Alternating Current For Peak Performance?

[safe]

I'm going to start the thread and that's it for the moment...

Alternating Current is in the opinion of "the pro's" the only way to go for top end performance. (look at all the people getting into it now) Is there any way we could do this on our smaller electric vehicles?

 

[knightmb]

I'm going to start the thread and that's it for the moment...

Alternating Current is in the opinion of "the pro's" the only way to go for top end performance. (look at all the people getting into it now) Is there any way we could do this on our smaller electric vehicles?

Well in a technical sense, that's what a brushless DC motor does to make it more efficient than a brushed DC motor. It's a trade-off between converting DC to AC (with power loss) vs. taking the DC you already have and "simulating" a three phase AC motor. It's also a matter of cost and space. There are plenty of affordable DC to AC converters that will do 2000+ watts of power, but these things are so big, bulky, and produce a lot of heat.

More practical for something that you scale up than down in my opinion. For Electric Cars, it can work great. When you want to fit it on a bike, that's when it gets a lot tougher. That's when these brushless controllers come in to give you a cross of both the AC and DC world.

 

[barts]

Do the existing controllers produce AC wave forms into the coils, or do they
just turn DC on and off to each coil at the right time; eg do they reverse polarity?

- Bart

 

[Leeps]

I really dont look at brushless dc motors as a sort of ac motor. I think of them exactly as a brushless dc motor. Its a dc motor with the brush commutation taken out and electronic commutation put in. This gives you the benefit of being able to reverse the position of the coils and magnets, thus heatsinking the coils to the case of the motor. On a brushed dc motor theres no argument that its dc, but the brushes are giving power to appropriate coils at the appropriate time, here we are doing the same thing with a seperate control unit, seems to be the same thing to me, unless brushed dc motors are actually ac but nobody knew
Joe

 

[safe]

People seem to suggest that a DC to AC converter needs to be heavy, but I doubt it NEEDS to be, it's probably more like they "tend to be" because they are designed without weight in mind. (if weight doesn't matter then it's easy to be let things get heavy)

I know that the "Electric GPR" offers an AC option and they claim an advantage over their DC motor. They're talking about a 15 hp motor however and not the 1-2 hp we are allowed by law on the street.

So it would be interesting to discover if there have been examples of AC conversions that were proven to be a success that were targeted at the 1-2 hp category.

 

[fechter]

Well, a brushless motor is AC by definition. What you're talking about is an induction motor, vs a permanent magnet motor. The controller uses a full bridge to reverse the polarity of the output.

The controller for an induction motor is more expensive.

The motor itself, since it has no magnets, should theoretically be cheaper, but in scooter sizes, they are not.

One cool feature of an induction motor is the rpm is not really limited by the voltage like a permanent magnet motor. It's like having gears. The max rpm is limited by the rotor flying apart.

You can get an induction motor quite cheap on ebay or surplus, but wound for 120v or higher. 480v ones are very common.
I wouldn't be that hard to rewind one for a much lower voltage.

In theory, a permanent magnet motor will be more efficient because there is no slip. In larger sizes, an induction motor can exceed 95% eff.

I rode the GPR with the induction motor. Unfortunately, I was confined to a parking lot and couldn't really open it up. The low speed response is extremely smooth and the motor is very quiet. The motor was custom from Highperformace Golf Cars.

Here's a pic of the motor and the Curtis AC controller:

 

[safe]

Awesome pictures.

So you know Todd Kollin over at Electric Motorsports? You live just half an hour away (Oakland verses SF) so I would guess you have been to his store.

It's an interesting idea to switch to AC because you do seem to get a wider powerband and MUCH better efficiency than with the brushed or brushless DC motors. It seems that the best you can expect for each type is:

DC Brushed - 80% efficiency.

DC Brushless - 85% efficiency.

AC Brushless - 95% efficiency.

Rather than the "Battery Pack Throttle" idea you are far better off spending your time switching to AC power...

 

[Leeps]

Just to throw a monkey bar in the works, i know im a pain, but what about the brushed agni motor 93% efficiency.
Joe

 

[safe]

One cool feature of an induction motor is the rpm is not really limited by the voltage like a permanent magnet motor. It's like having gears. The max rpm is limited by the rotor flying apart.

The permanent magnets seems to "define" the powerband of the motor. If you want to get the most out of it you have to stay within a pretty narrow powerband and that means you need gears.

With a "dynamically configurable powerband" you don't have the same "issues" and then the case for gears gets weaker again. (because the powerband becomes very, very wide and efficient)

When it comes to "real breakthroughs" for the "street legal" category we are involved in a well designed AC system could really "rock everyones world"! (or in my case Polka everyones world )

 

[safe]

Just to throw a monkey bar in the works, i know im a pain, but what about the brushed agni motor 93% efficiency.

