Pure horse power DC vs AC

140205 Views 350 Replies 49 Participants Last post by toddshotrods, May 23, 2010

Ioku

Discussion Starter · May 24, 2008

I know its hard to really find the max house power of an electric motor but I'm wondering what is generally more powerful an AC motor or a DC. This cite here http://www.electricvehiclesusa.com/category_s/67.htm states this "The WarP 13" diameter series wound DC Motor is a high performance motor used in some racing Electric Vehicles and could be used in very large vehicle. We have heard of uses of this motor, developing over 2,000 hp!" Over 2000 hp so what would that mean for the warp 11, 1000 hp. And how about the AC motors found here http://www.metricmind.com/index1.htm, some for them look very powerful but it seems hard to compare them to the DC motors. So if I'm just looking for high power what do you think would be better AC or DC.

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lazzer408

· #2 · May 24, 2008

You can't beat a series wound DC motor for pure low-rpm torque. On the other hand, AC motors can run at very high rpm and geared down to make torque. There's pros and cons for either. The DC motor is usually cheaper and easier to control. The AC motor has a much more complex controller adding to the cost. DC motors have brushes and a commutator to wear out. The AC does not. I think DC motors even sound better. =)
If one style of motor had any serious advantage over another we wouldn't have a choice. An example of this would be PM motors. It's not very cost effective to make a 13" PM motor for use in an EV. That's why we have series wound and AC motors. AC motors are begining to gain popularity as the "technology" comes down in price. Their already replacing series wound motors in golfcarts.
If I could afford it I'd be using AC. I think it has more advantages. Higher voltage being one of them. I can't fit 20+ lead acid batteries on a motorcycle chassis or afford lions so I'm using a series wound.

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major

· #3 · May 28, 2008

Ioku said:
This cite here http://www.electricvehiclesusa.com/category_s/67.htm states this "The WarP 13" diameter series wound DC Motor is a high performance motor used in some racing Electric Vehicles and could be used in very large vehicle. We have heard of uses of this motor, developing over 2,000 hp!"
<snip>
So if I'm just looking for high power what do you think would be better AC or DC.

Hey Ioku,

Don't believe everything you read. 2000 hp. NOT!!!!! And in an EV context? What do you think a 2 megawatt battery would weigh? 2000 hp? I don't think even railway locomotive traction motors will hit that, and they are 5 to 10 times the motor.

"just looking for high power" you say, AC or DC? Well, you can always go bigger for more power. So let's compare on size or mass. Call it power density. The high power dense motors are AC. This is primarily due to the fact that AC motors can operate at much higher speed. Some of the development has impressive hp/lb at 50 to 60,000 RPM. Even those used in the Prius are AC at 12,000 RPM and have great power density.

But at some point you have to get real and look at the hp/$. Then the line becomes fuzzy, AC vs DC. And a lot depends on you application constraints.

Regards,

major

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mattW

· #4 · May 28, 2008

The other major advantage of AC motors is that a lot of them are liquid cooled which would be the major limiting factor for serious hp. The more amps you throw in a motor the hotter it will get, but an AC is more efficient so you don't need to dissipate as much and is liquid cooled so it can dissipate more. Also the higher voltages mean its easy to get the amps to the motor... I cast my vote for AC, though the current records for both MC and car are with series DC so I could be wrong or that could be a cost thing.

lazzer408

· #5 · May 28, 2008

Most of the DCs I see at the track are air cooled. If AC was naturally more efficient, why would it need to be water cooled? If it was more efficient it would run cooler for the same output as a DC. More voltage to the motor means less current draw for the same output. In an AC motor, the source of the repelling magnetic fields, for both the armature and stator, is provided by only the stator windings. In DC both fields have their own windings. That could account for a stronger field? Motor theory isn't my cup of tea. Someone else would have to chime in on that thought.
On the other hand, radio controlled cars and planes are switching to the brushless motors and they are FAST.

Wirecutter

· #6 · May 30, 2008 (Edited)

major said:
Don't believe everything you read. 2000 hp. NOT!!!!! And in an EV context? What do you think a 2 megawatt battery would weigh? 2000 hp? I don't think even railway locomotive traction motors will hit that, and they are 5 to 10 times the motor.

Thanks for that, major. Great laugh. I've seen that number before, too, but when you put it that way, it really does sound ridiculous.

A coworker once ran the numbers on an EV with 200+ mile range and a 5 minute (or similarly very short) recharge time. Using conservative estimates for vehicle weight, it turned out that it took an incredible amount of power draw to acheive the stated recharge time, simply by virtue of replacing enough energy to move a car 200 miles in such a short time. (edit: Tesla -> 53kWh battery. To replace that 53kWh (or 3.18 megawatt-minutes) of energy in 5 minutes takes a 636kW draw, assuming everything's 100% efficient, which it's not. My house has 200A service, which at 220v works out to 44kW.)

It reminds me of the stereo maker back in the '70s that rated their stuff in "Maximum Music Power" and gave insane numbers for wattage. They got this number by putting the amplifier on a perfect power supply, driving a half cycle spike through it, then claiming that this instantaneous power was the fabled "MMP".

