4Q Motor control paradox

Most physicists these days would want to talk about energy balance, and would only resort to the details of the forces, currents, whatever, when they had to.

If the kinetic energy goes somewhere and it doesn't dissipate as heat in the motor and it doesn't dissipate as heat in the amplifier then it must come out of the amplifiers power terminals -- which it will if the amplifier has synchronous rectification and is sufficiently efficient and if the motor is actually pumping energy into it as it slows down.

Were you bored with your day to the extent that you had to make that up.

The energy goes back into the supply, if the supply can't deal with it it goes 'bang'.

DNA

Other than losses in the supply and motor, the energy will go back into the power supply. With a battery, it can recharge the battery, which is generally good. With simple rectifier/filter capacitor supply it will charge the capacitors, quite likely to an unhealthy voltage for the capacitors.

Best regards, Spehro Pefhany

Succinct, accurate, profane -- how could this answer be better?

The punctuation is a bit sloppy.

Best regards, Spehro Pefhany

The energy goes back into the supply, if the supply can't deal with it, it is f***ed.

There you go. Does the extra comma help?

DNA

Helps me. Spehro is right, even if he was being every so slightly perfectionist.

I think Speff (if I may be so bold) was commenting on the lack of a profanity.

However, what I should have said was....

The energy is returned to the supply. If the supply is unable to cope with the returned energy then the supply will be cunted.

DNA

I supose you won't believe me if I suggested the omission was deliberate.

DNA

I s'pose I missed out the apostrophe..... Mind you, that was not deliberate either, I'm just an opportunist. Having pisspelt it, I left it in.

DNA

Well if you made a mess of the punctuation, I clearly made a mess of the question ;-)

For the record, as you probably know, regenerative drives which cannot absorb current have resistors that can be switched onto the DC link and do not go bang. This was what I was meaning by conventional wisdom.

The **question** is why things don't work like the space rocket analogy. The space rocket is 'braked' by applying a force in the opposite direction. Passing current througth a motor armature in the opposite direction also creates force. So where is the difference between the two analogies?

There isn't. It's just that when you brake a motor there's a chance that it'll deliver power out instead of just burning it up in the armature. The _torque_ on the motor will be proportional to current, whether there's _power_ coming out of the motor will depend on a whole bunch of stuff.

Fucking hell, are you having a boring night as well?

Space does not go Bang!

There you go.

DNA

I think that "bunch of stuff" is generally called voltage ;-)

If we think about a motor that is being driven at a constant speed by an infinite mechanical force, then the equivalent circuit of the motor would be a voltage source with source impedance equivalent to the armature resistance.

So let's cut things down to basics: 2 voltage sources connected + to -, with just the armature resistance to limit the current, which will be (Vs-Vemf)/R. Clearly the I2R losses will be much less than the power being delivered by the supply, so where is the energy going? Answer, into the "other" voltage source (our armature), and hence to the mechanical load. If the load were a rotating mass of iron the energy I put into the motor would eventually make the iron rotate in the opposite direction. In a lossless system I would need twice the kinetic energy of the iron to make it turn in the opposite direction with the same kinetic energy.

My problem is that I arrive at the concept of "negative energy" :-((((((((((

Where the hell am I going wrong here!

Well, perhaps just once in it's lifetime, usually at birth ;-)

BTW, not bored....just been looking at an old chopper cct that I used in the past and am thinking of using again and I wondered why I need the braking resistors.

The answer is simply to "recapture" the ebergy used during braking and return it to the storage medium (battery) The purpose, of course, is to extend the range of the vehicle. The EM device switches from motor to generator during the braking process and an H bridge configuration allows for full 4 quadrant operation. The wheel that is braking continues rotating in the same direction, so by switching the current flow through a different configuration in the H bridge, the energy can be returned to the battery as opposed to being drawn out.

Dave

Papist.