[rfengineers]

I am starting this thread so I will stop hijacking other peoples conversations in order to sell my step-start delay.

There are a lot of opinions on how to apply power to the controller. There are safety considerations for both the equipment and people. If we can work out some generally agreed upon ideas here then I will consolidate them and put them in the WIKI.

Some of the questions I have seen asked are:
How many contactors to use and where to put them.
Precharge, what is it, why do I need it, how do I do it.
How do I apply HV to the controller?
Do I need an emergency disconnect?
Fuses, Circuit Breakers or both?

I think I have the answers to these questions, but am open to having my ideas adjusted by other's opinions and experience.

[rfengineers]

Oh Heck, I meant to put this under 'Technical Discussions'. Would one of you Moderator types please move it over there?

[TX_Dj]

Done.

I think it's a great idea to gather as much info about these things as possible and update/add to the wiki where necessary. As such, I'm also going to make this a sticky.

[rfengineers]

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.


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.


MORE TO COME.

[rfengineers]

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

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.

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.

MORE TO COME

[rfengineers]

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

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.

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.

MORE TO COME

[electriKAT]

RF - please forgive my ignorance, but what is the drawback to leaving the pre-charge resistor on all of the time? Is it bad for the controller? Or dangerous? Initially I was thinking that it would drain your pack, but a capacitor is essentially an open circuit to DC current once it's charged, so that part should be negligible.

[rfengineers]

Originally Posted by electriKAT

RF - please forgive my ignorance, but what is the drawback to leaving the pre-charge resistor on all of the time? Is it bad for the controller? Or dangerous? Initially I was thinking that it would drain your pack, but a capacitor is essentially an open circuit to DC current once it's charged, so that part should be negligible.

That is an excellent question.

Many DIY'ers use a high-Voltage cutoff of some type, a circuit breaker or Anderson plug for example, and normally turn off their HV when the car is parked. The step-start prevents them from powering up in the wrong order.

For systems that always have HV applied to the contactor (such as my EV) it can be a shock hazard since it leaves high Voltage on the controller terminals even when the car is turned off.

There is nothing wrong with leaving it on all the time as long as your controller terminals are well guarded.

Joe

[micheljaw]

Thank! Very nice answer...thanks.
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[alex]

A couple of questions:

Precharge resistor. Is this some kind of special resistor, or just an ordinary resistor? How many ohms?

Also won't the resistor decrease the voltage to the controller which may damage it? Or if it limits the amperes to much, the microprocessor in the controller might not start properly?