[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

 

[rfengineers]

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

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.

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.

End of Precharge stuff.

So, how about some feedback, comments, flames, dissenting opinions or support from all you other experienced and knowledgeable DIY'ers out there.

 

[DIYguy]

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

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.

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.

End of Precharge stuff.

So, how about some feedback, comments, flames, dissenting opinions or support from all you other experienced and knowledgeable DIY'ers out there.

Thanks RF! I think it is very helpful. Can you describe what the step-start device is comprised of and how it works?

Thank you.

Gary

 

[rfengineers]

Thanks RF! I think it is very helpful. Can you describe what the step-start device is comprised of and how it works?

Since you asked, I will take that as permission for a commercial.

Here is a picture of the step start device, the box that the circuit board comes in, and the two relays (precharge and contactor enable)

Here is how it hooks up:

There are six wires coming out of the box.

One goes to ground and the other to your 12V ignition circuit (only has power when the key is in the RUN or START position)

Two wires are input signals. One comes from the ICE starter wire. This wire will have +12 Volts on it when you turn the key to the START position. The other input comes from the vacuum pump switch. It should go to ground when the pump is not running. If you don't have a vacuum pump then simply tie this wire to ground.

The last two wires are the outputs, Precharge and Contactor. They supply ground to the precharge and contactor enable relays.

The device is controlled by a PIC 12f629 micro controller. The inputs and outputs are protected by optoisolators.

When the device receives a START signal the precharge relay closes and timer-1 starts. The contactor will not close during the first time period. This is your minimum precharge time.
After the first time period expires timer-2 starts. The contactor can now be forced to close by sending a second START signal to the device.
If a second START signal is not received then, after the second time period expires, the device begins monitoring the vacuum input. As soon as that input goes to ground (meaning that the vacuum pump has stopped) then the contactor closes.

I can custom program the device for any time period you wish. I use 10-seconds for timer-1 and 10-seconds for timer-2, but I also use a precharge resistor that is larger than the one Curtis recommends. This means that my capacitors charge slower than normal. The reason I did that is, that was the resistor I had in my parts box.

 

[DIYguy]

Thank RF! Very nice. I'm in the middle of some basic mechanical stuff, but will be getting to the electrical controls through the winter. I have thought briefly about this timing requirement.... but due to current tasks, haven't pursued it. Are you building any of these devices for sale by chance?

I was thinking.... what might be nice is an output to drive an "idiot light" (pardon the expression...but it works for me lol) when the precharge time is satisfied.

I assume you are monitoring the vacuum pump to ensure that the brakes are fully functional before starting? Could, perhaps, the verification of this be a signal from a vacuum switch verifying adequate vacuum presence?

Regards,
Gary

 

[etischer]

Another nice feature would be to monitor bus voltage. If for some reason the precharge contactor or resistor failed, you don't want the main contactor to close.

Are those cube relays switching pack voltage or are they used to drive another contactor?

Thank RF! Very nice. I'm in the middle of some basic mechanical stuff, but will be getting to the electrical controls through the winter. I have thought briefly about this timing requirement.... but due to current tasks, haven't pursued it. Are you building any of these devices for sale by chance?

I was thinking.... what might be nice is an output to drive an "idiot light" (pardon the expression...but it works for me lol) when the precharge time is satisfied.

I assume you are monitoring the vacuum pump to ensure that the brakes are fully functional before starting? Could, perhaps, the verification of this be a signal from a vacuum switch verifying adequate vacuum presence?

Regards,
Gary

 

[rfengineers]

Are you building any of these devices for sale by chance?

Yes, I am building them for sale. Please contact me off list for details.

I was thinking.... what might be nice is an output to drive an "idiot light" (pardon the expression...but it works for me lol) when the precharge time is satisfied.

A good idea. Assuming there is enough interest in the product, I will look into implementing it in a later version. Right now I just listen for the "clunk" of the contactor closing to tell me I am ready to go.

