Controlled Firing Angle BLDC Generator - Page 1
Forum section: electronics / projects

author: Electrical_Jeb
author_rank: Newbie
timestamp: July 31, 2015, 08:22:09 pm
content: I am trying to build a generator to output a controlled DC voltage from a brushless DC motor that can account for variations on the input shaft speed and maintain a relatively constant output voltage.I intend to build a 3 phase bridge using 6 solid state relays to rectify the output from the motor. A 12 bit absolute angle hall effect sensor will be used for commutation but I would use field oriented control on a future iteration. There will be 3 isolated current sensors measuring the current going to each phase of the motor which will be used to trigger each SSR to turn off when the current reaches 0, basically acting like a thyristor but at a frequency much higher than mains.The idea is to control the firing angle to control the average output voltage. My big question, which I hope someone might be able to help with, is whether or not I am missing something fundamental about whether or not this will work or whether it will be remotely efficient.I know that there is a more common method of using a BLDC motor as a generator that operates using the same circuit as just the motor driver. So far as I know this method basically uses the motor as an inductor as part of a boost converter circuit. The reason I am trying to avoid this is that the current output is discontinuous. The application is volume sensitive and I would expect that a capacitor on the output that could smooth the output voltage from the generator in this case would need to be able to handle a very high Irms, and so far as I know, those capacitors can be very large.I have done a lot of research and can't seem to find any example of a controlled firing angle BLDC generator being implemented before. Is there something I am missing that would make it obvious why I shouldn't attempt this, or is it possible that I could make a generator that wouldn't require as much passive filtering on the output as other BLDC generator systems?Any questions, ideas and suggestions are appreciated. This is my first post on the forum so if there are formatting issues or anything else, please let me know.Thanks.

author: C
author_rank: Super Contributor
timestamp: July 31, 2015, 09:20:25 pm
content: Think of what you are saying here.A BLDC when driven will produce three phase AC.A Motor drive when braking( BLDC is a generator) is switching on/off at a very high rate for each cycle of power. The faster the switching rate the smaller the capacitor.Switching by phase angle results in two switches each cycle of power. Bigger capacitor would be needed.. In addition this is harder on the generator an why Power Factor Correction is needed for AC loads.C

author: oldway
author_rank: Super Contributor
timestamp: August 02, 2015, 07:27:58 am
content: Hello and welcome to the forum.A short presentation would be nice.Anyway, as regards to your question, we would like you to provide technical informations and, if possible, a diagram of your project and some pictures.Without these technical informations, no one can answer your question properly.For example, you speak of a brushless DC motor, which motor is it? (type, brand, voltage, current, with or without electronic control, ... etc ...)What range of expected speeds?You also talk of using solid state relays ... what model?In general, a scr bridge can not be used for very low voltages because the voltage drop on scr's is too high. (2 x 2V = 4V with a high current)Another problem is the Toff thyristors that is too high to operate in frequencies much higher than the network frequency. (problem of overlap and short circuit windings).In the past, fast toff high power scr's were available for inverters applications, but they are absolete now.Solid state relays are not provided for this application.If it is a brushless DC motor without its electronic control, (3 input wires), it's actually an alternator with permants magnets.In this case, the easiest solution would be to use a three-phase bridge of fast or schottky diodes, a filtering electrolytic capacitor and a DC to DC converter.

