# Am i understanding DC vs AC correctly?



## frodus (Apr 12, 2008)

Yes, but for AC, you will have an encoder (and maybe temp sensor, depending on brand) connections to the controller.


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## feasty33 (Jun 22, 2012)

Thanks for the reply!

By encoder, do you mean sensor board so that the motor controller knows the rotational position of the rotor, so power is applied to the correct phase of the motor?

Dave

BTW my background is 16 years as an Auto-Electrician & 20 years playing with RC cars!!


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## frodus (Apr 12, 2008)

Yeah, encoder would give the controller a signal so it knows rotation direction and speed.

On series wound motors you'll also have another lead going from one of the armature windings to the stator windings, but you'll only have 2 leads from the motor controller.


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## somanywelps (Jan 25, 2012)

There's also brushless DC.


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## major (Apr 4, 2008)

somanywelps said:


> There's also brushless DC.


Make life simple and call it AC  It has the commutation done externally by an inverter which supplies the motor with pulses which in most cases are both positive and negative, so alternate---->so alternating current---->AC.


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## PStechPaul (May 1, 2012)

Also, most three phase induction motors (standard industrial type) do not have sensors or shaft encoders. They do usually have three wires for the three phases, and a ground wire for the enclosure. If it is dual voltage there may be three or more extra wires which need to be connected according to a wiring diagram on the nameplate.

The nameplate is very important, and has a lot of other information such as efficiency, nominal speed, horsepower, etc.


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## ruckus (Apr 15, 2009)

There is also to consider the different 'typical' performance characteristics of each motor type:

1. Brushed DC - Great low end torque even at lower voltages. Torque decreases rapidly at higher rpms (4-6000). Brushes are limited to medium voltages (2-400v) and can suffer arcing and wear issues with the increased amperage/voltage of EV use. 

2. AC Induction - Capable of high voltages (4-800v) and very high rpm (8-12,000+) which make good power ratings with the downside being much weaker torque. No wear items other than bearings. 

3. Bushless DC - Capable of high voltages (4-800v), but limited in rpm (4-5,000). High torque at low and mid rpms. Too much amperage can cause the 'permanent' magnets to become not-so-permanent. No wear items other than bearings.

Any motor type can be wound for higher torque at lower rpm or for less torque and higher rpm. The above are just generalities. Duty cycle, max amperage, and max rpm must be carefully regulated for all 3 motor types or you will end up with a very expensive boat anchor.

Cheers


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## Coulomb (Apr 22, 2009)

ruckus said:


> 2. AC Induction - Capable of high voltages (4-800v) and very high rpm (8-12,000+) which make good power ratings with the downside being much weaker torque.


I would say that DC and AC motors make about the same torque from the same sized motor. Not all induction motors are very high RPM. I know you didn't say they all were, but some readers could assume that.

Example: the AC50 motor makes 100 ft.lbs in a package about the size of a Warp 9", about the same as a Warp 9" at 500 A. Torque is roughly proportional to the size of a motor, AC or DC.


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## ruckus (Apr 15, 2009)

Hi,
Yes, AC can be wound for low-rpm torque as I mentioned, but it rarely is. I was trying to give the competitive advantage of each type. Brushed DC and Brushless DC are both limited to low-mid rpm with smaller motors generally achieving higher rpms (the smaller Glelec's go to 9000rpm). 

There are always exceptions to every generality.

Personally, I am not a fan of weak, high rpm motors with an impressive kw rating but little torque. This means digging down into the gears and winding the transmission far beyond it's design rpm. Not good.

Torque is what you 'feel' when you mash the go-pedal. Torque is what spins the tires. Dyno's measure torque. Power is merely calculated from torque and is extremely biased by rpm and confuses many people.

Two motors with the same torque will feel and accelerate exactly the same. But if one has a 3000 rpm limit and the other 6000rpm, the 'power' ratings will be double for the higher rpm motor. In reality they will feel (push you back into the seat) the same if the torque is the same. 

Electric motor power ratings are only useful for predicting continuous high speed driving ability (like on the freeway). The continuous hp/kw rating of an electric motor is purely based on heat dissipation and has nothing to do with the 'feeling' we think of as 'power' which is actually torque.

Seems a bit off topic, but very relevant if you want to compare motor specs and understand how those numbers translate to 'feel' (look at the torque).

Cheers


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## Coulomb (Apr 22, 2009)

ruckus said:


> Two motors with the same torque will feel and accelerate exactly the same. But if one has a 3000 rpm limit and the other 6000rpm, the 'power' ratings will be double for the higher rpm motor. In reality they will feel (push you back into the seat) the same if the torque is the same.


I agree that they will feel the same _at the start_, in fact right up to where the torque of the first motor runs out. What happens after that? The car with the 3000 rpm torque limit loses the acceleration, while the car with the 6000 rpm torque limit keeps that good feeling till twice the speed. Soon, the other car has to change up to a higher (less tall) gear, so the motor puts out adequate power, but the transmission will reduce the torque seen at the wheels, and hence the acceleration.

So torque is about the force you feel pressing you from your driver's seat, but power is effectively about how long that force lasts. Another way of looking at it is power sets what acceleration you get at the upper end of your speed range. A car where the force "just keeps coming" feels a lot better than "it was great while it lasted".


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## ruckus (Apr 15, 2009)

Coulomb said:


> ..."just keeps coming" feels a lot better than "it was great while it lasted".


Most women ..er, racers agree. 

The problem with electric motors is that you have to pick one winding or another. You can have low rpm torque, or high rpm power, or a bit of both. But not a bunch of both. Take two motors of exactly the same weight and construction. One is wound for 4500 rpm and the other for 9000 rpm. The 4500 rpm motor will easily leave the high rpm motor at the light (because it will have way more torque). Sure, maybe eventually the high-rpm motor will catch up, but by then the race is long over. I would take a neck-snapper over a high rpm screamer any day.

Now on the salt it's a different story...


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