I assume you are monitoring the vacuum pump to ensure that the brakes are fully functional before starting? Could, perhaps, the verification of this be a signal from a vacuum switch verifying adequate vacuum presence?

Yes, that is how it works. A signal from the vacuum switch (that turns the pump on-and-off in response to the vacuum level in the system) is used to indicate that the system is ready to go.

 

[rfengineers]

Another nice feature would be to monitor bus voltage. If for some reason the precharge contactor or resistor failed, you don't want the main contactor to close.

Yes, that can be done but is a bit more complex. I am trying to put together a basic, affordable device. If there is interest I can work on a more sophisticated model.

Are those cube relays switching pack voltage or are they used to drive another contactor?

The precharge relay switches the pack Voltage at low currents.
The contactor enable relay switches a ground for the contactor. The contactor Voltage could be 12V or pack Voltage. It depends on your EV's particular design.
The purpose of the external relays is to avoid bringing pack Voltage onto the circuit board.

 

[etischer]

I think it would be a good idea to check the rated voltage on the relays to make sure they are rated to switch high voltage. There is a certain liability when selling a product, it may be used in ways you haven't thought of, for which you may be liable if it fails. But like you, I am planning to sell my EV products once they are proven =) I'm gonna have to take out an umbrella insurance policy or something! I'm building an inverter for the Siemens/Ford AC motor.

There should be some feedback from the main contactor telling the drive that the contactor is closed. If the driver tries to drive off before the main contactor closes, all the power will go though the precharge resistor instead of the contactor. This could burn up the resistor, and cause some damage. I'm not trying to scare you off or be a jerk, just try to analyze the side effects of every possible failure. And remember no two EV's are alike in the DIY world.

Yes, that can be done but is a bit more complex. I am trying to put together a basic, affordable device. If there is interest I can work on a more sophisticated model.


The precharge relay switches the pack Voltage at low currents.
The contactor enable relay switches a ground for the contactor. The contactor Voltage could be 12V or pack Voltage. It depends on your EV's particular design.
The purpose of the external relays is to avoid bringing pack Voltage onto the circuit board.

 

[rfengineers]

I think it would be a good idea to check the rated voltage on the relays to make sure they are rated to switch high voltage. There is a certain liability when selling a product, it may be used in ways you haven't thought of, for which you may be liable if it fails. But like you, I am planning to sell my EV products once they are proven =) I'm gonna have to take out an umbrella insurance policy or something! I'm building an inverter for the Siemens/Ford AC motor.

There should be some feedback from the main contactor telling the drive that the contactor is closed. If the driver tries to drive off before the main contactor closes, all the power will go though the precharge resistor instead of the contactor. This could burn up the resistor, and cause some damage. I'm not trying to scare you off or be a jerk, just try to analyze the side effects of every possible failure. And remember no two EV's are alike in the DIY world.

Good point on the cube relays, I will check on that.

The controller will not run the motor on only the precharge resistor. If you demand any appreciable current the large drop across the precharge resistor will cause the controller to go into low-Voltage shutdown.

 

[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]

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

 

[dr01allen]

instead of a time delay, would it be useful to compare the voltage drop across the resistor? That way, regardless of resistance value, you know the capacitors are charged when it drops to 0.

 

[rfengineers]

instead of a time delay, would it be useful to compare the voltage drop across the resistor? That way, regardless of resistance value, you know the capacitors are charged when it drops to 0.

That is a good idea that I may develop later. I wouldn't bother letting it go to zero. Just let it drop enough, say by half-or-so, so that there is less chance of arcing of the contacts.

 

[engineer_Bill]

instead of a time delay, would it be useful to compare the voltage drop across the resistor? That way, regardless of resistance value, you know the capacitors are charged when it drops to 0.

I've heard of several people using a small wattage light either across the precharge resister, or as the resistor. When the light goes out the caps are charged.