Is that for real? (or marketing hype?)

A link? (the best I've seen is around 85% - 90% tops)

A dyno chart as proof?

Plus, the "issue" was "powerband width". If the brushless motor "peaks" at above 90% that doesn't mean that it has a wide powerband. The AC can have high efficiency across a larger percentage of it's rpm range. It's the "width" of the powerband that can allow you to avoid using gears in order to prevent losses... the main "issue" is "peaky power" verses "wide spectrum power" all at high efficiency. (because in the real world your rpm's vary a great deal)

 

[safe]

And I'll throw another "monkey wrench" into this...

On the Discovery Channel show "Future Cars" they talked about the speed record setting electric vehicle that uses AC current, NiMh batteries and GEARS!

So when EXTREMELY wide ranges of speeds are required you still need gears. (300 mph is hard to do with a one speed even if it has an AC motor!)

 

[fechter]

Yes, I know Todd.
I've been over to his shop a couple of times. It's like going to the candy store.

There's no way the induction motor on the GPR is 95% efficient under normal operating conditions. Probably under 90%.

I've seen brushed motors over 95% eff. I've seen BLDC motors up to 98% eff. Size makes a big difference, so to really compare, you need to compare motors of the same power range. Larger motors are inherently more efficient at their peak.

 

[Leeps]

http://www.agnimotors.com/home/ click on performance
The major killer in efficiency for a permanent magnet motor is I^2R losses in the coils. Thats what i love about these axial flux motors they just have copper bars instead of coils. So you can throw massive amounts of current through them without much loss. The major limitation is really the commutator in these things. The graphs will probably look odd to you the rpm ends up being on the y axis and the current is on the x axis. If they did it like a normal motor graph the current and torque at the lower rpms would just shoot off the graph so they did it this way i suppose to focus on the more normal parts.
Joe

 

[barts]

Here's a URL for some Curtis AC motor controllers:

http://www.curtisinstruments.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=8


Finding the right motors might be a little trickier....

 

[Leeps]

Safe this might be useful knowledge to you. The efficiency peak on a permanent magnet motor occurs when the I^2R losses in the coils is equal to the no load power requirements of the motor. The power peak occurs when half the voltage input is dropped within the coils, or in other words roughly when you bog the motor down to half its no load rpm.
Joe

 

[safe]

The efficiency peak on a permanent magnet motor occurs when the I^2R losses in the coils is equal to the no load power requirements of the motor. The power peak occurs when half the voltage input is dropped within the coils, or in other words roughly when you bog the motor down to half its no load rpm.

The second part is easy to agree with because the power curve is a simple parabola with the midpoint being "peak" power.

However, the "efficiency peak" occuring when resistance losses equals the no load amps doesn't sound right. "No Load" means the highest rpm's.... the "efficiency peak" occurs below that speed at roughly 85% - 90% of the maximum "No Load" speed.

Look at the chart... the "efficiency peak" is the top of the little "bubble" in the red curve...

 

[safe]

Here's a URL for some Curtis AC motor controllers:

http://www.curtisinstruments.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=8

I notice that this first one is 24 Volts.

Could it be possible that you don't need to use a high base voltage with AC because in the process of conversion the need for high base voltage disappears?

Might this allow battery packs to be built with a higher percentage of parallel subpacks and therefore make balancing and charging easier?

This might be an added benefit... lower voltage requirements...

 

[safe]

It's like going to the candy store.

I can relate to that!

 

[Leeps]

It is true, the no load current is there all the time, well friction and windage is proportional with speed but for all intents and purposes if it takes 6 amps at no load you can do efficiency calculations assuming your taking 6 amps and dumping it out the window. Now you load down the motor, the current and torque rises, the rpm drops. When you have enough torque and current to make the resistive losses equal to the losses from your no load current plot that point and you have the rpm point of maximum efficiency. See what i mean
Joe

 

[safe]

Basically I think it's your "philosphical orientation" about viewing the problem that tends to give me problems. I tend to think in terms of "final results" as in "what torque actually reaches the rear wheel" or "how fast does the battery have to drain to achieve a certain result". (or the easiest is "what does the dyno for the motor look like, what is it's shape?")

The only "hard reality" that can be said of a regular brushed electric motor is that it has a definite "powerband" where efficiency is WORST at low rpm's and BEST near the high rpm's. There are factors like PWM that bend the numbers a little, but the overall "hard reality" remains. (you can also use extremely large motors and make them run underpowered and this also manipulates things somewhat, but the "hard reality" doesn't change)

AC motors have the potential to expand greatly the region within the powerband that has high efficiency. So this is why we are interested in them.

