What on earth?  Variable field motors. - Page 1
Forum section: electronics / general

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2018, 12:01:41 pm
content: What on earth?What is the purpose and features granted of making a motor's stator do this?If I were to build a 3 phase PMA generator like this, can I extend my top efficiency for different drawn power levels at different RPMs without my existing mechanical chain multi pulley gear-box design?Or, is this just an elaborate useless hoax?

author: Gyro
author_rank: Super Contributor
timestamp: September 26, 2018, 12:19:09 pm
content: Without thinking about it in any depth, could it be a similar effect as a series wound brushed motor that runs away when unloaded, due to a very weak stator field. Irrc, they consume vast amounts of current when they do that though.Surely on the 'variable field' motor, the winding inductance would drop through the floor (as you withdraw it)!

author: Jeroen3
author_rank: Super Contributor
timestamp: September 26, 2018, 12:30:20 pm
content: It looks like it's purpose is to reduce eddy current braking when free-running.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 26, 2018, 12:34:21 pm
content: Impossible to say without knowing what the rotor and drive waveforms are..?Eddy currents may well be a concern, perhaps harmonics due to the sharp pole pieces.Maximum torque will of course be lower, which does sound very much like a gearing effect so that's not outright a bad thing. I suspect torque goes down faster than RPM goes up = total power goes down, but who knows.Tim

author: langwadt
author_rank: Super Contributor
timestamp: September 26, 2018, 12:42:46 pm
content: it weakens the field thus the emf so you can run at higher RPM for a given voltage , google "field weakening

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 12:57:00 pm
content: I would think its to make the permanent magnet seam weaker by pulling the electromagnets out and away.A strong magnetic field is nice when you want to produce lots of toque because the stronger the field is the more force the same number of amps trough the coils will produce. But when the motor is spinning at high speed those magnets are generating a voltage on its terminals. The faster it spins the higher this voltage is and once this voltage is as high as the voltage of your batteries then you can't put any more power into it, infarct if the motor is spun even faster the motor will start to charge the batteries.So if you reduce the strength of the field by half you also reduce this back EMF voltage by half and so you can run the motor twice as fast before you run out of voltage. As a consequence you also get half as much torque, but this sounds exactly like what a gearbox does so its essentially a clever 'single moving part' gearbox.They could also possibly have a different magnetization pattern on different parts of the rotor to change the number of poles on the motor and this would change the RPM of the motor when run at the same frequency.I think this is pretty clever now that i think about it.

author: dmills
author_rank: Super Contributor
timestamp: September 26, 2018, 02:11:19 pm
content: Meh, see the old separately excited machines where the speed (and torque) was controlled by varying the field current, this is technology that goes back to the very early days of electrical machines.Time was you had a motor speed controller that consisted of a set of contacts that cut resistors into the field circuit as you increased the speed setting to lower the field strength, often combined with an ammeter to tell you when the rotor speed was high enough to make stepping down the field safe (They also sometimes interlocked with the armature current so you could not switch it on until you had established full field).Think old DC powered trams, fairground rides, furnace blowers and things of that ilk, circa 1880 or so.Regards, Dan.

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2018, 02:35:46 pm
content: Looking at the tire and what I can see from the vehicle design in the second video, the technique appears to be adopted for a high efficiency bike or miniature 3 wheel car, like a solar powered vehicle for competition. I can assume they went through the trouble for a performance increase in efficiency. However, we are not told if they have achieved any overall performance gains.I wonder if the reverse can be applied for a generator, where, at times I may have a fast but low torque mechanical source, and, for the same device, I will at other times have a slow, but, high torque source. I would like to keep the generator in it's sweet spot of 92-96% efficiency without resorting to a mechanical gear box where I loose efficiency due to additional friction.The question is; is it worth my R&D efforts to build such a generator?My generator is also a pancake generator due to available space, so, this can get costly...

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2018, 02:49:16 pm
content: Yes, it's for a solar powered car, here:

author: langwadt
author_rank: Super Contributor
timestamp: September 26, 2018, 03:17:20 pm
content: Quote from: dmills on September 26, 2018, 02:11:19 pmhttps://youtu.be/tJNqpcsvlqI

