@ WheresRick,how is the AC done on a Prius? I can see several advantages to doing that way and am suprised that more manus dont do that,the condensor shouldnt be much of a problem-Kevin
The electric compressor is under the hood, the condenser and fans are the same as a traditional vehicle,
The Prius AC unit runs directly from the traction battery (~200 volts DC) and it is a variable speed compressor so current draw varies with the need for BTU capacity.
The compressor runs on 206 Volts AC nominally. It uses whatever voltage the battery provides, which varies relative to charge.
It is a 3 phase motor, modulated by the inverter based on control from the HV ECU.
The speed it is set to is determined by cooling or dehumidifying demand as determined by the AC ECU, Inputs include cabin temp, cabin humidity, outside temp, solar radiation detected, and your desired feels like temp.
The nice thing is there is no cycling on and off of the a/c its always able to run, even if the engine is off.
I assume that EVs such as the chevy volt, the leaf and others have a similar a/c setup.
The electric compressor on the Prius isn’t a new idea, I think the models as far back as 2008 used them. The condenser has nothing to do with how the compressor operates. I think an electric compressor is easier on the Prius because you’ve got a larger and more consistent electrical source. I think a conventional alternator system would need to be beefed up to handle the electrical load of a compressor.
Also, Priuses use a specific a/c compressor oil, since conventional oils can conduct electricity under certain circumstances.
GM has been using electric power steering on cars for at least 10 years now.
Yes, unless you have a high voltage power source it would be very difficult to have a car with a 12 volt electrical system run an a/c compressor. The main reason the Prius went to electric from what I understand is it allows a/c while the vehicle is in e/v mode. The prius has had an electric a/c compressor since the 2004 model year.
Diodes, the LM741 op amp/regulator.
What made the alternator practical was the invention of the silicon diode to change the 3phase output to DC. Before then, selenium rectifiers were used but they were so bulky and expensive that the DC generator was a better option.
Alternator regulators don’t need a cutout relay, a relay that opens when the generator’s voltage output goes too low and prevents battery current from going backwards through the generator and draining the battery, while driving the generator as a motor.
Advantages of DC generators, electrically rugged, a short high current shorted output is not likely to hurt it. If you have a completely dead battery, a generator will self excite and start charging the battery while you push the car to start it. Alternators will self excite also but have to really be spinning fast, fast enough for the residual magnetism in the rotor to overcome the .7 volt forward bias needed for a silicon diode to conduct.
Another advantage of the DC generator is that it can double as an electric motor, some old motorcycles and garden tractors used the generator as an electric starter. Old Lincoln SA200 arc welders could be jump started from another running welder by touching electrodes, the welding generator operated as a motor and cranked the engine. Yes, I’ve done it. A railroad employee told me that locomotive engines were often cranked by using the main generator as a motor just like a Lincoln welder.
Also, Beechcraft 1900 turboprop airliners come with twin “starter/generators” on each Pratt+Whitney turbine. So it’s not just an old-timey solution.
Add the Toyota Prius to the list of vehicles that use the generator to start the engine, although it’s not a traditional DC generator with brushes but a 3phase motor/generator driven by an DC to AC inverter.
Interesting thread. Just curious about one thing. Did generators of old (like on older pre-1960 VW Beetles) use permanent magnets to create the magnetic field, or did they use coils of wire with a current running through them, similar to the way alternators work?
Permanent magnets
That might be another reason to switch to alternators. Permanent magnets may be an expense the manufacturers wanted to eliminate.
The DC generators used in cars had wound fields. The regulator controlled the voltage by varying the amps through the wound field similar to the way an alternator is regulated. I have adapted a solid state alternator regulator to regulate an old VW generator on somebody’s dune buggy, I just had to put a blocking diode in series with the generator’s output so current couldn’t backflow when the generator was going too slow to match the battery voltage.
Permanent magnet alternators are common on small motorcycle engines.
If DC generators used in cars had wound fields, they would not self excite as they did. But I will say that it is possible that some did and some didn’t use wound fields.
All the DC generators that I recall working on used the field coils to regulate output, none had permanent magnet field coils. It’s been a few years since working on one though.
Residual magnetism got some of the old beasts going with a dead battery(roll start,push etc)-Kevin
The residual magnetism in the field poles self exites a DC generator.
The reason alternators won’t self exite is because the silicon diodes that convert the ac to the dc needed to exite the field have a 0.7 volt threshold that needs to be exceeded before the diode conducts. The current has to go through at least two diodes so 0.7 + 0.7 volts = 1.4 volt AC output before the rectifier conducts.
I have seen alternators self excite but you practically have to redline the engine before the residual magnetism generates enough voltage to exceed the diode’s 0.7 forward voltage barrier.
Carbon brushes on a copper commutator have no such barrier. If the residual magnetism generates .1 volts, that’s .1 volts to add to the residual magnetism and the bootstrapping begins. A generator can actually charge a battery at speeds achieved by push starting a car.
Large high voltage alternators all self exite, a 460 volt alternator will put out about 30 to 40 volts without excitation from the rotor’s residual magnetism. Plenty to overcome a silicon diode’s forward bias threshold.
Interesting discussion. I’m learning something from all of your comments. Thanks. I had a motorcycle years ago that would start up and run even if the battery was dead as a doornail. I’m assuming it had a permanent magnet generator as the electrical power source. My guess is that to generate the field required for a car generator, the permanent magnets would be too big and weigh too much. Plus it would be more difficult to regulate and maintain the needed voltage and electrical power output. So it appears to make sense that car generators used coils of wire with current varying depending on rpm & power output needed for the magnetic field rather than permanent magnets. Good posts all.