Simplicity, size, cost?
Brushes on slip rings last “long enough”.
So the armature rotates in the magnetic field of the DC portion of the stator, and that inductively generates AC current in the rotating armature, which is then rectified in the rotating armature to make a DC magnetic field in the armature, used to then generate AC in the stator’s other coils? Seems reasonable, but compared to a traditional slip ring design used in most cars the downside might be voltage regulation is a little more difficult, and there may be a weight disadvantage b/c separate coils for both DC and AC are needed in the stator.
Honda used a brushless alternator on many of its four cylinder motorcycles.
The magnetizing coil was fixed and did not rotate along with the alternator rotor even though it was positioned within the rotor. This allowed the alternator to be inside the engine case instead of external which requires another oil seal in the engine.
Yeah I think I’ve got one of those on my lawn mower.
Your lawn mower likely uses a flywheel magneto with additional battery charging coils and uses permanent magnets for the field. On some applications, the permanent magnet alternator can even start the engine by being used as an electronically commutated brushless DC motor.
A long time friend of mine has worked in an electric motor shop for 40 years; the last 15 or so as foreman. Repairs of small motors are not economically feasible for them or the customer so the bulk of their work is the larger stuff.
Most motor failures seldom involve the bearings. It involves insulation degradation, shorts, and so on. A lot of the oil field motors they repair are usually damaged to lightning strikes.
As for alternators, Harley has been using brushless alternators since 1970 and British bikes long before that. They’re all permanent magnet of course.
I was watching a TV show about hydroelectric damns on Sunday (Quest channel iirc). They were replacing a rotor in one of the turbines. The electrical part, not the part w/the fins that comes in contact w/the water. This rotor was big, appeared to be around 100 feet in diameter! Otherwise it looked just like the rotors in car alternators. It weighed so much they needed a very powerful overhead crane just to lift it out for repairs. This was the first time any repairs had been needed. It was original to when the damn was built 50 years ago. It hadn’t failed. It was still working ok, but producing 20% less electrical power compared to when it was new. They didn’t replace it, just did some re-work and put it back. B/c of the 20% increase in electrical power they’d get, It made $$$ sense to rejuvenate , removing oxidation from the wire connections, new insulating materials, etc.
Probably barnacles, mussels, or algae on the turbine wheel.
The old Brit bikes had a huge zener diode in a heat sink to ground the excess output. Today there are more efficient ways to regulate the voltage output of PM alternators and they are replacing the old excited field alternators rapidly. My current motorcycle has a PM alternator. Nice thing about a PM alternator is that it is actually possible to push start a bike with a totally dead battery. You’ll need a pretty long hill though. Especially if the fuel system is EFI but it CAN be done.
I’ve seen a few of the Brit bikes lose a Zener diode at night. For a few seconds the lights will become arc welder bright when the voltage becomes uncontrollable.
Some years ago 2 friends and I decided to cobble a bike together as an example of back yard engineering done cheap.
A 67 Triumph Bonneville fire job motor (carbs had melted) from the scrap yard, homemade frame, and a misc. collection of donated from the 3 of us Harley, Honda, and Triumph parts. Built in a week for less than 50 bucks.
It was small, light, and very fast; as some street racers found out the hard way. Due to the downsizing there was no place to mount a battery. The electrics were left up to me. So I tore apart a Delco alternator, removed the diodes, and soldered 4 of them together in a daisy chain. That served as a full wave rectifier. A Zener was added as a control mechanism and a large capacitor was used as a sort of storage facility.
The bike would always kick start on the 1st or 2nd kick. The only downside was that the lighting would dim a bit at idle.
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A lot of early Japanese bikes had a permanent magnet alternator and instead of a voltage regulator, it was simply engineered to trickle charge the battery as you rode. Switching on the headlight also connected an additional magneto coil to the charging circuit. I remember reading in the owner’s manual a procedure for riding the bike with a low battery. You put the headlight switch halfway between On and Off. This connected the extra magneto coil but left the headlight off.
Very informative on Tesla car motors:
Totally late to the show
Ac induction motor(asynchronous motor) came in because they are easy to build and easy to work with ac current. But the rotor always spins slower than the magnetic field and produces some wasted heat. This video describes how induction motor waste heat is dealt with
Bldc came much later. Instead of rotating the magnetic field without regards to rotor position, it rotates the magnetic field to lead the magnet by a certain angle. While this is an efficient method, it is only possible after the transistor has been created. You also can’t apply an insane amount of power or you would risk heating up those rare earth magnet and demagnetize them. This is why Prius don’t have ludicrous mode
Speaking of the Prius, it doesn’t use bldc. It has permanent magnet synchronous motor. As its name implies, it monitors the rotor speed and create a magnetic field that leads the rotor by a certain angle but always rotates at the same speed as the rotor, thus reducing waste heat. I spent a little more to get a ceiling fan with synchronous motor as I don’t want it to carry the induction motor’s waste heat in the air. Cordless tools are starting to use BLDC to save precious battery power
Tesla was a relatively small start up compared to established automotive company. Just making those batteries is a feat. They need to cut cost elsewhere, such as those rare earth magnet to produce a product. Hence they started with ac induction motor. And by using induction motor, they can provide the ludicrous mode as there is no magnet to damage
MAHLE, the major auto component supplier, has announced their design of magnet free synchronous motor. The rotor are cheap electromagnets powered by an axial transformer
Yes, an induction motor can withstand ludicrous mode, for about 3 or 4 seconds.