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 04:55:49 pm
content: Quote from: BrianHG on September 26, 2018, 02:35:46 pmYes generators do use this already.All the large generators in power plants or on large ships are synchronous 3 phase machines with a 3 phase wound stator and a DC energized electromagnet as the rotor that is fed via a slip ring (Not to be confused with a commutator on a DC motor). This is the same as a modern brushless motor but with the permanent magnet replaced by an electromagnet. By varying the current they can adjust the strength of the magnetic field and this affects the output voltage (That can go all the way down to 0V with it spinning). The power needed to magnetize the rotor is orders of magnitude smaller than the power coming out of the generator so its more convenient to control it. Here it has nothing to do with the speed tho since a synchronous generator has its frequency determined by the RPM, but when run on a isolated environment such as a ship it is used to regulate its output voltage or when tied to the electricity grid in a power plant it is used to regulate it into the ideal operating point depending on what power level it is being run at.Another common use for it is alternators in cars. That is also a 3 phase synchronous generator with external DC excitation but it has a diode bridge to turn its output to DC again. There is a little circuit attached to the alternator that controls this DC excitation current so that it regulates the output to be the perfect 12V lead acid charging voltage regardless of the RPM.Infact this was so useful in the old times that a lot of elevators had 3 phase motors belt driving DC generators (Called a motor-generator) because they could externally excite the DC generator to provide easily variable power for the giant DC lift motor using a small control current (Before modern semiconductors this was hard to do directly).

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2018, 05:11:26 pm
content: Quote from: Berni on September 26, 2018, 04:55:49 pmI knew about DC excited generators/alternators. Unfortunately, I need the super efficiency of neodymium permanent magnet generators. Sliding the stator in and out was something completely new to me. What I find confusing is, my current generator is 90-95% efficient between 100 watts and 300 watts, but, that drops like a brick below 100 watts.My question is:A) Will partially pulling out the stator give me 90-95% efficiency between 25 and 75 watts?orB) Will partially pulling out the stator give me still 90-95% efficiency at 100-300 watts, except, I just need to run the generator at a higher RPM with lower torque?orC) A mixture of A) and B)?

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 05:31:27 pm
content: Quote from: BrianHG on September 26, 2018, 05:11:26 pmWhen you speak about poor efficiency under 100W is that due to low input RPM, or you not drawing enough current from the output ? (The output load behavior is pretty important)The slower you spin it the stronger of a field you need to get the same output voltage from it.

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2018, 05:53:04 pm
content: Quote from: Berni on September 26, 2018, 05:31:27 pmI'm running the generator between 150rpm and 400rpm. The electrical load is a continuous restive load tuned for 100-400 watts.It's obvious that lowering the rpm below 150 will both kill efficiency and deliver less power.However, if I raise the load resistance, I will draw less power, but, the efficiency still drops due to some inherent drag from the magnets on the stator. In fact, without any load, there is still drag. (Note that the generator is designed to remove the denten effect...)Instead of raising the load resistor, will partially pulling out the stator, keeping the rpm at 150-400rpm, both reduce drag, and obviously power output, but keep that torque VS power output efficiency between 90-95% if I tune the stator's location so I get around 10-20% power? I'm trying to maximize generator efficiency at much lower wattage than what it needs to occasionally run at...

author: IanB
author_rank: Super Contributor
timestamp: September 26, 2018, 06:04:12 pm
content: Quote from: dmills on September 26, 2018, 02:11:19 pmWeak field in DC motors has been in use far more recently than 1880. For example on the "A" stock on the London Underground, which has only recently been withdrawn from service.

author: Berni
author_rank: Super Contributor
timestamp: September 26, 2018, 06:19:18 pm
content: Im guessing what happens there is that to keep providing 100W with the ever lower output voltage you force it to output a lot more current and that current is making it work like a motor in the opposite direction with a lot more toque in those operating conditions. But its expected to produce a lot more counter torque because it has to extract the same amount from power out of a lower rpm. Where the efficiency plummets is likely because of P = R*I^2 . A significant part of losses in a generator is the coils heating up due to having resistance. The formula tells us that if we increase the current by 2x we actually get 4x the power loss in the coils. As you increase the current the power loss goes up faster and faster.So if that is the case then using thicker wire in the coils should improve the efficiency at low speeds because there will be less resistance.Another way to reduce these losses is to increase the output voltage because then you need less current for the same amount of power. A stronger magnetic field would do this, but you are already using the strongest permanent magnets you can buy (So you need to use electromagnets and those use power to run, while your magnetic core might not be able to handle a stronger field). Another way to increase the voltage is to change the magnetic field faster, this can obviously be done by spinning it faster, or you can make it look like its spinning faster by giving the generator more poles. The more alternating pairs of magnets you have the faster the field flips at the same RPM. Then you can pull the coils out to reduce the voltage back down from being too high at high RPM.(But if your load can take the voltage you will be able to get even more power from the generator if you don't pull them out)Its difficult making generators efficient over a wide range of speeds as you have found out. At some point you run into the laws of physics that force you to make the generator physically larger and heavier or force you to use more exotic technology to sidestep the problem (Like superconducting coils to remove resistance, or superconducting electromagnets to make massive fields.)

author: amyk
author_rank: Super Contributor
timestamp: September 27, 2018, 02:23:10 am
content: Quote from: IanB on September 26, 2018, 06:04:12 pmI'm not even sure if field weakening was known or implemented in 1880. Probably closer to 1890s/early 1900s.