Reducing starting load on 3HP motors

[PaulL]

I finally have my solar power setup going in the new workshop. The lights work great! The grinders work great! The hand power tools work great!
The 240V split phase 3HP motors, not so much. The compressor threatens to start before the inverter trips off, and the power hammer manages to wiggle its flywheel momentarily. Sigh.
Though rated at 9kw continuous, and 26kw for 3 seconds, or 16kw for 10seconds, my loads are just too hard for this thing to start them. Annoyingly, my generator (Honda EU7000i) just gets them started, but of course, not when it's in line with the inverter.
So I'm looking at my options, and wonder if anyone has experience with split phase soft-starters. These look dodgy from all I can read.
I have an awkward feeling I'll be replacing the motors with 3-phase plus a VFD but would like to see if there's lower cost options to fail at before failing at starting the 3-phase motor successfully ;-)

 

[Mcgyver]

What about a thermistor? They aren't very much money. I've used them to stop inrush current tripping a breaker, in my case a variac, however I think the problem is the same. If not familiar, they are a resistor that varies by temperature which limits the in rush current. Then after a second or two of current conducting they're warm enough that they drop in resistance dramatically, like to milliohms depending on which one.

 

[Susquatch]

I finally have my solar power setup going in the new workshop. The lights work great! The grinders work great! The hand power tools work great!
The 240V split phase 3HP motors, not so much. The compressor threatens to start before the inverter trips off, and the power hammer manages to wiggle its flywheel momentarily. Sigh.
Though rated at 9kw continuous, and 26kw for 3 seconds, or 16kw for 10seconds, my loads are just too hard for this thing to start them. Annoyingly, my generator (Honda EU7000i) just gets them started, but of course, not when it's in line with the inverter.
So I'm looking at my options, and wonder if anyone has experience with split phase soft-starters. These look dodgy from all I can read.
I have an awkward feeling I'll be replacing the motors with 3-phase plus a VFD but would like to see if there's lower cost options to fail at before failing at starting the 3-phase motor successfully ;-)

I like @Mcgyver 's thermistor idea, but if that doesn't work, you could play with various fixed resistors and some relays. The start load would be through the resistor and then a relay would kick in to bypass the resistor.

Neither one of these will work if your motors see full load right away (eg pumps). But if you are just spinning them up, the right size resistor can choke off the in rush current enough to get them spinning.

 

[Former Member]

All AC motors single phase or 3 phase have 30% slip which means the motor lags behind the peak voltage approximately 30% at which point, that is your peak operating current. In rush occurs because the motor is being passed several times before the before the motor comes up to full operating speed, and this causes increased resistance. Ohms law, since voltage is fixed the only thing that can change is current, this is in rush current. Adding a thermister (increased resistance) to the circuit in series would only increase the current.

Personally from experience years ago the best and only solution is VFD as your peak current draw is maximum operating load current with a properly programed VFD. Here the motor slippage stays at 30% as the motor the frequency is ramped up as the motor only lags behind by 30%. This is one of the main reasons companies trying to save electricity costs are switching all 3 phase motors to VFD drives and run all high current draw motors 3 phase because it virtually eliminates in-rush which reduces current draw which reduces electricity costs.

If you are trying to save money, bite the bullet and make the switch and do it right, now! Anything else only increases your cost long term as you must pay for the parts and wiring to do it twice, not to mention the grief and aggravation of having not done it in the first place.

 

[Susquatch]

I fundamentally agree that a VFD is the better solution @Degen . But I think you had a brain fart on the impact of adding a resistor. A resistor in series with the motor windings does not increase the current flow, it decreases it. Which is why @Mcgyver and I both recommended it.

One thing I neglected to mention to @PaulL though is that it takes a resistor size (wattage rating) (= $) appropriate to the application. The cost does add up.

But ya, vfd is the way to go. Not just because it solves the current inrush problem but also because it adds other functionality.

Ps - who can't love the soft smooth starts, stops, and direction reversals that a VFD facilitates!

 

[Former Member]

The problem with that is that it stresses your motor, particularly those that start under high load, like compressors.

 

[Susquatch]

The problem with that is that it stresses your motor, particularly those that start under high load, like compressors.

Yup, I already pointed that out. But most of our machines are not compressors or pumps. Pumps are a no-win problem that require a VFD, but compressors can be set to kick back on at lower air pressure where the load isn't as high. It doesn't take much. 5 or 10% would prolly do it.

Besides that, with the exception of high load applications (pumps etc) reducing the in-rush is actually easier on the motor than slamming it full current.

