36V Soft start - Page 1
Forum section: electronics / projects

author: scroz
author_rank: Newbie
timestamp: June 17, 2020, 08:29:15 am
content: Hi All,Im new here because I need some assistance with a project, Having studied Mechanical engineering I have a basic understanding of electrical but I have hit a road block with my current project.Basically I'm building a substantial battery operated wet vacuum system, which seemed simple enough but the motor is tripping out the BMS on startup - (inrush current).Battery is a 36V 15.6A Li-ion pack from an ebike, the manufacturer has informed me that the peak discharge current is 50AMotor is rated at ~600W (17.5A@37V 1min after start, tested with a potentiometer)I'm looking to make many of these, and they need to operate with a simple switch, ON or OFF.#What is a simple method I can use to reduce the Inrush current?From my stumbling around the web Thermistors look like they might work for what I'm looking to achieve, if I'm on the right track id like some assistance working out the specs i need for said thermistor.Thankyou in advance.

author: ocset
author_rank: Super Contributor
timestamp: June 17, 2020, 09:56:21 am
content: Take out the BMS trip device, (since as you describe, its tripping.)Then add a current clamp instead.This could either be a controlled pass transistor.But more efficient would be a SMPS controlled to be a current clamp.It could be eg a LT8705 based 'buck-or-boost converter.Or maybe some vicor module could do the trick, with some circuitry added to make it act like a current clamp.Ill have a deeper think for you....i am now thinking of a constant off time SMPS with a fixed peak current limit, and then whenever the inductor current hits 50A, the FET turns off for an "off time", and then back on again...current rises back up to 50A...then back off again........repeat.......This last method could be good because it would be very fast and the current clamp trip level would be bang on right away, instead of relying on an error amp output to slew into "the zone".Actually, all that has to be respected is the 50A discharge current....so i think i would put in a parallel bank of "n" Constant-off-time peak current boost converters...with source current limit set to ~50A/n each....they woudl boost up to just 40V...so as to give a "shallow" inductor discharge. This way i woudl stop the discharge current ever going above 50A......then from the 40V otuput...i would have a Constant off time buck converter feeding 36V to the motor...(possibly a paralleled bank of such bucks)..... i woudl arrange it so that the transient response was very quick....may even do some kind of on/off control type thing, as the motor/drive will suddenly draw the 50A. (or whatever the motor/drive starting current is).Constant off time control works well with on/off type converters (hysteretic ones) since it doesnt suffer sub harmonic oscillations.Than again, maybe a simple dropping resistor (Big one) and a relay or FET to quickly short it out when the motor starts turning could be ok.

author: bin_liu
author_rank: Contributor
timestamp: June 18, 2020, 04:34:07 am
content: The starting current of the motor is several times that of normal work, which is too normal, you can use MCU+MOS to do PWM soft start.

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2020, 08:55:47 am
content: QuoteDo you mean the micro controlling the transistors in the electric drive such that the inrush can be limited like that?

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2020, 03:51:07 pm
content: Here is a simple solution..its a current clamp.I have scaled your case down to 5A max discharge current, for the purpose of the LTspice simulation. (attached)...so you would need a beefed up version of this circuit for your 50AI added the TLV431 model and symbol which you can use if you run the sim.To beef it up, you would use a bigger NPN, and possibly multiple in parallel to split the current down

author: ocset
author_rank: Super Contributor
timestamp: June 18, 2020, 04:02:32 pm
content: Here it is x10 for 50A current limit...but you will still need to find a better NPN (actually ill try to post later for 50A)

author: ocset
author_rank: Super Contributor
timestamp: June 19, 2020, 09:30:21 am
content: Sorry, my above examples dont use the right transistor, and so it doesnt exactly work........but you get the drift.........just need to find a "high hfe NPN or Darlington, that still has high hfe at high collector currents....then put that in there, with the correctly set base current.The circuit i show here is the standard current clamp circuit...........there is a limit on the current that the TLV431 can take, though , hence i say that a high hfe transistor needs to be found.Ill come back soon. Ill have a look for a nice power darlington to go in there.....then a few of these can be paralleled (ie, as the schem above but more in parallel) to give the 50A clamp rating.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 19, 2020, 10:03:56 am
content: You need to get some numbers on the behavior of the motor first. Brand new motor, worn motor. Jammed motor.Then you can see how long it takes for the current to settle. I wouldn't "take out the bms", since that is the only thing preventing a vacuum motor from being a fire motor.With those numbers you can then decide if the battery can't even try to start it (peak >80A) or if the battery may be able to (peak 30A)

author: Zero999
author_rank: Super Contributor
timestamp: June 19, 2020, 08:48:56 pm
content: The simplest way is to use a timer relay, in series with the load, with a resistor across its contacts. The resistor value can be calculated using Ohm's law.R = V/I

author: ocset
author_rank: Super Contributor
timestamp: June 19, 2020, 09:15:43 pm
content: ...yes, and just to be a nauseating copy cat, the same as above but with an NFET switch...or two in parallel.DC relays, above 24V, as you know, tend to be a little more expensive due to the sparking on the contacts as they bounce closed.(I think this was one of the reasons that cars have stayed with 12v batteries for years...instead of the move to 48V....youve seen the amount of relays in cars.)

author: james_s
author_rank: Super Contributor
timestamp: June 19, 2020, 11:02:05 pm
content: I solved a similar problem with inrush in the primary of a very large power transfromer that was frequently tripping the breaker when we turned it on. I found a big old power resistor and wired a contactor across to bypass it, but instead of a time delay circuit I simply wired the contactor coil across the primary of the transformer. As soon as the core magnetizes enough for the current to fall and allow the voltage to rise the contactor closes and bypasses the resistor. In practice this happens almost instantaneously.

author: ocset
author_rank: Super Contributor
timestamp: June 20, 2020, 10:35:23 am
content: ..yes we had that inrush problem too and i think that inrush is caused by the residual magnetism in the core being left with a certain polarity such that when turned back on there's inrush.Anyway, in this case, RC timer followed by switch out of inrush resistor sounds best in this case, would Scroz like a circuit?, if so ill send one in (or others will b4 me)