Will Induction Motor Inrush Trip Battery BMS?

[JohnnyQ]

Hello DIY Solar Power Forum Members.
I am currently building a mobile power cart for my off-grid workshop. I did some inrush current startup tests using the peak function on my multimeter (klein Tools CL800). The largest inrush current came from my table saw (2 hp running) 55 amps. The motor code says it should be 59 amps so my test is close but a little unconservative. I expect the MM to be unconservative as from what I have read the peak function is not a true inrush reading like on the fluke MMs. I read the inrush is already dissipating by time the multimeter takes a reading using the peak function. I purchased a 24V 6k/18k sigineer inverter. I would like to get a Lithium iron phosphate battery but I am concerned about the BMS tripping. Will the batteries see the same inrush the inverter sees, 59 amps * 120VAC / 24 VDC = 295 amps? Or, will the inverter act as a sort of current limiter and pull much less amps? Because of this I may go with lead acid as my golf cart routinely pulls 350 amps going up hills. Or build a battery pack with a BMS and solenoid relay?

 

[mikefitz]

Consider a soft start to reduce the inrush, locked rotor current, the motor takes on startup.

Controls -> Soft Starters -> Single Phase Soft Starters

blackhawksupply.com

My estimate peak current without soft start, 60 amps at 120, with soft start 20 amps.
It's possible to get high current FET based BMS. An option with a diy battery is to take power direct from the cell pack and 'bypass' the BMS for discharge.
24 volt batteries with 250 amp BMS are available. A single battery could easily power via a soft start unit, or two batteries in parallel without soft start.
I would consider the soft start regardless, as besides reducing the inrush, is mechanically less stressful.

It's also possible to have a soft starter incorporating energy saving. These work well and in an offgrig application would be worth considering.

Fairford Soft Starters HFE | Fairford Soft Starters

Fairford Soft Starters HFE Sale, Repair and Service At Specialized Electronic Services, Call Toll Free 1-800-681-7374

fairfordsoftstarters.com

This is one area I am confident in. An energy saving soft starter is worth the investment. (I designed the system).

Mike

 

[JohnnyQ]

Thanks Mike,

Your numbers agree with the calculations and tests I ran.

I spoke to ?Bill? at Specialized Electronic Services (one of the links you provided). Super sharp guy and extremely helpful. We chatted for 15 minutes; I wish I recorded the phone conversation. A ton of great information. Bill (again I hope I got his name correct) recommend I swap out the 1 phase capacitor start motors on the table saw and jointer with 3 phase motors and VFDs, even if I was not running off an inverter. I did more research, and this is what I found:

“Soft starts on single phase motors is a craps shoot. The SCRs in the soft starter see the starting capacitor charging as a short circuit, which stresses the SCRs, the starting capacitors see the chopped waveform of the SCRs as a non-linear load, which heats up the capacitors. It's a race to see which one fails first.”

Now I thought about rolling the dice anyway with a soft start since “the starter only stresses the caps during the start”. However, many of the soft starters have a limit of ten starts per hour. I could easily do 20-30. I guess they are meant for motors that run continuously.

The next issue I have is running the Miter saw, planner and some other standing machines with heavy hitting universal motors that are pushing about 50 amps at startup. There is a product by Innoraymond, GS10, Universal Soft Starter. Basically, it seems like it is a sophisticated thermistor or a resistance assembly that shuts off after a few seconds.