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  #81  
Old 09-29-2011, 01:52 PM
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Ziggythewiz Ziggythewiz is offline
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Default Re: EV High Voltage, Turning it on and off:

The key switch (usually) closes the contactor as a relay for pack voltage.

It is also used by the controller as a safety, so if the contactor welds when you try to turn it off, the controller should not try to power the motor anyway.
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  #82  
Old 11-15-2011, 09:44 AM
Conelrad Conelrad is offline
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Default Re: EV High Voltage, Turning it on and off:

I may be talking out of my hat, but in my years of high-power electrical work, relays/contactors get burns when de-energizing under load.

When they close, and they should close in a hurry, very little arcing is visible as long as all circuits through them are timed the same.

It doesn't seem there would be much going on when time to open the circuit in an EV. No demand, eh, since the controller would've already ramped the current down?

The other item I have not seen mentioned is most professional DC contactors have permanent magnets adjacent to each contact set to blow out the opening arc.

DG
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  #83  
Old 02-12-2012, 09:10 PM
Lee Hart Lee Hart is offline
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by rfengineers View Post
There is nothing wrong with leaving it on all the time as long as your controller terminals are well guarded.
There are two problems with leaving the controller precharged all the time.

1. The controller is "on". If anyone touches the accelerator pedal, the car will MOVE! It can run for a few feet just on the energy stored in the controller's capacitors. It's not safe to leave a car in this condition.

2. The controller is drawing power from the pack all the time. For example, a Curtis controller will be drawing 20 to 50ma continuously. This runs down your battery pointlessly. 50ma will kill a pack of 50ah batteries in about 5 weeks.
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  #84  
Old 02-12-2012, 09:28 PM
Lee Hart Lee Hart is offline
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Default Re: EV High Voltage, Turning it on and off:

I'm coming in late to this discussion. Please forgive the delayed responses. I'm answering comments in the order found, so as to avoid one huge response that covers many comments.

Quote:
Originally Posted by rfengineers View Post
It is not necessary to wait for the light to go out, that could take quite a while for some systems. Just let the light dim sufficiently to indicate that the voltage across the relay contacts is low enough to close without arcing.
I've been using light bulbs as precharge resistors for decades. They are just about ideal.
  1. They are very common and inexpensive.
  2. They have a low cold resistance to precharge quickly (under one second with a 60 watt bulb).
  3. If the controller fails to precharge (such as when the throttle is already depressed, or the controller has failed, or some other load like a DC/DC converter is preventing it), all that happens is the light stays lit. A normal resistor will burn up if it can't precharge.
You want to wait for the bulb to go OUT. If it is "dimly lit", there will still be a substantial voltage across it.
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  #85  
Old 02-12-2012, 09:35 PM
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Default Re: EV High Voltage, Turning it on and off:

Wouldn't some kind of a thermistor be ideal for a pre-charge? At least with my controller the precharge acts as a voltage divider, so the controller will never get near pack voltage (gets a little over 90V from 130 on pack). If you had a resistor that would drop in resistance after warming up a bit it should allow that to get higher.
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  #86  
Old 02-12-2012, 09:59 PM
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by kittydog42 View Post
Those relay contacts are rated for 30VDC max. You will have a very hard time finding an affordable relay with contacts rated over this. Since the current is so low, most people just use these relays anyway for KSI, with no problem, switching pack voltage.
There are parts that are designed and tested at higher DC voltages. For example, the Potter & Brumfield KUEP-3D55-12 is rated at 10a 240vac or 150vdc because it has blowout magnets.

Switches and relays with AC-only ratings are generally good for 1/4th that on DC. A 120vac contact is good for 30vdc; a 240vac contact is good for 60vdc, etc. Since the voltage ratings of contacts in series add (when they're all in the same device), you can also put two 240vac 60vdc contacts in series to switch 480vac or 120vdc. For example, I've used the Schrack PT52B012B (a 4-pole relay with 250vac 6amp contacts) to switch 240vdc at 6a with all 4 contacts in series.