-Mark

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Dennis

· #7 · May 30, 2008

Most of the DCs I see at the track are air cooled. If AC was naturally more efficient, why would it need to be water cooled? If it was more efficient it would run cooler for the same output as a DC. More voltage to the motor means less current draw for the same output. In an AC motor, the source of the repelling magnetic fields, for both the armature and stator, is provided by only the stator windings. In DC both fields have their own windings. That could account for a stronger field? Motor theory isn't my cup of tea. Someone else would have to chime in on that thought.

On the other hand, radio controlled cars and planes are switching to the brushless motors and they are FAST.

While an AC motor is more efficient it will over heat just like any electric motor would if you demand it's peak horsepower for too long and therefore the insulation will start to breakdown. AC motors have whats called service factor rating which means they can be overloaded above their continuous horsepower rating for short durations. Usually the service factor is between 1.15 to 1.25 for air cooled AC motors. Now if it was water cooled then the AC motor can be under severe loading and not overheat since the water will keep the stator windings cool.

xrotaryguy

· #8 · May 31, 2008

Also, the AC motors that are water cooled were generally designed to be EV motors. In fact, no one actually designs a DC motor from the ground up to be an EV motor. Most large electric motors do duty as hydraulic pumps, elevator motors, industrial air compressor motors, CNC machine motors, etc. These applications don't demand much of the motor and so the motor doesn't need a very capable cooling system. A motorist will place much more demand on his motor than a than a lift equipment operator, so the motor needs to be designed in a more rugged fashion from day one.

AC motors can run at higher voltage because they have no commutator to arc or melt. This means that AC motors can run lower amperage to get an equivalent amount of power. That's why AC motors are more efficient.

Can you guess that I like AC motors better?

Most drag cars use lead acid batteries. Is that because lead is better?

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lazzer408

· #9 · May 31, 2008

Why do so many industrial vehicles use dc motors? You'd think an industrial vehicle would want power, longevity, and low maintenance. Yet they chose dc. My guess would be cost. That doesn't make much sense because AC motors cost less to manufacture.

saab96

· #10 · May 31, 2008

lazzer408 said:
Why do so many industrial vehicles use dc motors? You'd think an industrial vehicle would want power, longevity, and low maintenance. Yet they chose dc. My guess would be cost. That doesn't make much sense because AC motors cost less to manufacture.

AC controllers cost more.

Dennis

· #11 · May 31, 2008

Also, the AC motors that are water cooled were generally designed to be EV motors. In fact, no one actually designs a DC motor from the ground up to be an EV motor. Most large electric motors do duty as hydraulic pumps, elevator motors, industrial air compressor motors, CNC machine motors, etc. These applications don't demand much of the motor and so the motor doesn't need a very capable cooling system. A motorist will place much more demand on his motor than a than a lift equipment operator, so the motor needs to be designed in a more rugged fashion from day one.

AC motors are put under much more strain than a car would be. Examples are AC motors used to lift large crushed rocks up a steep inclined conveyor into a silo, drilling, train propulsion, shredders, rock crushers, ultra high viscosity mixers, mills, and mining truck propulsion. Non of these motors though are water cooled, but are large and heavy. With water cooling you can stick with a small motor and peak horespower the living daylights out of it compared to one being air cooled. So weight reduction is possible.

Dunkard

· #12 · Jun 1, 2008

With industrial equipment size and weight are not usually such an important consideration, nor is efficiency for long range, but cost is certainly very important.

J_R_S

· #13 · Feb 23, 2009

Every time I read a comparison discussion between DC and AC motors it seems that there is something missing. The logic usually goes like this:

A series wound DC motor is tops for pure low-rpm torque and a DC motor is usually cheaper and easier to control. AC motors can run at much higher rpm and be geared down to make torque, are more expensive and complex to control. The higher rpm factor always seems to win out and push people towards AC motors. But, wait, follow the logic: If a high-rpm AC motor can be geared down to make torque, then doesn't that mean that a lower-rpm DC motor can be geared up to make more rpm?

If so, doesn't that put the DC motor back on top again? Particularly when you consider that if you're going to have storage batteries on board then you must store that energy in DC form and then waste 10% to 20% of it converting it to alternating current for your AC motor to use. An on-board generator/alternator creates electricity as DC, which then must also be converted to AC as well. But with a DC motor, both sources of electrical "fuel" can go straight to the motor with no conversion.

I can't help but think that the current enthusiasm people have with high-rpm AC motors might not be leading us down the same garden path that Edison did when he convinced the public that Tesla was wrong about DC current being the way to go. At least Edison was right that you need AC to transmit power over long distances -- but that's not an issue with EVs.

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Dunkard

· #14 · Feb 23, 2009

I agree with your opinion about DC EV motors, except you've got Tesla and Edison confused with each other.

