A/C design/engineering question

Hi Timo,

No, it does not. Its a feed-back control system. If the setpoint (you set) is well below the actual temperature in the car it will come on full speed, and as the car cools the compressor speed and fan speed reduce until the AC power just coasts along at a low output maintaining the temperature/humidity. Its more like a houshold unit, but its continously variable system, not one that is off and on. If you drive around a corner, out of the shade into the sun, it will automatically increase its cooling power.

 

Tim,

I can verify that you can noticeably influence your fuel consumption by setting the AC at a different temp. Put it on MAX COLD and you will see your consumption skyrocket. I always put it at 23 C and I do not notice the difference in consumption as compared to AC off.

 

I find it curious that the AC compressor motor runs on high voltage DC, rather than AC from the inverter. Is the motor brushless? Surely with all the cutting edge technology used on this car, they would not use a DC motor with brushes that need to be changed periodically?
Any Toyota Electrical Techs out there?

 

The A/C (air-conditioning) compressor motor is a three-phase AC (alternating current) motor. The A/C inverter is a separate section of the Power Control Unit (also called 'the inverter' but there's a lot of power electronics in there besides MG2's inverter).

A 'DC brushless' motor is a permanent magnet synchronous AC motor with the necessary control electronics integrated into the motor housing. However, it's easier to cool all the power electronics with one coolant loop integrated into the single PCU. Small DC brushless motors (like the electric water pumps) don't dissipate enough power to require liquid cooling of the electronics, but the A/C compressor requires a couple of kilowatts of power!

 

Ah, that makes perfect sense. So the AC Compressor Motor has is own, dedicated inverter?
I'm really dying to see the schematics for the Prius electronics!!!
Even a peek would be nice...

 

Mike's explanation underlines a common point of confusion. All motors are AC. A true DC motor would be a solenoid - all that would happen is that the motor would pull into one position and stop. What differentiates motors, from an AC/DC perspective, is how they are commutated. DC and universal motors use brushes to produce an AC voltage. AC motors use the inherent alternation of the line voltage. Brushless DC motors use electronics to produce AC. If the electronics are moved to a separate box then it becomes an AC motor powered by an inverter. In the end, all of the motors are driven by an alternating magnetic field.

Tom

 

Tom is correct, in that currently all DC motors that you will meet are powered by AC, or more correctly, the fields that drive them are alternating.

Careful though Tom with the "all", as there are motors coming to market that do not alternate the fields. I don't pretend to understand them, but they use "strange" physical properties that cause the parts in the motor (the magnetic media) to invert the fields, not electronics or brushes. The ones coming soon are micro-motors, but larger ones are in the works.

 

The 2010 AC is way better than the gen 2's. My 2006 had trouble keeping me cool when the outside temp went much above 90F. It's been over 100 the last few days (with bright sun) and the car has remained a comfortable 72F without appearing to try hard. This is easily the best car AC I have ever exoeruenced,

 

The block diagram about the A/C inverter is shown at page-14 of following PDF.
http://www.autoshop101.com/forms/Hybrid02.pdf
The A/C compressor is shown on the following PDF.
http://www.autoshop101.com/forms/Hybrid06.pdf

Anyway, the site has a lot of good information.
AUTOMOTIVE TECHNICAL ARTICLES

Ken@Japan

 

Tim

I'm late to this party and the technical questions have already been answered.

To clarify: the Prius A/C, either 2nd generation or current 3rd generation, is truly variable speed. Rather than use a magnetic clutch, like on my FJ Cruiser, to cycle the compressor on or off, the compressor is on almost all the time

The variable speed feature, which you will also find in the top-line Mitsubishi Mr Slim inverter heat pump ductless wall mount units, dramatically increases efficiency. The Bryant Evolution in my home has a two speed outdoor unit and a variable speed furnace fan, that also dramatically increases efficiency

The "reheat" used on conventional automotive A/C is a good way to dehumidify. It sounds counter-intuitive to cool AND heat air at the same time, but that is the only way an on/off compressor can be used

My 2004 Prius had superior A/C and dehumidifying capability, compared to my FJ Cruiser

jay