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Default Precharge, what is it, why do I need it, how do I do it.

The PWM motor controllers common in EVs have a sizable bank of capacitors on their input. When you apply a Voltage across a capacitor it initially appears to be a short-circuit, that is, the Voltage across the capacitor is zero. If there is very little resistance in the circuit, e.g. a closing contactor with no precharge, then the current will be very high. Nearly all of the traction pack voltage will be across the closing contacts. The large Voltage difference and sudden high current (known as an inrush current) can cause damage to, and in extreme cases, welding of the relay contacts. Also of concern to some is the stress on the controllers electrical components caused by the inrush current.
{see Contactor with no precharge.}

This can all be prevented by the use of a precharge resistor across the contacts of the main power relay. The precharge resistor allows the capacitors in the controller to slowly charge BEFORE the contactor closes. This means that there is less voltage across the closing contacts and little or no inrush current.
{see Contactor with precharge}


The problem with having a precharge resistor across the contactor is, there is high Voltage on the controller terminals even when the car is turned off. This is because the capacitors remain charged all of the time.
I've heard it argued that keeping the caps charged all of the time keeps them 'fully formed' and thus, extends their life. While this is technically true, it is not really an issue with modern capacitors. Unless you plan on putting your controller in storage for years, the capacitors will likely outlast their associated active components (transistors and diodes) whether you keep them fully formed or not.

Many DIY'ers add some sort of power switch, circuit breaker or disconnect to remove the high Voltage from the controller when the car is parked.
{see WithPowerSwitch}

This solves the 'high Voltage on the controller' problem BUT introduces a new wrinkle. You must now turn things on in the correct order or you will defeat the purpose of the precharge resistor.
For example, if you first turn on the contactor and then close the power switch there will be no precharge. You will have reintroduced the high Voltage/large inrush current problem.
In this case, you must first close the power switch, wait an appropriate precharge delay period, then close the contactor.


If a precharge switch is added in series with the precharge resistor it can be used to turn the high Voltage on without switching a large current flow, as is done with the contactor or power switch.
{see WithPrechargeSwitch}

In this configuration the power switch becomes an emergency disconnect that is normally left on. The precharge switch is turned on first and then, after a delay, the contactor closes.

This is different than the previous design because now the "on switch" (the precharge switch) can be a relatively small relay and the turn-on sequence can be easily automated to avoid closing the contactor before precharge.


Here is how I did it. I have a Step-Start device that turns on the precharge relay when the start signal is received (the ignition key is turned to the START position). After a time delay the contactor is turned on.
{see StepStart}


There are additional safety and convenience features of the Step-Start Device, but the basic function is to make sure that the precharge relay is always turned on BEFORE the contactor and that at least some minimum amount of time passes between the two events.
Attached Images
File Type: png NoPrecharge.png (1.6 KB, 1119 views)
File Type: png WithPrecharge.png (1.7 KB, 1104 views)
File Type: png WithPowerSwitch.png (2.1 KB, 159 views)
File Type: png WithPrechargeSwitch.png (2.7 KB, 164 views)
File Type: png StepStart.png (3.1 KB, 180 views)


Contributors: rfengineers
Created by rfengineers, 12-21-2008 at 08:09 AM
Last edited by rfengineers, 01-10-2011 at 04:54 PM
52 Comments , 29419 Views
  #2  
Old 12-23-2008, 09:33 AM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

Nice! Thanks for the contribution!
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Old 12-27-2008, 07:02 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

Quote:
Originally Posted by TX_Dj View Post
Nice! Thanks for the contribution!
Yes, something I definitely needed help with! Thanks!

Looks like I got a second contactor for Christmas. Is there anything wrong with using your 2nd to last schematic, and having my second contactor be the "precharge switch"? When I turn the car to "on" it will engage the precharge switch. When I press the pedal my PotBox will engage the 2nd resistance free contactor? As long as I wait a few seconds before hitting the "gas" (what most people probably do anyways), then I am precharging correctly?
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Old 12-28-2008, 09:42 AM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

Quote:
Originally Posted by ClintK View Post
Yes, something I definitely needed help with! Thanks!