But my guess is that for our "small motors" (1-2 hp) geared bikes are always going to have advantages over non-geared bikes of the same size... (so all things being equal gearing always yields an advantage)

 

[safe]

Here's a URL for some Curtis AC motor controllers:

http://www.curtisinstruments.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=8

Could it be possible that you don't need to use a high base voltage with AC because in the process of conversion the need for high base voltage disappears?

Picking back up on the core thread about "AC Motor Performance" again...

So what is it? Can you take a lower 24 Volt DC base voltage and then when you convert it to AC be able to expand the "perceived" voltage as being wider and taller and thus promote better overall efficiency across the rpm range?

After all what you need to do is throw enough energy into each "cycle" of the AC current so that the peak is high. You could store energy coming from the battery (which is at a lower voltage) between cycles and then "spit" it out in a "burst" (somehow) to get a voltage peak that EXCEEDS the value of the source. Am I correct here? Is this possible? Or no?

This idea (AC) seems to "dwarf" all other efforts for improvements!

We should be doing this.... shouldn't we?

You guys ought to download the pdf manual for the AC Curtis controller... it's pretty awesome the things you can do with it...

 

[safe]

Thinking "philosphically" about AC verses DC motors it would seem that DC is very closely dependent on Voltage to be able to function because electricity naturally resists moving as a current. Electricity is actually more like "thick oil sludge" and less like "water" because it doesn't move well as a current. However, long ago, they realized that you can simply send a "wave" through the medium of the wires and those "waves" will carry the energy so that the electrons can stay in "their own neighborhoods". AC for motors would also behave with this "advantage" because you aren't so worried about high voltage to get current to move so low voltage should still be able to get energy transmitted to the motor very efficiently.

You also don't really need to worry about motor control so much because all you have to do is vary the AC cycle rate and the motor would simply run on that...

 

[Leeps]

I have some issues here. Correct me if im wrong.
thick oil sludge and water are acceptable philosophical terms to apply to electricity, most people dont realize that in a wire the actual speed of the electrons on average is slower than the minute hand on a clock.
Im under the impression that you are suggesting that on an AC system low voltage and high current can be more efficient. How and why is this possible with AC and not DC? Granted there is no net electron displacement, but there is a current, just because you have a negative and positive current that balance out does not negate any resistave losses that you will have.
Motor control is a magnificently major worry on ac systems. Usually you end up with something that resembles a brushless motor control with a much more complicated pwm pattern. The pwm duty cycle goes up and drops in time to form a sign wave, but the peak voltage is proportional to the sign wave frequency. The max voltage drops with frequency because the inductance of the motor is fixed and with a low frequency the current would get out of hand quickly. Im not as well read on ac drive as perm magnet motors but this much i do know and it will keep me from making an ac motor controller for the present time.
Joe

 

[knightmb]

I have some issues here. Correct me if im wrong.
thick oil sludge and water are acceptable philosophical terms to apply to electricity, most people dont realize that in a wire the actual speed of the electrons on average is slower than the minute hand on a clock.
Im under the impression that you are suggesting that on an AC system low voltage and high current can be more efficient. How and why is this possible with AC and not DC? Granted there is no net electron displacement, but there is a current, just because you have a negative and positive current that balance out does not negate any resistave losses that you will have.

AC is more efficient at lower voltages for power transmission, but when you get up really high in voltage, DC becomes better due to the AC problem of corona discharge. Because the motors we use are way under when AC has a problem, it's more efficient to use AC on a motor than DC. For the same reason the power company uses AC and not DC to send power to your home. So think of the windings in your motor like a power station. AC will carry better than DC and thus the AC motor has a higher efficiency than a DC motor of the same voltage.

The problem with using a portable AC motor in my view is the power source. We can't carry around true AC generators on our e-bikes, so the next best thing is batteries which are DC only. So then then we need an inverter to change the DC to AC. There is some power loss in the conversion, so it's a matter of figuring out which works best.

Do you stick with DC and take the loss from the motor or do you switch to AC and take the loss from the inverter? With so many variables and the low voltage involved, I think DC is better suited for this task than AC. I'm not sure how the power loss of DC to AC + Motor vs. DC + Motor would really pan out unless we did an experiment to see. The first problem is that you can buy or build a motor suited to many task for the same given power input, it leaves a lot of variables that would make for a difficult experiment.

 

[Leeps]

I disgress, i dont really see ac as being more or less efficient than dc. The same basic rules apply. Ease in stepping voltages is really the only reason the power stations apply ac over dc.
The inverter to drive an ac motor really ends up looking like that of a brushless motor controller, the control algorithim is vastly different but the circuit is essentially the same, there wouldnt be any more loss in the inverter than there would be in the dc controller. As far as peak efficiency goes, the best ive seen go to brushless dc motors.
Joe