> Apart from the inductor and rectifier simply being in different
>> positions, what is the difference between the boost and Buck-boost DC-
>> DC converter?
>>
>> Is one more reliable than the other? Is one more energy-efficient?
>>
>> Thanks,
>>
>> Michael
polarity on the output.
Crossposting to SED due to the underwhelming response...
there's a spam war going on and many may have battened the hatches too tightly, also usenet takes time, give it 48 hours so so before you decide everyone is ignoring you.
I googled it before posting, and it seemed that the preference is buck- boost for electric bicycles for some reason. I personally only built a (tiny, prototype) boost converter, and was wondering what I was missing out on.
Buck/Boost is good for use with batteries that dramatically change their voltage as they are used For instance, alkaline cells vary 3 to
1 as they are discharged. That is, you start at 1.5V, and the cells are dead at 0.5V.
Seems to me you would power an electric bike with lead acid or nicad. These batteries do not change their voltage greatly (or at least as much as compared to alkalines), so I would go for a buck converter.
Regenerative braking is not worth a lot of bother unless you have a large momentum that needs stopping in a controlled fashion. I'm thinking of suburban rail cars that accelerate to high speed, and then decelerate before the next stop. In the days of plenty, they used to use resistive braking by slowing the train down with electric generation fed into a great resistance that glowed red hot. These days that waste can be redirected back into the grid. For cars, RB is a moot point. For the few percent of saving, not much expense should be made. For smaller vehicles, like wheelchairs and old farts' scooters like mine :) it is a total waste of complexity. jack
There are very few vehicles that I ride that I'd be happy with them stopping in an uncontrolled fashion. Actually I cannot think of any off hand.
It also saves on brakes (and in some cases motors). I'd also say there was a very good chance that both the wheelchairs and scooters you mention have regenerative braking (less certain on the scooters). They probably use PM motors (vehicles that small usually do) and regenerative braking on a PM motor is a trivial addition to the proportional control, it's actually harder to prevent it than to use it.
Robert
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Umm, why would a buck-boost be useful for regenerative braking? Anything other than a straight PWM seems overkill for most vehicle motor driving applications.
Robert
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What I meant by controlled, was not at the whim of any pedestrian, or random traffic light or other vehicle that might force you to jam on your service brakes. Knowing that at a certain point on the track, a certain slowing is required to come to a stop at a station a certain distance ahead is what I meant by "controlled". Sorry I was not clearer.
And by regenerative braking, I meant putting the braking energy back into the battery. Of course, electric motors can be set up to save the service brakes by dynamic braking (if that is the term) without the regenerative element. jack
the voltage out of the permanent magnet motor is proportional to the speed and will be less than the battery voltage, hmm, straight boost is probably better suited.
We may be running into a terminology issue here as well. Motors in electric vehicles are usually controlled with some form of PWM, for a PM or BLDC in a vehicle of this size this would be a MOSFET based controller (1/2H, full H or multi-phase). Now it's obvious how that acts to buck down the voltage to drive the motor. What's less obvious is this also automatically provides regen. If the motor's speed is greater than that provided by the PWM'd voltage (the back emf is greater than the PWM) then you will generate a current in the motor and this current will feed back to the DC bus. This is used by commercial PM and BLDC vehicle controllers at least down to the wheelchair class size. I would be surprised if anyone went to the effort and cost to remove it from smaller controllers.
You could consider the regen operation a boost converter but I don't think that's what you meant and I usually don't think of it as such.
The regen happens because - the motor acts as a generator - The motor windings are an inductor
The latter point means that when the PWM turns off the voltage rises until current can continue to flow giving the boost action. Note that gives rise to a very real failure mode, if the battery is disconnected from the DC bus during regen the voltage will quickly rise high enough to blow up the controller power section.
The difference between a PM or BLDC motor controller with regen and one with out is the level the the regen current limit is set to(1). A robust controller also has trips on the DC bus voltage.
No external boost or buck required. Unless we consider the PWM/motor combination to be a buck/boost convertor. Probably technically true but not what I usually think of.
Teranews seems to be dropping/delaying posts so this reply is a bit delayed.
Robert
(1) I have seen controllers with a diode on the DC bus to prevent regen.
