| Electronics > Projects, Designs, and Technical Stuff |
| pinout puzzle on big BLDC drivers (repurposed automotive motors) |
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| james_s:
If you want the fun of building something then you'll certainly learn more by designing something yourself. You won't save any money over buying a mass produced brushless ESC though, there's no way. It's been a couple years since I've built an airplane but I was paying $20-$30 for a decent quality 300-400W ESC. I just checked and you can get a 100A controller for $44, there are some under $30 but I've used several of these Turnigy Plush models and they do work significantly better than the cheapest ones I've tried. https://hobbyking.com/en_us/turnigy-plush-32-100a-speed-controller-w-bec.html |
| max_torque:
--- Quote from: mtraven on November 30, 2019, 11:47:54 pm --- --- Quote from: max_torque on November 30, 2019, 05:30:03 pm ---These EPAS motors are not that useful in traction or continuous operation because they are only rated for short term operation, for steering assist, which on a road car is a very part time duty cycle. The driver is also fairly limited in terms of continous current. The large mosfet will be used as a primary power switch, to enable the power to the motor to be shut off for fail safe no matter what happens to the 3 phase driver itself. As steering systems as "safety critical" they tend to include dual switching capability and often dul processors, with one operating as a equizzer / monitor to check the performance of the system is appropriate. --- End quote --- I wondered if there wasn't some redundancy built in, that makes sense on such a safety critical system. I understand the term EPAS to refer to column mounted systems, which these were not. These were rack mounted with a 1:1 pulley driving a ball screw(pulley/nut). Just 1 per axle. I think most of your assertions about the duty cycle are still sound, just wanted to clarify. --- End quote --- There is no electrical redudancy, because the fail safe state is to simply de-power the driver. The steering column is still the primary mechanical link, so it's just important that should control of the motor be lost, the system can be effectively switched off.. Worst case would be the motor turning when uncommanded, but also if the driver mosfets fail short, the motor can fail locked.. Either case is, er non optimum in a car being driven at speed along a road! |
| mtraven:
--- Quote ---I'd bet the position sensor is the SO-8 component, smack bang in the middle --- End quote --- YES! good eye! that has been bugging me for a while now, but that chip is in fact a KMZ49-magnetic field sensor. --- Quote ---Regarding power, EPAS racks typically are rated at around 1.2 to 1.4 kW peak (100 amps peak supply current at between 12 and 14 volts) and will usualy have a continuou rating of around 300 watts --- End quote --- is that consistent with the measured resistance of the coils(0.058ohms)? Sorry to do this, but I have to go back to, this is not an EPAS, steering column mounted motor. Everyone of those I have seen are much smaller motors driving a worm gear with a bit of planetary gearing to prevent the lock up scenario you described since wormgears don't back-drive. --- Quote ---If you can get the datasheet for the driver IC, then you can reverse engineer the drive signals to the packageless fets, and keep using those (because they are obviously well matched to the motor) --- End quote --- oh how I would love to have that datasheet...the driver seems to be made by a company called "hella." I sent them an email about a week ago and have gotten no response. Good to know that those are called packageless FETs, i thought they were a bit goofy looking. Are those able to handle more current/ cool better since there is no package? While I agree those FET's are obviously well matched to the motor, I destroyed the connections between the two boards and that whole thing is covered in this clear goo, so I am not even sure I could get it clean enough to solder new leads. --- Quote ---If you add a large fan driven heatsink or water cooling system to the outside body of the motor, you will be able to significantly increase the continuous rating of the unit. --- End quote --- i agree and I see no reason not to do that...I can also spread out and better cool the FET's --- Quote ---These motors are designed to be low inertia, because they try to drive the rack without adding additional inertia and hence damping into the steering system --- End quote --- thats fascinating, I would have thought you'd want some dampening to replace the dampening normally done by a hydraulic steering rack. good to know. suppose I was to use these as servos, I would need to add a proper encoder to properly position, correct? --- Quote ---If you want the fun of building something then you'll certainly learn more by designing something yourself. You won't save any money over buying a mass produced brushless ESC though, there's no way. --- End quote --- I hear ya, and have experienced that many times. I look at it like this: if the DIY cost is less than the premade cost + cost of course work to learn the topic, I still come out ahead. But in this particular situation, there is a good chance I can do it cheaper. I while back I bought the inventory of an electronics shop that had gone out of business, so I have boxes and boxes of electronic components..got a whole crate of high power NPN's / mosfets ect. Thank you for the recommendation, I will certainly look into those. I do have my doubts that a 100a controller is even enough, but that's just a hunch based on the resistance of the coils. Then we get into repair...suppose I burn out a premade driver...if its something simple like the FETs I can replace them, but if its anything else, without a schematic, the board is probably shot....whereas if I have designed it, I can more effectively diagnose and repair problems. |
| max_torque:
The measured phase resistance of the coils means nothing. In order to build a high efficiency motor, that motor needs to be substantially reactive rather than resistive, ie to have a high inductance and a low resistance. For a motor controlled by an "Inverter" there is no such thing as "short circuit current" (i'm ignoring active short circuit functionality here) and phase current is controlled dynamically at all times. A EPAS system in order to provide the maximum fidelity must provide torque assistance to the driver, ideally without imposing any inertial or drag forces on the steering system, ie be completely transparent in its operation. Therefore the motor is designed for low inertia, and high peak current so it can very quickly change its velocity to match that of the steering system. EPAS simply means "Electrical Power Assisted Steering" and there is no relevance to how that assistance is provided, or the architecture of the system. Early, low power, high inertia systems were mounted directly on the steering column, but they were bulky, noisy, and very poor performing often having truely shocking steering "feel". A few "city" cars used them for a while, but the gen2 systems such as the ZF/Hella system you have moved to a rack mounted high performance brushless motor, with only the driver demand torque sensor mounted inline in the column itself. The rotor of the motor must be low intertia because it inertia is referenced to the rack by the overall gear ratio, which needs to be as practically large as possible to provide the greatest assistance force from the smallest (cheapest) motor! You won't get any info from Hella regarding the driver IC, as that is a proprietry automotive chipset, so you'd have to sign an NDA with them before they will release it to you. Realistically, peak motor currents of around 100 amps are used, perhaps as high as 150 amps for a few seconds, but the resistance of a lead acid battery starts to become limiting, ie the supply voltage falls with increasing current, limiting the power you can pull from the battery to a couple of kW at peak. If you have a look at the phase current measurement system, you may be able to get an idea of the peak phase current used, it may use one of three different phase current measurement techniques: 1) Voltage drop across the power silicon 2) Voltage drop across resistive current shunts 3) A hall effect non contact magnetic based measurement system There is also a possibility it doesn't actually directly detect phase current, just measures supply current and ratio metrically calculates the phase current from the effective duty cycle ratio |
| max_torque:
PS, the sytems try to be mechancially transparent, so that damping and inertia compensation can be calibratable parameters and allow the system to be tuned to the necessary atributes for the particular application |
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