Electronics > Projects, Designs, and Technical Stuff

36V Soft start

(1/3) > >>

scroz:
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 50A
Motor 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.

ocset:
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.

bin_liu:
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.

ocset:

--- Quote ---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.

--- End quote ---
Do you mean the micro controlling the transistors in the electric drive such that the inrush can be limited like that?

ocset:
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 50A

I 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

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod