this power being fed to the field will take away from the net energy recovered during braking. The efficiency will likely be so low that the system will be pointless.

So now you get the a system that could work, and that is using some form or inverter technology between the motor and the battery. You can already find this type of system in many of the readily available E-Bikes and RC controllers. You can use it to drive the bike and brake the bike.

 

[murcott]

Hi Lionel,

It is a three phase ,dual rotor, axial flux alternator that has permanent magnets. So it is not an induction motor. This type of alternator is commonly used by wind turbines. It is standard practice to rectify the current to DC.

With regards to braking, I agree , the system will be useless below a certain RPM. However, this is not an issue as I can use the friction brakes when the RPM is too low.

I'll have a look at the E-bikes and RC controllers, are you able to explain the technology?

Thanks,
Ben

 

[jraef]

BTW; Had a discussion about old meggers the other day. The other fellow thought that the electronic meggers with an analogue meter instead of a digital display were old school. He had never cranked a megger. It made me feel old.

Bill, I had a similar experience a few years ago. I was telling a motor technician that he had to megger a motor, not just use a VOM, and he pointed out to me that the device he was using WAS a megger. I didn't see a crank, so I assumed it was just a VOM...

"Will work for (the memory of) salami"

 

[waross]

Thanks for sharing, Jeff. I don't feel as old now. grin
When the company supplied me with my first electronic megger, for the first several years I had a resistor in the meg-Ohm range taped to the front of the instrument. I would regularly check the resistor to reassure myself that the instrument reading was valid. I still miss the "feel" of the crank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I'll have a look at the E-bikes and RC controllers, are you able to explain the technology?

http://en.wikipedia.org/wiki/Electronic_speed_control

See also;

http://en.wikipedia.org/wiki/Variable-frequency_drive

You interested in the output circuit - the part between the capacitors and the motor.

 

[mikekilroy]

how about posting a schematic so we can see exactly how you are hooking up the full wave bridge rectifiers and shorting it out?

also, why not just short the wires at motor instead of dc side since it works?

http://www.KilroyWasHere<dot>com

 

[murcott]

Hi All ,

Mike, I have followed your suggestion. The following link shows a schematic of the rectifier.

http://en.wikipedia.org/wiki/File:Tridge_(three-phase_bridge)_rectifier_(symbolic_diagram).png

I think I've figured out why i'm losing so much resistive force in the brake. My shottky diodes each have a forward drop of 0.65 volts. I presume this means I am losing 6 x 0.65 volts? So if my AC current has an amplitude of say 10V, then I could be losing about 40% of the voltage when converting to DC. Could I just double the rpm of the motor? Bit of a pain mechanically.

I want to use a transistor as my switch. Is it possible to use transistors as on/off switches in AC circuits? As it stands, the DC circuit will be shorted. I am converting to DC so that I have the option to either dump the current in a load bank when it gets too high, or use the system as a generator . This is the only purpose of the DC circuit. By shorting the circuit, the aim is to retain the transfer of torque between the rotor and the stator. How significant are the consequences such as heat generation? What proportion of the energy would be lost to heat. 10%, 50%, 90%? For obvious reasons I want these losses to be as low as possible. Are there preventative measures available?


Lionel, thanks for the links. I am particularly interested in the electronic speed controllers. They could negate the need for DC. Not sure yet, need to do some more reading.

Ben

 

[mikekilroy]

Look at the total path for each current: short dc link and path is thru 1 diode to dc side then thru a 2nd diode back to stator coil; hence you have TWO series diodes so 1.3v drop.

A transistor is like a diode so will exhibit same result.

A FET has on resistance down in milliohms so v drop can be negligible.

http://www.KilroyWasHere<dot>com