# What are the Pros and Cons of AC vs DC?



## einherjar (Nov 8, 2010)

I keep seeing this battle between AC and DC electric motors. Hopefully someone here can set me straight. 

Are there clear benefits to using one over the other? And in which situations?


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## DavidDymaxion (Dec 1, 2008)

Cost forces most folks to go with DC.


einherjar said:


> I keep seeing this battle between AC and DC electric motors. Hopefully someone here can set me straight.
> 
> Are there clear benefits to using one over the other? And in which situations?


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## Sunking (Aug 10, 2009)

If money were no object, most all would use AC. However money matters and DC is inexpensive and very easy to control and regulate speed.


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## Georgia Tech (Dec 5, 2008)

Sunking said:


> If money were no object, most all would use AC. However money matters and DC is inexpensive and very easy to control and regulate speed.


Actually it seems if you want SUPER RAW Horse Power your better off going DC too..... I think AC is more over blown than what its worth personally...
If you can find a good sep Ex motor with interpoles to do Regen I really don't see that much more advantage to AC...


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## Sunking (Aug 10, 2009)

Georgia Tech said:


> Actually it seems if you want SUPER RAW Horse Power your better off going DC too.....


I could careless about HP. Hp sells cars, torque wins the race.


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## Georgia Tech (Dec 5, 2008)

Sunking said:


> I could careless about HP. Hp sells cars, torque wins the race.


 
Well DC is THE way to go for Torque. by nature the DC motor makes nothing but Torque.......series especially


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## EVfun (Mar 14, 2010)

Sunking said:


> I could careless about HP. Hp sells cars, torque wins the race.


Torque goes *nowhere* without horsepower.


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## Sunking (Aug 10, 2009)

Georgia Tech said:


> Well DC is THE way to go for Torque.


Sure at 0 and low RPM and decays proportionally as RPM increases. AC motors can be ran at constant torque or HP throughout the operating range of RPM.

Both have advantages and disadvantages, but it really comes down to denero.


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## Qer (May 7, 2008)

Sunking said:


> Sure at 0 and low RPM and decays proportionally as RPM increases. AC motors can be ran at constant torque or HP throughout the operating range of RPM.


I thought this horse would be kicked to minced meat by now.

You can't run any motor at constant HP because at 0 RPM there isn't any HP to begin with. It takes a while before HP will hit the plateau or peak (depending on pack voltage and settings). The flat torque is proportional to current (absolutely so for DC, pretty much so for AC) so a typical DC torque current will also be flat until it starts to drop due to back EMF, pretty much as for AC.

With a Soliton and a big enough pack voltage the torque curve will be flat up to wrecking RPMs or until you blow it up due to excessive power for the motor. With a small Curtis it won't, but that's not the motors fault.

And I agree, both have advantages and disadvantages but it ain't flat torque which is the difference.


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## steven4601 (Nov 11, 2010)

I think AC motor advantages over DC has been discussed a few times, but hopefully will my understanding help.

I know a little about large DC motors, this what I know about these motors: DC motors used (like in forklifts, series wound) are ideal for non continuous (over)loads. Plenty of starting torque, relative low maintenance, best is that their are relatively cheap. Controlling the speed of a DC motor is the simple but elegant. Varying the (avarage) voltage, varies the rotor speed. Varying the current varies the torque. This can be done with a chopper circuit. I won't go into detail now how chopper circuits work. 
Inside the DC motor an electro-mechanical solution called the commutator, provides the field rotation/phasing needed to keep the rotor spinning after the first step is made. 
Detail: 
Soft carbon brushes press against the commutator contacts to conduct the electricity which magnetize the rotor coils during each step. Mechanical abrasion & arcing of the carbon brushes against the copper commutator wears both components. Mechanical abrasion is nearly irrelevant compared to a second source of wear, electrical arcing. 
Electric arcing due to commutator contact impurities & magnetic energy stored in the rotor during each step represent them selves as a high voltage kick-back. I wont go into detail way they kick back now.

AC motors wear only at their bearings. (Not to confuse with ageing, a motor also ages, insulators age due to dielectric stress, time & heat) No serviceable wearing components such as brushes are incorporated in AC motors. Watercooled & Oil cooled design allow them to be used at much higher continuous power levels than size & mass compared DC counterparts.
AC motors used in the industry are made in various Nema Design classes. A,B,C,D, possibly even more today. Each class actually describes how the motor behaves from start, 0 RPM to max RPM. The reason they made classes is that AC motors can be designed to respond/react specific to load-changes. The higher the class name (D), the higher the available starting torque. There is a catch, higher starting torque designs trade motor slip for torque. This results less torque when the engine speed is near the rotational field speed. This draw back of high-torque motors can be 'ironed' out by using a variable frequency drive, upping the frequency forces torque to be produced at elevated speeds. 
Detail:
AC motors, and brushless DC motors do not have commutators. That means something else will have to do the phase-rotation for the motor. In the industry, the 3-Phase mains power provides all the required commutation. An AC Motor driver/inverter/vfd, or motor controller is required to make the motor turn at various speeds from a DC source. AC motor inverters are technically 3 chopper circuits linked together to create 3-phase AC. The luxury with an advance motor inverter is that complete control of the motor is at your finger tips. Programmable start/spin-up behaviour, custom torque curves, regenerative braking. etc. 
Down side is that ac motor drivers are still quite expensive above certain power ratings. Safety: if an AC motor inverter blows up and shorts the DC-Battery to the motor, it is NOT possible to generate a motor run-away situation where the vehicle speeds out of control. 

//Steven


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## major (Apr 4, 2008)

Hi Steven,

Nice summary, but:



steven4601 said:


> AC motor inverters are technically 3 chopper circuits linked together to create 3-phase AC.


It actually takes 6. Although the diodes can be common.

Glad to have you aboard. Welcome.

major


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## steven4601 (Nov 11, 2010)

Hello major,

I think you are right. I hope i can throw in some more information:

The (3 phase) AC motor drivers/inveters have indeed a complementary set of choppers/switching components for each motor output. Top switchers connected to the positive side of the DC rail and Bottom connected to the negative . 

Each-output stage is capable of sourcing current and sinking current. Push-pull / half bridge, what ever name satisfies the application. Modulating the on-off as in puhs-pull ratio of each stage allows Sine wave's (*1) to be reproduced. Generation of the sine-waves is done in electronic helper circuits which are often of a multi magnitude more complex than the output stage itself.

Add two more of these push-pull / half bridge circuits and there is the output stage of your 3 phase AC motor driver/inverter taking shape. 

//Steven


(*1) sine waves are not magically reproduced by raw on-off switching. Square shaped output signals need filtering by means of filters (Inductors possibly also combined with capacitors). AC Induction motors are already highly inductive, little filtering is needed to create Sine wave currents. The voltages however are still square waves without filtering.


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## Georgia Tech (Dec 5, 2008)

Sunking said:


> Sure at 0 and low RPM and decays proportionally as RPM increases. AC motors can be ran at constant torque or HP throughout the operating range of RPM.
> 
> Both have advantages and disadvantages, but it really comes down to denero.


Well actually then the Sep ex motor trumps you there...


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## JRP3 (Mar 7, 2008)

Don't forget the higher potential RPM's of AC motors. Regarding sepex, no high power sepex controllers exist that I'm aware of.


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