 

[PaulL]

Newly informed by the thinking on this thread I've paged back in too much (or is that not quite enough?) of my EE classes 35 years ago (yikes!). I remember why I switched out of electrical now...
That said, I like Degen's direction - it makes total sense, and puts the least abuse on the whole system.
Now to scope out a motor and a VFD. I expect I'll be able to find something locally - we have a couple of small motor shops I can ring up on Monday.

Thanks!

 

[PaulL]

When did VFDs suddenly become this cheap? $133 including shipping? Am I missing something?

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[Former Member]

There are lots of options out there, I spent a lot more for mine but I bought it from a motor shop I've known for years.

 

[trevj]

Compressors are big ugly loads at best times, and short of making sure the bleed down system is working so that the compressor does not start against a load, or swapping the pulley on the motor for a smaller one, got not much to suggest there. Does it need to be that large?

Being on solar is a kicker, if it's killing off the inverter. About all I have to poke in your general direction, is to see if you can set up the motors to start under no load, then once they are running, let the momentum aid in bringing the flywheel up to speed on the hammer. A basic overcenter toggle lock on the motor mount, to slack the belt, is what comes to mind.

 

[PaulL]

About all I have to poke in your general direction, is to see if you can set up the motors to start under no load, then once they are running, let the momentum aid in bringing the flywheel up to speed on the hammer.

Yeah, I tried that. No go. Quite disappointed, really, as I sized the system for the nominal starting load, and it's simply not providing it. So now I'm going to see how much I get to squeeze the electrical contractor who implemented the design. Should be good for at least a bit of labor setting up VFDs...Not that I'm sure I'd trust him now.

 

[trevj]

Good luck with that!

Honestly, if he's a solar contractor, he is likely working off assumptions, that fit well with folks that don't have things like Power Hammers, and big (ish) compressors to deal with.
Normally stuff like a microwave or coffee makers, maybe a well pump, is 'peak draw'.

I'd be interested in hearing how it goes, and whether the root cause is an under-rated inverter, battery bank, or what...

 

[Former Member]

Years ago when I sold the first 2 VFD's on a hospital elevator modernization. The hospital found that on standby power both elevators could run and not take more than one before the upgrade. They still one used one as it gave them overhead of power that they didn't have before.

The biggest change over 30 years has been the improvement in the electronics in terms of power handling (ie higher currents and voltage), higher frequency (smoother cleaner power).

 

[Susquatch]

The biggest change over 30 years has been the improvement in the electronics

And lower cost!

 

[PaulL]

I just got off the phone with a local motor shop - this isn't going to cost nearly as much as I was afraid it would!
A new Techtop aluminium 3PH 3HP 182T motor is quoting at $550. Plus $135 for the VFD, I'm in under $800 including taxes and shipping.
And I'll be able to install it the next time I get to the cabin.

Any thoughts on Aluminium vs Cast Iron housings? I can get cast at the same cost, but with a 2 week wait.

 

[Susquatch]

Any thoughts on Aluminium vs Cast Iron housings? I can get cast at the same cost, but with a 2 week wait.

No knowledge.

But I would think that if it's available, it's ok. Prolly only there for certain environments. Yours is zero issue. So I would not fear aluminium at all.

Is that motor vfd rated? If so, do they sell them in Ontario?

 

[PaulL]

Is that motor vfd rated? If so, do they sell them in Ontario?

Yes, "vfd-ready". And they probably are sold in Ontario: http://www.techtopcanada.com/
They claim 48,000sf of facility in Toronto!
My retailer is quoting under the web site price, so you might want to call around.

 

[Susquatch]

Yes, "vfd-ready". And they probably are sold in Ontario: http://www.techtopcanada.com/
They claim 48,000sf of facility in Toronto!
My retailer is quoting under the web site price, so you might want to call around.

Thank you!

If that motor is anything remotely like mine, you are gunna love it!

 

[PaulL]

An update: I replaced the 3.7HP motor on my compressor as well, with a 2HP motor that my motor tech recommended instead of going larger. I have a 4KW inverter on it, which sounds like too big, but the smaller inverter that drives my power hammer's 3HP motor isn't happy pushing the compressor at lower speeds. And in a fit of brilliance, past-self put away all the air things in one box when leaving Kitcherer 3 years ago, and all the air bits went back together on the first try! Magic!
So here's the shop with more of the ply and partitions up, the striking hammer, and the Anyang hammer, and my little propane forge. I actually got some work done in there, though nothing photo-worthy.

Next is to get a box built around the batteries.