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So did I (or at least the DC bus). See my longer reply to Jasen. It's harder to avoid regenning a PM motor when using PWM than it is to implement it.
Regenning with a PM motor to the battery is just a matter of allowing regen current to flow to the battery.
That's one term. Although that's also used to describe braking using regen to the DC bus and then using a resistor load to preven the DC bus from rising. A method used commonly on industrial drives.
I think I've left the quoting properly intact.
Robert
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the probelem I have with this is that the back EMF of the motor will always be less than the supply EMF, and it's back EMF that does the regen... so how do you get regen without boosting.
but how does the voltage get that high when the PWM is stopped. I can see the flyback voltage producing regen current when the pulse ends, but it seem to me that that's just returning some of the pulse energy,
I just can't picture what you describe, how would I go about modeling it?
should I treat the motor as an AC voltage source with less than battery voltage in series with an inductor?
Basically the same way it does in a boost circuit.
energy,
First I'll take as a given that the motor responds to the mean applied voltage. The PWM is fast enough that current ripple is minimal. In practice minimal current ripple is easily achieved, except perhaps for some very low inductance motors. That being the case the motor back emf can easily be larger than the applied voltage and the motoe will be generating current.
Now consider a MOSFET switching on the B- side of the motor being PWMed at 50%, and the motor running at the equivalent of 100% applied voltage.
- When the MOSFET is on the battery voltage applied across the motor will act to decrease the current and reduce the torque, increasing the speed of the motor. Due to inductance this change in current is small, at high enough frequencies it's insignificant. - When the MOSFET is off the voltage will rise since there is now no longer a complete circuit path but the inductor will 'want' the current to remain constant. The voltage will continue to rise until it's high enough to complete the path through the integral body diode of the MOSFET. Note that without the integral body diode or similar the voltage will continue to rise until something conducts.
Now if you just shut off the PWM to zero duty cycle then you do just get a short pulse. It's the PWM establishing the operating point that provides the reference, without that the motor is just floating.
An inductor with a voltage source in series, a flyback diode a across the two of them and a MOSFET switching the lowside. Maybe another voltage source to act as an ideal battery. I've never tried it, there never seemed to be much point.
Robert
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in a boost circuit there is no power out unless power goes in...
energy,
with nothing at all acting to _increase_ the current in the motor windings how can this work?
where does the current come from?
hang on! if the input drive is in the opposite direction acting to increase the current in the motor, the generated EMF is helping the input power, and on the flyback (when the input current switches) off the generated EMF this time again adds to the inductor voltage, boosting the energy coming out.
the motor itself is acting somewhat lika a buck-boost converter boosting during acceleration (more current flow in than is returned) and bucking during decelleration (more current flows out than in).
Remember the back EMF. The ultimate source being the vehicle's kinetic energy of course.
energy,
The back EMF of the PM motor
Nope, input is in the same direction (ie it will tend to accelerate the motor). The back EMF is the same polarity as the DC bus (+ to +).
I was reviewing my explanation and trying to develop a simulation in LTSpice. I realized I left out an important bin on the off cycle (sorry about that). I'm assuming a full H or 1/2H control and the flyback switch shorts the motor during the off cycle, there's where your current build up takes place.
Here's an LTSpice model if that's any use. Switches with anti-parallel diodes substitute for the usual power devices. The motor is modelled as an inductor with a voltage source for the back EMF.
On 2007-05-27, Robert Adsett wrote: To: Robert Adsett Subject: Re: boost / Buck boost In-Reply-To: References:
X-Face: ?)Aw4rXwN5u0~$nqKj`xPz>xHCwgi^q+^?Ri*+R(&uv2=E1Q0Zk(>h!~o2ID@6{uf8s;a+M[5[U[QT7xFN%^gR"=tuJw%TXXR'Fp~W;(T"1(739R%m0Yyyv*gkGoPA.$b,D.w:z+> > - When the MOSFET is off the voltage will rise since there is now
the back EMF is less than the rail voltage, there's no way it's going to increase the current, unless the motor's shorted out instead of connected only to the bridge.
ah! yeah, that'd work, it'd be more efficient too, freewheeling the coils instead of feeding their energy back into the supply (when driving) and having the mosfets conduct instead of the protection diodes when braking.
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