WHY USE HIGH VOLTAGE DRIVE SYSTEMS

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nino500
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Postby nino500 » Tue Sep 09, 2008 1:01 pm

mainly because AC is a killer esp at high volts - and i dont like messing around with it


I'm no engineer but I'm pretty sure that DC is a whole lot more dangerous than AC at high voltages. If you touch a high voltage DC terminal the muscles tense up and you can't let go, so you fry. With AC you get a buzz in time with the supply frequency but it usually throws your hand off (not that I'd like to test it!!!).

Sticking to below 50volts on either system would be safest, of course.

I have a feeling Jeremy's muti motor, multi controller project might be very interesting to those investigating low voltage setups.

JonSpence
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Postby JonSpence » Wed Sep 10, 2008 1:32 pm

nino500 wrote:
mainly because AC is a killer esp at high volts - and i dont like messing around with it


I'm no engineer but I'm pretty sure that DC is a whole lot more dangerous than AC at high voltages....


To be honest at the sorts of power required to get an EV to work the power system IS dangerous. More than 50mA through the chest will kill you and remember that we regularly talk in terms of 100's of amps.

The "is AC more dangerous than DC" debate dates back to Edison and Westinghouse and provided the Americans with the electric chair (Edison demonstrated it to convince the public of the dangers of his rival's system).

It's also unwise to think that because the motor is a "DC" motor and that the controller is for "DC" control that there won't exist AC voltages in the system. There WILL (due to the way the controller works), and they may be quite high. This is of course only the case when you run the motor, but that's true of AC systems to.

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Jeremy
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Postby Jeremy » Wed Sep 10, 2008 4:50 pm

The bottom line here is that AC voltages over about 50V and DC voltages over about 75V are dangerous, in that above these values sufficient current can be forced through the rather resistive human body to interfere with heart conduction systems, disrupt the central nervous system and disrupt autonomous bodily functions.

In practical terms, there is less likelihood of getting an accidental fatal shock with an AC supply than a DC supply, as the reversing muscle contractions that result from a low frequency AC shock may throw you clear of the connection. DC tends to have the opposite effect and may result in a muscle contraction that retains the connection or even makes conduction somewhat better (by inducing a tighter grip, for example).

In terms of lethal effect, low frequency AC may well be slightly more likely to quickly disrupt heart conduction patterns than DC. It may well be that AC is more effective as a way of killing people who are strapped to an electric chair, as Edison so effectively demonstrated all those years ago.

Jeremy

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qdos
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Postby qdos » Wed Sep 10, 2008 5:11 pm

certainly is dangerous stuff no matter what flavour it is. Because people handle small batteries everyday it's not surprising that people get complacent to it all. Ask yourself would you mess around with the plug even for one second you charge the car up with for several hours pumping all those amps into those little boxes ?

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nino500
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Postby nino500 » Wed Sep 10, 2008 7:12 pm

Might be an idea to discuss safe working practice while we're here.

My usual trick is, if I have to work on a high voltage system when it's powered up (checking voltages at test points etc.) I make sure I'm not grounded (plastic or rubber soles) I use one croc clip and one probe on my test leads, so I can make and break all connections using only one hand. If I'm at the bench, I sit on my spare hand (the right in my case, as I'm a lefty). If I'm working on a machine in situ, I put the spare hand firmly in a pocket. This ensures that a circuit cannot be completed between my two hands (one touching live and the other neg) which would go through my chest.

I know you can get insulated spanners for battery and motor connections but if you can make the connections removable with one hand(captive nuts etc.) that would probably help.

I'm not a professional so any useful tips would be appreciated.

JonSpence
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Postby JonSpence » Thu Sep 11, 2008 1:44 pm

nino500 wrote:Might be an idea to discuss safe working practice while we're here.


I'd also add to your list that you should keep a co2 fire extinguisher handy.
Use safety specs, if working with Pba batteries have a solution of bicarb handy and make sure that you have good ventilation.

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EVguru
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Postby EVguru » Thu Sep 11, 2008 1:46 pm

arnolde wrote:I'm building an EV with a 660V pack (400V AC motor), and I'm a little worried about what kind of fuse and contactor to use. At upto 800VDC/20A (during regen) the fuse and contactor between batterypack and controller must be high-voltage, spark retarding type. Any ideas?


I'm worried about you building a vehicle at that voltage.

My question would be why? I've been involved with several high voltage EV's, but noting remotely at that level. You're going to be struggling to find cable rated at above 600 volt and DC rated fuses are going to be really scarce.

The best I could find so far is a solid-state switch 25A/800VDC which costs 150 EUR.


There are contactors rated at suitable voltages, the Killovac EV200 for example http://relays.tycoelectronics.com/datasheets/ev200.pdf
or the Gigavac (guess how they came up with that name) GX12 http://www.gigavac.com/products/contact ... /index.htm

Unfortunately the controller (SEW MDV60A) has no power-off or standby mode, so I need to break the battery DC circuit manually when shutting down.


Which model of MDV60A? The largest 440VAC model only has a maximum phase current of 100amp. Typically industrial AC motor controllers only have a low speed torque boost of 2:1, EV practice is more like 4:1. This is usually achieved by reconfiguring the motor windings in parallel, turning a 220volt motor into 110volt or lower. That way you can keep the V/F ratio normal further up the rev range.
Paul

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