Soft start solutions for 1P induction motors?

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I have a 3 phase reactor rated at 8 amps on my shelf.
I have a 1.5 HP 1Ph motor, but not a load for it. FLA: 8.6 @ 230

Fluke 43B on the inrush average of three starts:
No Reactor: 83 amps With Reactor: 82.6 amps

Run NR: 6.13 Run WR: 5,38
VD across the loaded reactor 9.6 volts. (246.2 - 236.6)

Fluke 337 One Sample each time
Inrush NR: 36.6 WR: 36.7
Max: 28.1 WR: 27

37.5 KVA POCO transformer with about 100 foot lateral to 150 Amp service. Pretty stiff.
OPs MMV.

Totally scientific. :rolleyes:
 

kwired

Electron manager
Location
NE Nebraska
I have a 3 phase reactor rated at 8 amps on my shelf.
I have a 1.5 HP 1Ph motor, but not a load for it. FLA: 8.6 @ 230

Fluke 43B on the inrush average of three starts:
No Reactor: 83 amps With Reactor: 82.6 amps

Run NR: 6.13 Run WR: 5,38
VD across the loaded reactor 9.6 volts. (246.2 - 236.6)

Fluke 337 One Sample each time
Inrush NR: 36.6 WR: 36.7
Max: 28.1 WR: 27

37.5 KVA POCO transformer with about 100 foot lateral to 150 Amp service. Pretty stiff.
OPs MMV.

Totally scientific. :rolleyes:
Now tell us how much current limitation it provides if you supply a short circuit or ground fault through it ;)
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Is this a new project being designed from a blank slate, or trying to adapt things to existing equipment?
Are there higher-end air conditioners available that already have a soft-start or variable-speed drive integrated?


OK, so I watched one of the videos -- the one with the nerdy-looking guy in an electronics shop with at least one oscilloscope in the background. I wasted four minutes learning absolutely nothing more than I could have learned from one (still) frame.
View attachment 2555558

The deceptively-simple wiring diagram there raises more questions than it answers. If the start winding isn't used, how is any starting torque developed? With only one winding in use, there would seemingly be only an alternating magnetic field, not a rotating one.
View attachment 2555559
I see a very faint yellow line between controller and motor.
 

fastline

Senior Member
Location
midwest usa
Great data point! I'm curious what reactor you used? It would seem to be oversized for this motr application? I know they work because some of our big CNC machines should pop the line for 1000A or more when the VFDs charge, and current is gentle when you throw the big switch. The impedance is a function of current and the reactor stays in series all the time but does practically nothing during operation since current stays much lower.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
One thing to consider is that inverters have a limited peak current output capability relative to their RMS ratings because of the limitations of their output devices, e.g., IGBTs. And so if you use a phase-controlled SCR soft-starter to reduce the RMS current drawn by a motor it will not provide a proportional reduction in the peak current. Instead it will make the ratio of peak current to RMS current worse, not better.

Another thing is that typical VFDs will not have a sinusoidal input current but it will be drawn in relatively short peaks. An active front end (AC-to-DC switching supply) would make the input current more sinusoidal, but it would cost more. It might be cheaper just to have a larger inverter handle the peak input current of the VFD if cost, rather than the instantaneous power drawn from the source, was the primary constraint.

To the OP, do the photovoltaic systems under consideration have battery storage? And if so, is your concern therefore mainly on the size of the inverters required and not so much on the peak power available from the energy source(s)?
 

fastline

Senior Member
Location
midwest usa
OP here. I guess I am not quite sure if the above is talking more about VFDs for loads or more talking more like solar inverters? IGBTs and SCRs both have some wiggle regarding current as they are largely heat limited like many electronics, but they are very voltage sensitive. Ringing in VFD line set will kill both the VFD and the driven motor if not arrested.

As to the PV systems, yes these are battery supported off-grid applications. I realize I preach to deaf ears most of the time but people in the know have to realize inrush hits have to become a thing of the past! That is 100yo tech! I don't care if we are taking about PV or just grid connected industrial settings, designs hover around motor starts. I have watched countless devices die due to the violent acceleration of motors. It is hard on everything, including the grid!

But getting off my soap box (not due to you guys), a typical PV inverter is bringing in higher voltage DC from an array that is modulated down efficiently to a battery charge level. that DC is effectively the "DC buss" of your typical VFD. A good battery system is rarely the bottle neck for load starting. It is typically the inverters, which usually either have current limiting built in, or will just go offline if you hit them too hard. Junk ones will just blow up. In most inverter, they will handle 2-3x their FLA for a short period, usually 1-5sec. The main issue with an inverter is load starts WITH some level of base load. Consider a 3KW rated inverter. Say it will do 6kw for an inrush. But if you already have 2kw of loads running, you don't have enough reserve for that inrush hit. The solution currently is to stack inverters that consume power constantly and add cost.

I do think the problem is multi faceted. We are moving towards more and more DC. We have brushless DC motors which are highly efficient, yet we are still designing only for AC because that is the grid. There will need to be some new thinking. To date, the largest loads to even consider in normal home are the electric kitchen appliances, and clothes dryer....unless your wife dives deep into industrial 240V 'toys' for her pleasure....lol Most of these are resistive, but are huge in terms of kW requirements.
 

fastline

Senior Member
Location
midwest usa
I think dealing with a simple air compressor is pretty simple to adapt. However, other loads like an HVAC compressor are different in that you are pretty much married to that configuration and 3P residential equipment is extremely rare. Even at that, most already have 1P good so making that work is paramount.

As for water wells, that again has to moved to DC so easily even though it can be easily modulated and likely much better efficiency. I can tell you from experience AC submersible motors are probably like 60% efficient. My general rule on smaller pumps is double the rating for power requirements so a 1HP pump may need about 1500W. And they kick like a mule on your equipment. I struggle to get a 1HP to start on a 5000W generator..... A few vendors started 1P variable drive devices but they were extremely proprietary so I stayed away. When someone won't tell you how it works, that means it is probably simple. Their drives are thousands, and need 'only' their pump motors. I can tell you Franklin 2 wires do not have a capacitor to start them, which has been a huge issue for other pumps.
 

kwired

Electron manager
Location
NE Nebraska
I think dealing with a simple air compressor is pretty simple to adapt. However, other loads like an HVAC compressor are different in that you are pretty much married to that configuration and 3P residential equipment is extremely rare. Even at that, most already have 1P good so making that work is paramount.

As for water wells, that again has to moved to DC so easily even though it can be easily modulated and likely much better efficiency. I can tell you from experience AC submersible motors are probably like 60% efficient. My general rule on smaller pumps is double the rating for power requirements so a 1HP pump may need about 1500W. And they kick like a mule on your equipment. I struggle to get a 1HP to start on a 5000W generator..... A few vendors started 1P variable drive devices but they were extremely proprietary so I stayed away. When someone won't tell you how it works, that means it is probably simple. Their drives are thousands, and need 'only' their pump motors. I can tell you Franklin 2 wires do not have a capacitor to start them, which has been a huge issue for other pumps.
Some those can power either a single phase or a three phase motor, you just have to set a parameter and it will know how to regulate output accordingly. Something a general purpose VFD will not have incorporate into it. Otherwise the basics are mostly the same it is mostly software differences to be able to properly run the single phase motor AFAIK. With a single phase capacitor start motor you eliminate the capacitor and the potential relay, the drive does all the regulating of current in the two windings to get desired results.
 
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