Generator for hybrid gokart - Page 1
Forum section: electronics / projects

author: mtzm
author_rank: Newbie
timestamp: July 09, 2016, 04:15:41 pm
content: Hello,this is my first post in this forum.I have a project going on: I am building a hybrid gokart.Attached you can se the connection diagram.The generator controller actuates the gas engine throttle. The engine spins a bruschless motor used as generator. Attached there is a rectifier(and smoothing caps). This setup implies that whenever the voltage of the generator is higher than the battery voltage, the current flows to the battery and motor(which spins the wheels). When the generator voltage is lower than the battery voltage, the current flows from the battery to the motor.Because of the "fixed" RPM/V of the generator, it is only generating electricity at "fixed" RPM (from 6200 to 6800RPM dependung on the generated current). Under 6200 RPM the voltage of the generator is lower than the battery voltage. At 6800 RPM the power generated is equal to the maximum engine power at those RPM.So the problem is that the system can on one hand not reach the maximum power of the engine (at about 7500RPM) and on the other hand it needs to reach at least 6200 RPM to generate a small amount of power.The maximum power of the engine is advertised as 6.3kW.The voltage of the battery is ~39V.Attached you can see a drawing of the maximum power curve of the gas engine and the curve of the generated power. I would like to make the generated power rise slower with RPM.As I understand it the voltage drop is due to the impedance of the generator coil.As I understood it, the impedance is:X=2*PI*f*Land the voltage drop is:U=I*XMy generator has an inductance of about 16 uH. My thought was to add a inductor at all three phase cables, to increase the inductance an thus the voltage drop.Attached you can see a screenshot of the calculation I made on wolfram alpha with following formula:(f/150)-c*(2*pi*f*0.25*26*10^-6)150 is because I have a 150 RPM/V motorf are RPMc is the current in Ampf*0.25 is roughly the frequency i measured between two poles of the generator at given RPM.26*10^-6 is the inductance of the motor plus a 10uH inductor. This seems to give me the desired voltage drop as RPM and current rise.I made a test with some toroids I had, but I the inductors saturate really quickly.An alternative path I was thinking about is to use a DC-DC converter to step down the voltage at higher RPM.What do you think of the setup with the inductors?Can you tell me what kind of toroids I need to avoid saturation?

author: oldway
author_rank: Super Contributor
timestamp: July 09, 2016, 05:59:52 pm
content: If you choose an alternator instead of a brushless motor as generator, you will solve your problem.

author: calexanian
author_rank: Super Contributor
timestamp: July 09, 2016, 06:21:44 pm
content: +1 gor automotive style alternators. Already very lightweight, efficient, run in the RPM range you need, and cheap or even free if you scrounge them up from a dead car. If you are going to series them I recomend a small battery across each one to balance the charge and provide the power for the field coil. Remember you can leave them turning and they will not load down and generate power until the field coil is energized.

author: TurboTom
author_rank: Super Contributor
timestamp: July 12, 2016, 07:08:18 am
content: An automotive alternator is way less efficient than a good brushless motor used as a generator. Internal resistance is much lower, magnetic field strength is higher and there's no additional energy required to produce the magnetic field in the BLM. Yet, properly controlling the brushless motor in generator mode is difficult. A BLMC with synchronous rectifier an regenerative operation mode will be required in order to properly control output, for example see this one:http://kellycontroller.com/keb72301x24v-72v150a30kwe-bike-brushless-controller-regen-p-274.html] http://kellycontroller.com/keb72301x24v-72v150a30kwe-bike-brushless-controller-regen-p-274.htmlIf you want to go this way or just prefer the "simple" three-phase rectifier with its restrictions "RPM vs. voltage"-wise is finally your decision.Good luck and all the best,Thomas

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 12, 2016, 02:39:01 pm
content: What RPM range does the engine operate most efficiently in? With a series hybrid, you have the option to stay in a narrow operating band.

author: johansen
author_rank: Super Contributor
timestamp: July 12, 2016, 07:12:52 pm
content: simplest way to do this is get three inductors in series with your brushless motor.Find three mosfets, configured in WYE fashion (common source) to short out all three motor phases together. you can use a single gate drive transformer to drive all three mosfets together.You will need a fast three phase rectifier, but even the 3 phase rectifier out of a car alternator may work for you. also need a robust capacitor with low internal inductance wired directly to the rectifier, and the three mosfets must be very close to the rectifier.anyhow, now you have a boost converter. you will need some kind of brains to figure out what percentage of pwm to load the alternator and engine at an optimum rpm and load.

author: mtzm
author_rank: Newbie
timestamp: July 12, 2016, 10:28:34 pm
content: Thank you very much for your replies!Both power and efficiency are important to me, so I prefer to go the BLM, as I want to get out the most of the engine.Can you maybe recommend me an inductor or a toroid capable of the currents generated by the BLM? (should be about 100A P-P)I don't know much about inductor design/toroid selection. Do I need one with low permeability and high flux?I have these capshttp://it.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=15001&langId=-4&urlRequestType=Base&partNumber=2284004&storeId=10165and IPB021N06N3 mosfets.

author: johansen
author_rank: Super Contributor
timestamp: July 12, 2016, 10:49:04 pm
content: so you actually don't need the inductor, however the inductance of the generator is of particularly high loss materials.. iron, air gap.. nickel plated magnets.. etc.what kind and how big an inductor you need is dependent on the frequency of the pwm. if you're willing to build the gate drivers needed to pwm the mosfets at 100khz, then you need some very nice ferrite inductors, but very small ones. three ferrite cores from CRT flyback transformers may work, along with 2 pounds of litz wire.sbut if 10khz works out then you can use iron powder toroids, several pounds of them. .if you can get your hands on a 400Hz 1KW three phase transformer, you can use the core from that as a three phase inductor and set the pwm frequency to as low as 4Khz and it should work fine.the rms ripple current generated by the boost converter will be on the order of half of the alternators output, reducing to zero at full rated load. so maximum ripple current might correspond to half of, half of your generators full rated output. the reason why of course is because at full rated load, 6800 rpm, there is no pwm going on and the capacitors don't have to pass any ripple current except what is generated by the alternator itself (not zero, btw)