PWM smoothing - Page 1
Forum section: electronics / beginners

author: j_car
author_rank: Contributor
timestamp: November 16, 2020, 11:36:27 am
content: Hi.I have a 200v 140a 20Khz speed controller and need to smooth the PWM. What would be the best way to do this?Thanks John.

author: Ian.M
author_rank: Super Contributor
timestamp: November 16, 2020, 11:57:16 am
content: *NOT* to !The only feasible option at that power level would be a LC filter, but a capacitor bank capable of handling 140A of ripple current, with >200V working voltage is going to be absolutely massive and pretty expensive, and a choke capable of carrying 140A without saturating is going to need a very large core area, and at 20KHz it will probably need to be ferrite or powdered iron to keep the losses low enough to avoid excessive core heating, so again massive and expensive.Usually in PWMed motor drive applications, the motor windings self-inductance provides all the current smoothing required, and the resulting shape of voltage waveform is irreverent so only just enough filtering to mitigate EMI from the voltage waveform edges is used.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 16, 2020, 12:06:20 pm
content: "Speed Controller" indicates it's used for some kind of motor.I agree with j_car. Quite a lot of power.Instead of asking for a PWM filter solution, tell us more about the system you have and what the problem is you want to solve.Is it some 10kW+ mains powered motor, or is it more like a small model car which has an ESC that has those numbers printed on it?

author: j_car
author_rank: Contributor
timestamp: November 16, 2020, 01:35:22 pm
content: HiThe board works OK when powered direct from the batteries, but when it's connected to the pwm speed controller it stops working at 14v. So I want to smooth the pwm to see if this solves the problem. The maximum I will be going to at the minute with the speed controller is 48v 10a, the values I gave before are the maximum for the speed controller.Thanks John.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 16, 2020, 04:32:34 pm
content: Afraid we don't know how to play the pronoun game -- what board? What does your system look like, from battery or other source, to all the modules involved, to the motor?Why do you need PWM filtered?Tim

author: pwlps
author_rank: Frequent Contributor
timestamp: November 16, 2020, 07:56:50 pm
content: Quote from: j_car on November 16, 2020, 11:36:27 amAs I understand "speed controller" means that you apply pwm to a motor coil ? In this case you should not smooth it.

author: j_car
author_rank: Contributor
timestamp: November 17, 2020, 09:55:51 am
content: Hi TimThe attachment it the last post is a schematic of the board, this is the commutator for the motor. When connected directly to batteries 12v, 24v, 36v etc everything works OK. When the PWM speed controller is used the motor will runs up to 14v and stops, turn the power to the board off and on then it will work again up to 14v. I think it might be the ICs that are latching, the only difference is the PWM speed controller so I want to smooth the PWM to see if this solves the problem. Hence the question, what is the best way to smooth PWM? Thanks John

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2020, 12:41:01 pm
content: Commutator? Like, a BLDC motor? Wait, A1104 isn't a transistor, that must be a... Hall effect sensor then?https://www.allegromicro.com/~/media/files/datasheets/A110x-datasheet.ashxBut why is the pull-up 82 ohms? It's rated for 25mA max.And what's CRE1S? 15?And no pull-down on 'S'?Cute, a two transistor multivibrator, so it'll stay in place until one or the other sensor twiddles it. Complementary outputs, so it won't start in quadrature or anything, it has to magically kick on. So, single phase BLDC I guess.AUX is powered separately, not from ⊞?What does the layout look like, is this a board someone else has produced, is this something lashed up, or?So then, I take it you're treating this assembly like a DC motor, and just bolting that onto a speed controller?You're leaving us to guess an awful lot here...If that's the case, then yeah, that won't work, at least not just any speed controller. Get one made specifically for BLDC. Better still, get a whole BLDC controller and don't faff around with switching circuits.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2020, 10:26:20 am
content: You don't need PWM filtered. You need a motor controller which works with your requirements. There usually is no output filtering.

author: j_car
author_rank: Contributor
timestamp: November 18, 2020, 01:50:03 pm
content: Hi Tim.You are right it is a BLDC using mosfets and hall sensors with a PWM speed controller bolted on. As you say this won't work so it would be better to get one off the shelf. Do you know where I could get a BLDC controller from or who could design one, as the ones I have found are too small (50v) it needs to be much bigger, my PWM speed controller is 200v 140a 80a continuous. Thanks John.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 18, 2020, 01:58:12 pm
content: Look at companies specialized in electric vehicle conversion products. "Inverter" is a better search term. Expect the cost to be around $5000.