Alternator generator topic

So a bud was asking why cars use alternators instead of generators, and my response was they are more energy efficient. His thought was something must use alternating current in a car, and I said not a thing I can think of. We are each convinced the other is wrong, but thought it might be an interesting topic, and I might learn something.

I think it is b/c alternators aren’t quite as complicated as generators, as there is no electrical commutator needed to reverse the current flow (something required in generators). That makes alternators more reliable. Plus alternators may have a better power to weight ratio. So I think you are right, it is at least in part about energy efficiency.

An alternator is a generator, meaning that anything that is driven by an outside force and makes electricity as a result is a generator. An alternator is simply a generator that makes AC and has a rectifier (or diode bridge) that converts that to DC.

I suppose you could say that they are more energy efficient because they tend to be smaller and lighter than generators and produce electricity at idle speeds that generators don’t.

I can’t think of anything on a car that operates on AC. There are several things on a car that make AC current of their own, like sensors and the pick-up coils in some old distributors.

That’s my take on it anyway.

I can't think of anything on a car that operates on AC.

On my 70’s Ford truck the automatic choke heater is powered from an otherwise unused AC port on the alternator. It produces 8 volts AC or something like that. AC works fine for heating up resistor heaters.

Nothing in a regular car uses AC. The alternator’s primary job is to keep the battery charged… and since AC cannot be stored, the battery is by necessity DC. Everything in the car then operates off the DC battery.

Special use vehicles can be bought with inverters that can convert DC to AC for purposes like camping. Everybody knows one cannot have a successful camping trip without the espresso maker. Seriously, they’re generally used by contactors to operate saws, drills, etc.

So a bud was asking why cars use alternators instead of generators

Two advantages of alternators include:

  1. They put out more current at low rpms (i.e. idle) than generators. This was needed as cars’ electrical demands grew. The bandaid of putting a smaller pulley on the generator to fix this caused the generator to spin excessively at higher engine rpms.

  2. In a generator, 100% of the current produced needs to pass through the brushes, which doesn’t lend itself to long brush life. In an alternator, only a small fraction of total current passes through the brushes (for the field windings).

The friend is wrong. Nothing in a car needs AC current.

“The friend is wrong. Nothing in a car needs AC current.”

I’ll echo this statement because it’s true. I’ve owned vehicles with generators and with alternators and believe me…alternators are far superior. This same battle, basically, was fought years ago between Westinghouse/Tesla (AC) and Edison (DC). Edison lost and so did the generator.

Joe Mario is right. When alternators were introduced this was the advantage touted; more current at low RPM to cope with increasing electrical demand. If cars only operated at highway speeds, we would likely still have generators.

I’ll give an application where a generator was better suited than an alternator. I had a 14 HP MTD lawn tractor when I had a 2 acre lawn. Since a generator will act as a motor, the generator served two functions: 1) it served as a starter motor to start the engine and 2) once the engine was running, it charged the battery. If the lawn tractor had been equipped with an alternator, it would also have to have had a starter motor.

With today’s emerging DC to three phase inverter technology, an alternator could easily be used as a motor, in fact, a three phase induction motor can easily be used as an alternator, eliminating brushes and slip rings entirely, and when you need extra passing power, the inverter could drive it as a motor helping the engine instead of being a load on the engine.
Perhaps this could even be programed to charge the battery mostly during braking, when you are scrubbing kinetic energy with the brakes anyway.

@B.L.E. now I was a little mystified @Triedaq having a generator as a starter motor, this takes it one step further now I am totally mystified. Closest concept I can think of is using speakers as a microphone.

@Barkydog, a DC motor and DC generator are basically the same machine. I have worked on a lot of 25kW DC generators used on magnet cranes at metal salvage yards. How do I test them in the shop? I use our welder to run them as electric motors. You can also run those old car generators as motors.
We used to have a really old Lincoln SA250 gasoline powered welder. The engine was really worn out and hard to start so to get it started, we would hook another welder to its output and the welding generator would motor the engine and get it started.
If you ever have an opportunity to visit an oil field where pumpjacks are pumping the wells, watch the meter at the power pole. The little disk spins really fast as the pumpjack lifts the sucker rods and oil up and spins really fast when it lets the sucker rod drop while lifting the counterweights. At the top and bottom of the stroke, the decelerating sucker rod in the pipe transfers its kinetic energy to the pumping machinery and it tries to overrun the motor and the motor is actually holding the load back instead of driving it. During this time, the little disk on the electric meter backs up a little as the motor temporarily functions as a generator.
A lot of small wind turbines use induction motors as the generator and so do a lot of co-gen/gas energy recovery plants.

I think probably the sole advantage of a generator is that it can charge even a stone-dead battery, while an alternator can’t, and can even be damaged in the effort. Otherwise it seems that alternators are just plain superior.

Probably the main reason generators were not damaged by charging a stone dead battery is the fact that generator regulators had a cut out relay and a overcurrent relay as well as a voltage control relay.
The cutout relay was an absolute necessity, it disconnected the generator from the battery anytime the generator’s output voltage was less than the battery voltage, preventing the generator from drawing current from the battery and trying to spin the engine as a motor.
The overcurrent coil was a few turns of heavy wire that along with the voltage coil opened the field if the output current went above a certain level, thus protecting the generator.

There’s no reason a current limiting relay couldn’t be incorporated in an alternator regulator. Alternators don’t need a cutout relay because the output diodes won’t conduct backwards to discharge the battery when the alternator spins too slow.

No wonder I’ve never been able to find the generator on my mower-its the starter motor. Dope slap.

Searching the memory banks, I remember when Chrysler first came out with the alternator instead of the generator. It was supposed to be advanced engineering. It was either late 50’s or early 60’s. Maybe 57, 58. Then everyone else followed in the years to come. Then GM recently started calling them generators again but they still look like alternators. So I guess I’ll just remain confused.

Even with DC generators, the current in the armature is actually AC, the commutator and brushes are nothing but a rotary switch that does what diodes in an alternator does.
The invention of the solid state silicon diode able to carry the amps needed for a car is the enabling technology that allowed alternators take over. If we still had to use selenium rectifiers or vacuum tubes for this, generators would still rule.

The two motor/generators of the Prius and similar hybrids are AC, though I don’t know what sort. Clearly they work pretty well at moving a car. I’ve always found it fun watching the power flow diagrams when we’ve had Priuses for drives (from Zipcar, and I was in the passenger seat.). There is some clever management going on to combine the output of an ICE and two electric motors and do so smoothly. I couldn’t reliably tell when the engine started without watching the diagram, though when accelerating hard it does get a bit loud.

The starter/generator on air cooled lawn equipment and golf carts was a real Rube Goldberg contraption. Some would start the engine clockwise or counterclockwise.

Just to muddy the waters, there are a few things on some cars that use AC power, but the power is made by inverting the DC–cars that have that blue vacuum-fluorescent display for their gauges and clocks, etc., which require a few hundred volts I think to light up the tubes. Also, some types of sensors use or generate an AC signal, which is interpreted by the ECU.

And if the voltage regulation for the alternator is handled by the ECU (as all modern cars are I think), the alternator is termed a ‘generator’, even though it generates AC, which is converted by a full-wave rectifier to DC, like any alternator.