Edison was the guy who promoted DC for everything because he owned the patents on it - in spite of the fact that he knew as well as anyone that AC was better for certain things. Edison promoted a DC town-distribution system that was very dangerous, and stupid by todays thinking. Edison also promoted the use of the electric chair as a method of execution - but only when using AC power - as a false tactic to spread lies about the "dangers" of AC power. Edison was smart, but was also a nasty skunk.

Tesla was the guy who promoted AC, invented AC motors, got screwed out of a truck load of money while he was an employee of Edison, then died broke because he had given up his per horsepower royalty on the sale of AC motors by Westinghouse, to save Westinghouse's butt when they were in trouble. Later when Tesla was broke, Westinghouse never returned any favors.

But on the real point, I am starting to reconsider my thoughts about DC for electric cars because I've recently seen some nice DC motors with better efficiency than I would have thought possible a couple of years ago. Upwards of 88 to 90 percent.

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CroDriver

· #15 · Feb 23, 2009

Dunkard said:
I agree with your opinion about DC EV motors, except you've got Tesla and Edison confused with each other.

Edison was the guy who promoted DC for everything because he owned the patents on it - in spite of the fact that he knew as well as anyone that AC was better for certain things. Edison promoted a DC town-distribution system that was very dangerous, and stupid by todays thinking. Edison also promoted the use of the electric chair as a method of execution - but only when using AC power - as a false tactic to spread lies about the "dangers" of AC power. Edison was smart, but was also a nasty skunk.

Tesla was the guy who promoted AC, invented AC motors, got screwed out of a truck load of money while he was an employee of Edison, then died broke because he had given up his per horsepower royalty on the sale of AC motors by Westinghouse, to save Westinghouse's butt when they were in trouble. Later when Tesla was broke, Westinghouse never returned any favors.

But on the real point, I am starting to reconsider my thoughts about DC for electric cars because I've recently seen some nice DC motors with better efficiency than I would have thought possible a couple of years ago. Upwards of 88 to 90 percent.

...and he was from Croatia. Just like me

Bowser330

· #16 · Feb 24, 2009 (Edited)

A DC motor purposefully refurbished for High Voltage EV use could compete very well with an AC system....and for a fraction of the price...

240V Interpoled DC motor with 800A for 5 seconds would have 257hp...of course you have to take back emf into the picture, but the point is, 257 hp is nearly as much as the tesla has....

and for the price of the tesla drivetrain (approx 25,000$) you could get 2 high voltage DC systems and have plenty of left over cash to invest in some lithiums...

So even with back-emf you would have 300-400hp with a dual dc setup...

etischer

· #17 · Feb 24, 2009

An AC motor has constant torque up to base speed, and is constant horsepower from base speed to max speed.

This is a huge advantage. I think if you look at the area under the hp*rpm curve, the AC motor will be doing more work.

Wirecutter

· #18 · Feb 24, 2009

Bowser330 said:
257 hp is nearly as much as the tesla has....

Actually, it's quite a bit more. The Tesla is closer to 200 HP.

Bowser330 said:
and for the price of the tesla drivetrain (approx 25,000$) you could get 2 high voltage DC systems

Uh, how do you figure this price? I'm thinking it's much more. The base vehicle (Lotus Elise) runs $60,000 well optioned, and a Tesla is what, $110,000? Anyway, I tried to run the numbers with the AC-150 drivetrain, the 6831 lithium ion 18650 cells, and wild guesses for the modifications to the base vehicle. The only way I could imagine that Tesla Motors could make money on the car was with volume purchasing discounts.

I suppose it's a moot point, since AC Propulsion won't sell its drive system to anyone who doesn't know the Secret Handshake...

-M

Zer0

· #19 · Feb 24, 2009

He said price of the Tesla Drivetrain. The Drivetrain itself includes only the motor and the controller, really, and it's a well known fact that the AC-150 from AC Propulsion is $25k.

That's not counting the batteries.. which have been estimated to be in the $20k range.

No-one really knows how much the glider (car without batteries and Drivetrain) costs. You can't use the Elise as your ruler because
a) the price you quote includes a combustion engine drivetrain
b) the Tesla is designed with a similar architecture, but is, in fact, NOT an elise. they only share a handful of interchangeable parts, which means that price includes almost no Tesla parts. It would be a joke to buy an elise to turn it into a Tesla. It's like buying a Camaro to turn it into a Corvette.

He was simply quoting a price for the drivetrain, nothing more.

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Bowser330

· #20 · Feb 24, 2009

Zer0 is correct I only meant the drivetrain and thats because just as you said, Wirecutter, I thought they used the AC-150 for 25K$...

However...after looking at this... it says 248hp....so....maybe its an upgraded version?

http://www.teslamotors.com/performance/perf_specs.php

Also I thought I read an interview with Tesla where they said that half the cost is the battery pack, 50K$...i believe it too...but maybe Im wrong and youre right..20K$...seems too cheap for a 200+ mile pack though! doesn't it?

and for sure a lot of cost comes from the carbon fiber body and custom design R&D cost...they're trying to get their investment back!!! haha...makes sense...

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