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Old 02-26-2009, 05:00 PM   #1 (permalink)
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AC vs DC for DIY

Okay okay... i am sure the questions "what's better, AC or DC" would bring a million hits on google by now. I have read many pages discussing the differences and they all make assumptions that i don't like.

First assumption - AC controllers cost more. NOT TRUE! At least not if i DIY my own controller! AC or DC does NOT matter on the controller side. +/- a hundred bucks in components, whatever.

Second Assumption - AC drive systems cost a lot, so you'll probably want $50k in top of the line batteries. HUH? Why? What diff does it make what batteries I have driving the motor?

So the real questions... If i have a 144v battery pack, plus a DIY controller... what is the real difference between an AC or DC drive?

From what I read, AC is more power efficient. True/false?

What about motor cost? If two cars had the same pack/controller, and they both had the same speed and range, but one was AC and the other was DC, which one has the more expensive motor?

I can't seem to find a 3-phase 110v motor. There are 1-phase 110's or 3-phase 240/480's. Can someone explain some of the differences?

I'd like to make a lengthy discussion of this. So the terms of car A vs car B - same battery pack, DIY controller, one is AC, one is DC... go.

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Old 02-26-2009, 05:12 PM   #2 (permalink)
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Overall, I believe AC is a more efficient setup. Don't ask me why, but thats what I've read/heard too. It is also much better for regen setups (again not sure why). Almost all OEM electric cars are AC.

I don't think three phase 110V motors are too prevalent. Three phase is normally used in more industrial setups and they generally run 240V or higher. Single phase 110V would be any 'ol motor you can plug into a normal wall socket obviously. Finding one large enough to power a vehicle may be a problem.
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Old 02-26-2009, 05:39 PM   #3 (permalink)
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A 208v 3 phase motor (208v because it is the lowest voltage common industrial electric motor) can be run from higher frequencies and voltages for a short time for more power or from lower frequencies and voltages for lower speed. Combine that with the fact that motors are rated by continuous horsepower and it actually isn't that difficult to find one that will outperform the original engine.

All modern production electric cars use 3 phase AC motors.
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Old 03-02-2009, 01:11 PM   #4 (permalink)
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So i was doing some research into motor control for AC drive circuits. I figured I had all the knowledge i needed for the control section. Apparently there are big gains to be had by making it complicated instead of easy. Funny that.

So i was planning on using a control type called "constant volts/hz". So at rated frequency (60hz) you apply rated voltage (208/phase), and you scale it up/down according to speed. Apparently this is not very good and the latest and greatest is called Field Oriented Control.

Here is a paper about some different control types. I think you need to be an engineer or some other kind of geek to understand this, though:

http://www.ab.com/drives/techpapers/PWMDrives01.pdf
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Old 03-02-2009, 03:18 PM   #5 (permalink)
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I'm going to turn this into sort of an idea-bouncing thread on AC control. Any EE's out there want to help me out with this one... I want to use an Hbridge to produce a sine wave. I understand the idea of PWMing the values of a sin table, it is the bridge that is confusing me. Lets talk about a 1-phase motor for now. The H-bridge has 2 high and two low mosfets. Lets call them AH, AL, BH, BL to represent side A and B of the motor and High and Low connection.

To generate a sin, is this correct:

Turn on AH 100%, BH 0%
PWM BL from 0 through 100% duty and back to 0 (sinusoidally)
Turn on BH 100%, AH 0%
PWM AL from 0 through 100% duty and back to 0 (winusoidally)

To me, this looks like the way to produce a full sin wave across the load in the H with minimal switch loss (high side staying on). Is there any problem with the high side staying on through a 180* and switching for the next 180*?

How does synchronous rectification play into this? Or does it? Would sync.rect. make my H-bridge turn into 6 mosfets? Or 8?

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Old 03-02-2009, 04:22 PM   #6 (permalink)
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Out of my league! I can only parrot things I've heard about AC benefits elsewhere, and they've mostly already been mentioned here.

AC advantages: 1) can be more efficient; 2) easier to implement regen; 3) seem to be higher RPM capable, so a 1 speed gearbox can save some weight/reduce drivetrain losses; 4) cool Jetsons sound!



Aside from all the AC drive hybrids, I've driven one OEM AC EV (Solectria Force sedan), and I liked it a lot.
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Old 03-02-2009, 04:28 PM   #7 (permalink)
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I'm interested in the often repeated advantage number one. "AC can be more efficient". Why is this? Is it because the proximity from the winding to the pole is actual a perfect sinusoid? I have read that driving an AC motor with a square wave, or a 6-step is inefficient - isn't a DC motor just the same as an AC motor except that it mechanically transforms the DC signal into a square wave AC? (what's that part called, the commutator or something?)

This Field Oriented Control concept is very interesting. It would be worthwhile to design a controller that has all the inputs required to use FOC, but start with a simple Volts/Hz controller. That paper shows massive advantages to FOC over V/Hz - specifically, it shows "DC motor-like" low-end torque... which is one of the main benefits of a DC motor.

btw, LOVE the motor sound
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Old 03-02-2009, 08:35 PM   #8 (permalink)
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FYI, the place to go for AC/DC consideration is the EVDL archives. A quick search found this thread, for example:

EVDL Archive / Forum Interface - Electric Vehicle Discussion List

Hope the link works. There's much more where that came from.
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Old 03-02-2009, 09:32 PM   #9 (permalink)
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From my short looking around on the subject it seems like AC has a wider power curve so it can put out good power higher up in the rpm range. Where DC goes down pretty consistently from peak torque at 0rpm. This gives you a much wider usable rpm range compared to a dc motor. So for example the motor I have I think has a workable range of 0-4000 and it actually has a good bit of power at 4000.

Plus you could do neat stuff with your own controller like mess with the timing so you could effectively have variable timing on the motor. A DC motor you have to rotate the brushes to change timing. I am not sure that would help anything but it is a possibility with an AC motor.

For regen, a DC motor is a generator so it puts out a voltage based on the shaft RPM so it needs a DC-DC converter and other junk to boost the voltage to something that can charge the pack and is generally not going to get you much extra power. AC motors are an alternator so you just adjust the field current to vary the voltage output so they can generate high voltages even at low RPM.
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Old 03-02-2009, 11:19 PM   #10 (permalink)
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Quote:
Originally Posted by Coyote X View Post
For regen, a DC motor is a generator so it puts out a voltage based on the shaft RPM so it needs a DC-DC converter and other junk to boost the voltage to something that can charge the pack and is generally not going to get you much extra power. AC motors are an alternator so you just adjust the field current to vary the voltage output so they can generate high voltages even at low RPM.
Actually, it is possible to use the motor windings as an inductor in a boost converter. That is true of both AC and DC motors.
http://techno-fandom.org/~hobbit/cars/boost-hack/

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