Also, BLDC motors are essentially permanent magnet synchronous motors powered by an inverter. Many have a rotor position sensor that tells the electronics when to switch polarity but there are also sensorless controllers that read the induced voltage of the unpowered leg to determine the switching point. These are common on electric model airplanes. The only quirk is that until the motor is turning, the sensor does not know where in the sequence the motors rotor is and that means the prop sometimes turns backwards a little before going forwards when you turn it on. Large models that use two motors geared together either have to have one motor start first, or the pilot has to spin the prop as he advances the throttle so the motor’s electronic speed controllers don’t fight each other on the startup. This is something that can break the gears.
On a Tesla, the rotor of an induction motor is often spinning faster than synchronous with the magnetic field. This happens whenever the motor is braking the car. This forward slip overspeed generates reverse torque to brake the car as well as causing the motor to act as a generator to recover the kinetic energy, recharging the battery.
Also, during very low power use, (most of the time in a car) the slip is actually very low and the voltage delivered to the motor can be lower than the constant torque voltage, reducing hysteresis and eddy current losses in the motor’s core. The slightly increased slip from the lowered voltage is often smaller than magnetic core and copper loss saved in the stator by the lowered voltage.
In a permanent magnet synchronous motor, even in a zero torque situation, like coasting down a grade, the magnetic core of the stator experiences the same eddy current and magnetic hysteresis losses as it does at full power. The induction motor running at zero volts would just freewheel.
A totally hysteresis and eddy current free magnetic core would be to the magnetic circuit of a motor what a superconductor would be to the electric circuit.
Synchronous motors have excitation losses that are constant at all loads. The slip of an induction motor essentially is the rotor’s excitation and at reduced load, the induction motor may be the more efficient motor.
That’s true. Slip is not negligible in a house hold fan as you power it at a low voltage with the rotor fighting aerodynamic drag at low speed while the stator runs at line frequency. This is why I went with a synchronous motor fan. Tesla has a fancy variable frequency drive to keep the slip to the minimal. But even still, Toyota chosed the synchronous motor, as did Tesla later on
It would be nice to turn off the magnets when you don’t need them. I guess that’s why we have this design
You can simplify all that coil design by using a switched reluctance motor. No slip and no eddy current loses. But those have hysterisis lose in the rotor
A variable reluctance motor is essentially a rotary solenoid. You still have eddy and hysteresis losses in the stator, however, you can run at a lower flux density at light loads, reducing those losses.
Some synchronous motors with a salient pole design in the rotor can continue to run as variable reluctance motors if the DC excitation is removed, however, this lowers the power factor increasing the resistance losses in the stator winding and the motor may no longer have the torque needed to run at full load.
Once a variable reluctance motor pulls out of step with the stator, it comes to a stop, even under no load. The principle has been known for a long time but it took variable frequency drive to make the design practical.
UPDATE: Many thanks again to everyone for information and suggestions regarding replacement of the trimmer/edger.
I was delayed almost a month doing hands on shopping but used that time doing lots of online research, referencing what was posted here, and narrowing down what particular trimmers to look at and handle at several stores.
I ended up purchasing a combo kit on sale of a Worx WG163 cordless trimmer and WG547 cordless blower with includes charger and two 20V batteries.
I’d had some concern whether or not 20V battery power and a single line string would be sufficient but so far it seems quite adequate to my suburban yard needs. It isn’t as powerful as the corded dual line B&D trimmer was but the VERY nice trade off is no longer having to deal with fifteen pounds of multiple heavy duty three-pronged extension cords to unwind, drag around, and rewind to put away.
The trimmer is lightweight, about 5.5 lbs total including battery. It has wheels! So for edging that is quite arthritis friendly. It also is slightly quieter than the old trimmer.
The potential negative trade offs are what runtimes for the 20V batteries will prove to be, how fast the trimmer does or doesn’t use up line, and how well it’s lightweight construction holds up over time. Fortunately the blower doesn’t need extended runtime because it does seem to go through a battery charge quickly.
Bottom line, for my needs of tools that are light in weight, arthritis friendly in design and function, and budget friendly, this seems a reasonable and useful compromise.
Again, thank you all for good advice and suggestions regarding trimmers. And thank you for all the interesting and educational information about electric motors of all sorts!
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Both the trimmer and leaf blower run on the same battery. It was only $10 more for the combo kit, meaning I got a blower for $10. A total of $130 for trimmer, blower, charger, and two 20V batteries seemed a good deal.
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That seems a pretty good price for both the trimmer and blower plus the charger and 2 batteries. Good for you. Keep an eye out for sale prices on replacement batteries, then buy an extra one or two, so you’ll have a backup battery if you ever need one. One disadvantage of battery powered equipment, several years from now the replacement batteries may be hard to find.
I have a Makita battery powered drill/driver for 25 years. It still works like new, but the battery is no longer stocked at Home Depot. I can still purchase them mail order though.