Low current *may* let you get away with a higher DC voltage. The KSI input of a Curtis controller draws less than 50ma, so higher voltages can be switched without problems because the energy in any arc would be reduced. Schrack is good about showing their DC voltage rating versus DC current. For example, the data sheet for the above relay (www.digikey.com) says:
  • 30vdc at 6a
  • 50vdc at 1a
  • 100vdc at 0.4a
  • 300vdc at 0.2a
Note that this is per contact, with a resistive load.

Finally, it is good practice to put a series RC "snubber" across any contact that is switching DC. The values are about 10-100 ohms and 0.1-1uF. They dramatically reduce contact arcing, thus extending the life of the relay.
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  #87  
Old 02-12-2012, 10:08 PM
Lee Hart Lee Hart is offline
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by rfengineers View Post
I would recommend using a relay or similar device to avoid routing high-Voltage into your instrument cluster.
Your wiring practices just need to be appropriate for the voltages involved. If you put a precharge light (or any other device connected to pack voltage) on your dash, just be sure it is wired the same way you could wire household 120vac or 240vac devices. I.e. all wire, connectors, and components rated for the voltage involved, and preferably UL, CSA or CE listed so you *know* someone has tested them.

Also, no exposed wiring or "live" terminals. Everything should be double insulated or in boxes or other enclosures. For example, use double-insulated wires (insulated wires with a second independent layer of insulation, like a round extension cord has). Put the light or meter or whatever in a standard electrical box.
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  #88  
Old 02-12-2012, 11:02 PM
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by Otmar View Post
Precharging is certainly an important safety topic...
Fifteen years ago I made a simple precharge controller for use with Curtis style controllers. You can see a schematic of it here:
http://cafeelectric.com/downloads/precharger_1.1.JPG
It's GREAT to see Otmar posting on this topic. He's the Grande Master of controller design! Read his post carefully, folks.

I like his circuit. It's not fail-safe, but it's pretty darned good. It properly senses voltage, and won't engage the main contactor if the controller fails to precharge. It is protected against reversed polarity (miswiring), is not sensitive to pack voltage, and doesn't need a relay to control the main contactor coil. It doesn't need a micro, and doesn't depend on assumptions about precharge times. Using a thermistor as the precharge resistor keeps it from burning up if the controller fails to precharge (as does a light bulb).

Improvements:
  • I'd use a better relay than the Radio Shack one at K5 (like the Schrack PT52B012B I mentioned in an earlier comment).
  • Add a 10 ohm resistor in series with C1.
  • I prefer a light bulb over the thermistor.
  • I'd connect the left lead of bridge D3 to the top of R4 instead of J1. This prevents current from flowing from pack to controller through D3-T1-D1 and U1 with the key off.
  • Move the upper lead of C2 from J3 to the collector (pin 4) of optocoupler U1. C1 should be about 0.2uF. This keeps the main contactor from pulling in for at least 20msec, to allow time for K5 to pull in and the precharge circuit to begin functioning.
  • Use a bidirectional 24v zener in place of D4 (1.5KE24CA etc). It lets the main contactor turn off much faster.
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  #89  
Old 02-12-2012, 11:14 PM
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by Lee Hart View Post
It's GREAT to see Otmar posting on this topic. He's the Grande Master of controller design! Read his post carefully, folks.

Thanks for the reminder and improvements. May need to upgrade from my 2 piece 'circuit' one of these days.
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  #90  
Old 02-12-2012, 11:20 PM
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Default Re: EV High Voltage, Turning it on and off:

Quote:
Originally Posted by wb9hco View Post
I was wondering if there would be a down side to relocating the [contactor] so it would be placed between the capacitors and the rest of the electronics in the controller.
This would work as long as you NEVER had current flowing in the motor when this contactor turns off. If it happens even once, the diodes and transistors in the controller would experience a hellish voltage spike.

The transistors, diodes, and capacitors form a "golden triangle". They must all be tightly connected with very short, low-inductance, low-resistance leads or you get horrible transients that will kill your parts.
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