Even gents,

We're working on designing the wiring diagram for our low and high power circuitry for an electric motorcycle.

We're contemplating a configuration like the one attached in the figure below,

I have a couple questions regarding the best way in which to power our electric motorcycles circuitry.

1) First question, is it problematic to place the ignition switch on the bike directly between the high voltage 48V bus and the 48V:12V isolated DC-DC converter? (I've been told we should not be doing this, and I'm not sure why) If so, how can we safely power our circuitry whenever the ignition is turned on?

We do have access to a switched isolated 48:12V DC-DC converter meaning that it can be turned on and off via electrical signal applied to a control pin on the device, but without the 12V supply from the output of the same DC-DC converter we have no means to generate the control signal to the DC-DC converter to turn it on. For example, if the control signal is to be sent from the microcontroller, the microcontroller would need to be powered on, and this wouldn't be the case if the 48:12V DC-DC converter was switched off.

2) Second question, how do I know whether or not a pre-charge circuit is necessary for our DC link capacitor connected across the terminals of our battery bank?

3) Third question, is there any advantage to using a contactor over a solid state relay?

4) Finally, can anyone spot any obvious flaws in the schematic above that we may have missed or not yet have noticed?

Thanks again!

 

I don't see a problem with the switch where you have it providing your FET driver has under voltage detect. You don't want to try and drive the motor as the 12 volt supply is coming up.
I don't know how big your motor is, but on my golf cart the 48 volts is always present. There is no precharge in it either, but it does scare you when you hook up the batteries after the caps have been discharged.
Again depending on how big the motor is and how far things are from the battery 200 Ufd. may not be big enough. You can see how hot it gets.

 

Your gate-driver circuit only has a 5V supply. Is the mosfet a 'logic-level' type?

 

I don't see a problem with the switch where you have it providing your FET driver has under voltage detect. You don't want to try and drive the motor as the 12 volt supply is coming up.
I don't know how big your motor is, but on my golf cart the 48 volts is always present. There is no precharge in it either, but it does scare you when you hook up the batteries after the caps have been discharged.
Again depending on how big the motor is and how far things are from the battery 200 Ufd. may not be big enough. You can see how hot it gets.

Hi rony!

This is good to know!

If the 200uF is not enough we have access to another 100uF as well as a 47uF cap for noise.

Your gate-driver circuit only has a 5V supply. Is the mosfet a 'logic-level' type?

Hi Alec_t!

The wiring diagram isn't 100% accurate to our current circuitry, so this actually isn't the case.

Our gate-driver circuit has a 15V supply relative to the battery ground. The controlling signals are fed to the gate-driver circuit from the micro-controller via opto-isolators. The microcontroller side of the opto-isolator is our signal ground, while the gate driver circuitry side is the battery ground.

I hope this clears things up.

Any other comments/suggestions?

 

If your primary 48 VDC driver circuit that runs the motor is designed right you shouldn't need any physical on/off switching system or precharge. At most a properly sized DC fuse or circuit breaker is all that it should need between the battery and the rest of the system.

The 48 VDC to 12VDC converter supply should be the only circuit that needs an actual physical ignition switch in it.

Given the whole system runs on a common ground plain if it was me I would change the 48:12 DC converter to a non isolated one being that given the common grounds on both sides the isolation is serving no purpose anyway.

 

If your primary 48 VDC driver circuit that runs the motor is designed right you shouldn't need any physical on/off switching system or precharge. At most a properly sized DC fuse or circuit breaker is all that it should need between the battery and the rest of the system.

What are you reffering to as the "driver circuit"? Also, can you be more specific than "designed right"? What would a correct design have?

The 48 VDC to 12VDC converter supply should be the only circuit that needs an actual physical ignition switch in it.

Good!

Given the whole system runs on a common ground plain if it was me I would change the 48:12 DC converter to a non isolated one being that given the common grounds on both sides the isolation is serving no purpose anyway.

The system isn't going to run on a common ground plane. There are going to be two separate grounds, one for the power circuit (e.g. battery ground) and one for the signal circuit.

This is to prevent any severe ground bus currents from disrupting any of the logic circuits (e.g. the micro-controller), in turn causing mis-commutation of the power switches resulting in a catastrophic failure of the entire drive.

 

This may be a useful link.

http://www.4qdtec.com/pwm-01.html

 

What are you reffering to as the "driver circuit"? Also, can you be more specific than "designed right"? What would a correct design have?

The Half Bridge power circuit that runs the motor.

By designed right that would imply that the power handling devices are being fully turned off when the logic and control systems are powered down.

I think running a fully isolated logic system is going to cause more problems than with one that shares a common ground plain.

In commercial power systems all logic systems usually have their grounds tied to the main power common ground system so that stray floating voltage differential issues don't create problems.