Looks like I got a second contactor for Christmas. Is there anything wrong with using your 2nd to last schematic, and having my second contactor be the "precharge switch"? When I turn the car to "on" it will engage the precharge switch. When I press the pedal my PotBox will engage the 2nd resistance free contactor? As long as I wait a few seconds before hitting the "gas" (what most people probably do anyways), then I am precharging correctly?
The only thing wrong is you are using a very nice, heavy duty contactor to do a job that a much smaller (less expensive) relay could perform. There is not much current flowing in the precharge leg of the schematic.
The precharge resistor will do a good job of limiting that current to a very small amount, so you don't need a big contactor there.
I am planning on addressing the question of: "Contactors, how many and where" in one of my next "lessons."
In answer to your other question, With a small enough (in resistance value) precharge resistor, a few seconds CAN be enough time. It is all a matter of how low should the voltage across the contacts be before they close (to reduce arcing and contact damage).

Joe
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Old 07-19-2009, 09:54 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

I am using a 60 watt 120V household lightbulb for my precharge resistor on my 144v EV. The key switch turns on the contactor on the negative side of the pack. The precharge circuit is connected to the hot side of the contactor at the positive end of the pack. Thus, turning on the keyswitch enables the precharge and the lightbulb lights. On the Raptor controller, once precharge is complete the controller turns on the positive contactor. With a 60 watt bulb, this takes about 5 seconds.
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Old 02-13-2010, 07:34 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

Can I precharge my caps with the 12v system during bootup.
If my main contactor low volt (coil) circuit is on a 4 second delay, I switch on the 12v system that parallels though the high voltage circuit via a diode instead of a resistor, or would I still need a resistor.
Then when the main contactor brings on the high voltage the diode stops the 12v system from nuking.
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Old 02-17-2010, 04:05 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

The "Pre-charge" is to charge up the capacitors in the controller input circuit to full pack voltage (144 V.?) At a controlled rate and not as an "Inrush" Surge. that is what the resistor or household 120 V. light bulb id there to do. It limits inrush or initial charge-up of the input capacitors as otherwise the initial surge will shorten the life of the power contactor it is connected across. To start charging with only the 12 volt is unnecessary, and potentially a problem because the 12 v. auxiliary supply,(Battery and/or dc/dc) is preferably ISOLATED from the high voltage of the 144 v. traction battery pack. If you have more questions ask them (I just LIVE to give answers.)
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Old 08-18-2010, 09:25 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

In case they didn't know Precharge is very important because when current-limits the power source such that a controlled rise time of the system voltage during power up is achieved.When high voltage systems are designed appropriately to handle the flow of maximum rated power through its distribution system, the components within the system can still undergo considerable stress upon the system power up.
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Old 08-25-2010, 07:02 AM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

To get around the correct sequencing of the precharge ono my tractor I have a main isolator and a conventional ignition switch.

The isolator has to go on first or nothing else works.
Then the ignition key is turned to the accessories position which allows power to the instruments and other low current circuits.
Then the key is turned to the running position where the controller main isolator has feed, but is not energised, and the precharge resistors are powered.
This allows the caps to charge up for a while before I flick the key to the momentary 'starter' position. That sets the main relay to latch closed powering the controller and all the high current circuits.

Any loss of power in any part of the circuit will cause the main controller contactor to unlatch but leave the precharge resistors on. A quick flick of the key to the starter position will restart the controller if all is well.

I don't know how useful this would be in a road car but it works well on the tractor.
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Old 08-25-2010, 09:21 PM
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Default Re: Precharge, what is it, why do I need it, how do I do it.

I can't see the schematics in the original posting. Are they still visible to others?
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