Generator set for an EV

[JCG]

Ok, so I've been starting to take the time to figure out what must go into a system such as the Chevy Volt, which is a range-extended EV according to some people's preference, others term it a series hybrid. Some just say PHEV.

I'm sure everyone is already familiar with it, but just in case: the car runs as a pure EV, in battery-depletion mode, until the batteries reach around 50% SOC (I was expecting 20% SOC for generator activation, but apparently, it's not). Then, the 53 kW generator turns on and does double duty, providing traction power and charging the batteries.

If we assume that the average cruising power is 20 kW, then the gasoline generator will still have 53 kW - 20 kW = 33 kW for battery charging. I'm not an electrical engineer (or a physicist) so some of the real-world implications of a system like this are confusing me.

In terms of control: is this a simple system? Say you could flip switches to make things happen, if you were paying attention. Let's say that your monitor is either displaying that you've spent half of your battery's amp hours, or you're detecting a current-interrupted terminal voltage that is associated with 50% discharge (a little tougher to do). You turn the key and the generator starts.

The battery pack might be 350 V or so. I suppose that ideally you'd like the full-wave rectified generator voltage to be around 380 VDC to charge the batteries, and allow the motor controller access to a high enough voltage for its own use. You'd need a 3 phase generator with a phase voltage of around 305 VAC (I think?).

The generator won't be up to full speed right away, so I guess it'll take some time before the full voltage appears. But, here're my main questions to open for discussion:

1) Let's say the batteries and driving requirements are asking for a lot of current at the time you ask the generator to turn on. The priority is to send current first to the motor as demanded, then to charging as available. Can you control those current magnitudes?

2) How do you limit the current that the generator is sending out, or can it self-limit?

3) What happens to a generator when it effectively is throwing its current into a short circuit? Will it overload and switch itself off, or damage something (or itself)?

The answers to these questions determine how complicated such a control system must be. Looking forward to hearing some thoughts on these items!

 

[fechter]

Hmm... well a lot depends on the batteries. Are we talking Nimh?

Regardless of the charging source, the batteries need to be monitored to prevent over charge and over discharge. The generator needs to have some kind of current limiting to prevent it from burning up or stalling the ICE. In a series system, the current will just take the path of least resistance, so under heavy load, most of the current will go to the traction motor. Under light or no load, most of it will go to charging. I don't think you need to try to direct it. The generator must also reduce current output if the batteries reach full charge. I don't think the control circuit will be that complex. The battery monitoring part will be more complex.

 

[MitchJi]

Hi,

Ok, so I've been starting to take the time to figure out what must go into a system such as the Chevy Volt, which is a range-extended EV according to some people's preference, others term it a series hybrid. Some just say PHEV.

Not exactly. A "range-extended EV" and a "PHEV" are the same thing. But a "series hybrid" is one form of PHEV. For example the Prius PHEV Conversions are "Parallel Hybrids".

 

[JCG]

The generator needs to have some kind of current limiting to prevent it from burning up or stalling the ICE.

Yes, this one is the killer - definitely bothers me the most of any one concern. How does a generator limit its current, when it's designed to spin at a given speed, and is simply hard wired into a load with a widely varying current demand? I don't have a feeling for that. Voltage regulation is one thing, current is a whole new ballgame.

 

[fechter]

If it's a permanent magnet generator, then you would need something that resembles a motor controller to throttle the output. If it has a field winding like a standard car alternator, then you just control the field current to vary the output.

 

[JCG]

If it has a field winding like a standard car alternator, then you just control the field current to vary the output.

I assume that it would be field wound, since it might get hot and heat can kill PM generators. I'll have to follow up with someone in the business! Computer control of the field winding strength sounds easy, but I'lll follow up with an industry rep to see how this might be done in a "flip the switch" or "turn the dial" way. Thanks for the advice, fetcher.

 

[jag]

If it's a permanent magnet generator, then you would need something that resembles a motor controller to throttle the output. If it has a field winding like a standard car alternator, then you just control the field current to vary the output.

If a permanent magnet generator rotates with a constant speed, the voltage output should be relatively stable even under varying load. The torque in the input shaft will increase, so the ICE motor power needs to be properly regulated to not drop rpm under load.

In the old days, one would probably put a mechanical speed governor type of control (like on Diesel engines) on the motor, and then if needed provide additional regulation independently on the generator/alternator output.

These days all is likely to be computer regulated in a big multivariate control system.

 

[christopherf15e]

If you look at some small aircraft systems, you have a voltage regulator, current limiter, and reverse current cutout (keeps the batteries from turning the generator into a motor). standard stuff.

 

[swbluto]

I thought the chevy volt was as simple as simply charging the battery during use, no? Wait... *thinks* that's more complicated than I originally thought. *sits back and listens*

 

[JCG]

Chris appears to be right on the mark here, you can do most of what you need with the voltage regulator on the generator. Set it to the appropriate value for the high voltage bus, and then current will be drawn (as demanded) to the battery pack for charging, or to the traction motor. I spoke with a rep at a very good generator company (Mecc Alte), and they suggested that voltage regulation is the simplest approach, which will aim to change the winding excitation current to match the desired voltage produced. Draw too much current, and the generator's driver will usually cut off. For the Volt, that'd be the gasoline engine.