author: Electrical_Jeb
author_rank: Newbie
timestamp: August 04, 2015, 06:22:13 pm
content: Thanks for the input.The motor in question is the scorpion motors SII 5535http://www.scorpionsystem.com/catalog/aeroplane/motors_1/sii-55/S-5535-160KVIt is a BLDC intended for RC airplanes. It has no built in control or hall effect sensors. The hall effect sensor to be used ishttp://ams.com/eng/Products/Position-Sensors/Magnetic-Rotary-Position-Sensors/AS5048A.The motor has a Kv of 160 and the windings are connected in a delta configuration. It will be driven by a gas engine that has a speed controller on the throttle to keep it running at 7200RPM. The output voltage will then be 45V peak to peak.The motor has 14 poles so the frequency of the voltage on each phase is 14/2*7200RPM/60s= 840Hz.I said solid state relay but i should have said a pair of N channel MOSFETs arranged source to source to act as a switch that can block current in either direction. I am still trying to select the best MOSFET for this application but I am looking at these ones from TIhttp://www.digikey.ca/product-detail/en/CSD19531KCS/296-37480-5-ND/4437459.I agree that the easiest solution would be to use a bridge of schottky diodes and to control the output voltage with a DC to DC converter. I had built a prototype to do exactly that however the filtering capacitor on the input of the DC to DC converter was very large and expensive in order to handle the Irms. It just didn't seem practical to scale up to handle a peak of 100A.Here is a picture of the diode bridge and switching regulator I made. I'm sorry it looks like crap, it is for an undergrad project and a lot of the thermal management was done after the fact.I dont have the prototype with me at the moment but these are pictures I have of it.The filtering capacitor between the diode bridge and switching regulator is this capacitorhttp://www.digikey.ca/product-search/en?vendor=0&keywords=338-1923. It has a 160microfarads and about 1milliOhm ESR. It can handle 74amps Irms. Since the motor is so inductive, the capcitor sees nearly the full current of the system going in and out of it.The inductor in the DC-DC converter is this coil from coilwshttp://www.coilws.com/images/ES55246-300M-180AH%20Rev%20B1.pdf. The combined weight of just those 2 components was over a kilogram and I am trying to find the lightest solution.Here is a schematic of what I am trying to design. Basically a 3 phase bridge with 6 bidirectional mosfet switches. The gates of the mosfets are driven by a TI DRV8301 which is intended for driving a 6 mosfet inverter.The output from this bridge to each of the phases would pass through a shunt resistor in order to measure the current. Since the shunt resistor isn't tied to ground or the high voltage I intend to use an AMC1200 as an isolated amplifier to measure the current. This current measurement would be used to detect the zero crossing of the current and would trigger whichever switch was conducting to turn off. Basically act just like a fast Toff thyristor.I understand that this would contribute mechanical noise to the system since the there would be a spike in torque every time one of the switches is toggled on. However I am trying to pursue this method of rectification since the application requires it to handle high currents, between 50A and 100A. I think that it would be difficult to have a filter to smooth out the chopping of such high currents.What I am asking for is if my assumptions are incorrect. That using a controlled firing angle rectifier on a BLDC is a viable option or if I am just wasting my time. C pointed out that a motor drive in braking is switched at a very high rate which would reduce the size of capacitor needed. Could I ask what size and type of capacitor are typically used in this application? Also, could I ask how the power factor would be a problem in this case? At least any more of a problem than it would be by using a simple diode rectifier?If there is anything else I can provide that would help or that I left out, please let me know.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 04, 2015, 09:41:51 pm
content: Quote from: Electrical_Jeb on July 31, 2015, 08:22:09 pmYou need a standard 3 phase inverter comprised of 6 MOSFETs or IGBTs to do this. Which is to say, you don't need switches capable of reverse blocking like shown in your last post. This is because the half-bridge can operate as either a buck or boost converter. In the parlance of motor control you will be operating the motor in regeneration mode, but any decent FOC (field oriented control) motor control code running on an appropriate microcontroller or DSP should be able to seamlessly transition from operating the BLDC motor as a motor (buck) or a generator (boost).If there is no need for high torque at 0 to low RPM in motoring mode (or, high output current at 0 to low RPM in generator mode) then a "sensorless" BLDC control scheme will be fine, otherwise you need to know the rotor position with high accuracy.EDIT - corrected "comprise" to "comprised"

author: nuno
author_rank: Frequent Contributor
timestamp: August 04, 2015, 11:24:13 pm
content: I was also under the impression that you could use the motor's windings as coils for the boost converter, each phase pumping current in turn into the output capacitor.

author: C
author_rank: Super Contributor
timestamp: August 04, 2015, 11:38:52 pm
content: expanding on MagicSmoker post,An alternator likes a resistive load. The three phase bridge causes current to go from almost 0 to very high on peaks of cycle, not a resistive load. The Phase angle control does same, just different place on the cycle. This causes extra load on the alternator preventing max power causing mandates for PFCA motor controller when breaking is working like an inverter. When the input is kept looking resistive, you have power factor correction happing while breaking.So you could also search for "3 phase power factor correction". Most shown are using Boost mode, but you can have buck/boost. PFC to keep the alternator seeing a resistive load while creating DC. You could think of a PFC as a switcher with additional control loop. Some are bridge-less, lowering power waste.There is a mode of switch control where in place of ob/off current, you use the switches to keep a more constant current on the inductors lowering the noise to filter out.You have three phases. In place of one large current pulse of the DC-DC converter, you can have three smaller pulses.All of this adds getting parts smaller and lighter as switch freq. increases. The Higher freq switching causes more heat in mosfets.You state 840Hz so your phase angle filters are working at ____ and weight=____.The simple thing would be to switch the low phase of alternator output by the cycle. The other two phase, switch at very high freq, One on while the other Off. Look at how the needed filters change.When PFC is added, The PFC changes the width of the on phase and/or freq of the two phases creating more of a resistive load on input. The DC output voltage controls average current of PFC. This is a good motor controller breaking.C

author: oldway
author_rank: Super Contributor
timestamp: August 05, 2015, 04:43:02 am
content: Your schematic with 3 phases bridge rectifier looks good but soft start Mosfet is not needed and you should use two buck converters with 180° phase difference instead of only one to reduce ripple current.You never said what is the load DC voltage you need.Be careful, this motor is intended to be used as an airplane motor and is then cooled by the wind generated by the propeller. (forced cooling)You will probably have to reduce the output power in your application.