soft starter ideas on single phase compressor.
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    Default soft starter ideas on single phase compressor.

    going to have to do something here guys. Have your run of the mill 5hp, 3600rpm compressor that has to be taken to an area with limited power and its going to be an issue on inrush. Already tested the motor under load at 16.5A/235V, and 78A inrush. Its not worth throwing in bigger everything just to cover that 100ms of inrush. I would be happy to reduce this to 2x FLA.

    I know it has been discussed here guys, and I am not new to the subject, but lets come up with an idea that makes financial sense. Obviously I can probably find a used motor and VFD to throw at it, but it would exceed the value of the whole compressor.


    I would think a means of reducing voltage for a very short time on the main motor winding should calm it down? Obviously a little longer start up but that is a non-issue.

    The VFD could be nice to overspeed the compressor just a touch and get a little more, or maybe get creative with the tuning, but honestly, I am mostly looking for a quick, cheap solution for now.

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    Quote Originally Posted by huleo View Post
    going to have to do something here guys. Have your run of the mill 5hp, 3600rpm compressor that has to be taken to an area with limited power and its going to be an issue on inrush. Already tested the motor under load at 16.5A/235V, and 78A inrush. Its not worth throwing in bigger everything just to cover that 100ms of inrush. I would be happy to reduce this to 2x FLA.

    I know it has been discussed here guys, and I am not new to the subject, but lets come up with an idea that makes financial sense. Obviously I can probably find a used motor and VFD to throw at it, but it would exceed the value of the whole compressor.


    I would think a means of reducing voltage for a very short time on the main motor winding should calm it down? Obviously a little longer start up but that is a non-issue.

    The VFD could be nice to overspeed the compressor just a touch and get a little more, or maybe get creative with the tuning, but honestly, I am mostly looking for a quick, cheap solution for now.
    Too complicated. Single-phase REFRIGERANT compressors - air-conditioning tribe - deal with this all the time.

    See: "hard starting capacitor" or "hard start kit".

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    3600 Rpm motor indicates non-commercial or not mentionable unit.

    Easy fix is to add a correct unloader so motor starts unloaded and thus starting current gas less for less time.

    Hard start caps may help motor start but still require source energy.

    Unloader reduces load on motor during start time.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    You want quick and cheap- install a gasoline engine on the compressor.

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    Thank you Bill (thermite) for bringing to light the hard start kits for A/C's. I googled that and think it might be just the ticket for an ageing central air at a loved one's house. I had no idea such a thing existed!

    I'm still mystified by the position the relay must be installed and find it hard to believe some of the explanations that insist it has to do with gravity assist for the contact lever arm... but that's just me I suppose.

    Anyways, thanks Bill!

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    I have a older quincy 325 and it has a really nice setup just like your asking about ,,, the unloader valve does not close tell the crank has oil pressure. I don`t know if you and a pressure pump or a splash pump but at any rate you should have a unloader you can hold open for a few seconds so the pump can free wheel up to speed before the unloader closes and it starts pumping air . you could even do it with a simple timed relay and a air valve ... hope that helps

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    Quote Originally Posted by gusmadison View Post
    Thank you Bill (thermite) for bringing to light the hard start kits for A/C's. I googled that and think it might be just the ticket for an ageing central air at a loved one's house. I had no idea such a thing existed!
    Don't be too optimistic, on that one. Any HVAC guru as is old enough to be able to chew solid food has 'em on the truck. Lots of packaged units, factory has a cap already there one just moves a spade lug over to a different terminal. That's all a routine part of the initial install and commissioning.

    Old unit? Struggling now, but didn't raise a sweat when new? An over-age-in grade NORMAL start cap in need of renewal is the more likely fault. Check for that first. They are cheap enough, and even easy to get at.

    Adding a new Hard-start cap could mask that ... OR... speed-up the demise of an already-dying refrigerant compressor. DAMHIKT, but it WAS overdue, so BFD, bought me a new one.

    I'm still mystified by the position the relay must be installed and find it hard to believe some of the explanations that insist it has to do with gravity assist for the contact lever arm... but that's just me I suppose.
    Not at all new.

    Vibration is a reality. Gravity is a dead-reliable helper, stable, barring the odd Earthquake, never gets tired and weakens or breaks as springs can do. Why not put it to work? For free, even.
    Mind - position-sensitivity is NOT what you want in a fighter plane. See Spitfire carburettor vs Bf-109 fuel injectors.

    Prolly not a concern on OUR dance-card, though. Average shop only LOOKS as if it gits turned clear upside-down every third day or so.



    I use a few no-longer-common - or never really were all that common - "Mercury Displacement" corn-tractors so as not to be fussed over arcy-sparky sources of ignition of truant solvent vapours nor contact wear.

    For damned sure THOSE buggers must be installed and operated "this side UP!"

    Not to mention properly sized and protected against severe overload. Seriously Politically Incorrect to turn toxic Mercury into a pyrotechnic device.



    "Don't try this at home" IOW. They ain't for overly casual application. At all.

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    Not that this helps, but you have a 2.5 to 3 hp compressor. A 5 hp compressor draws about 25 amps at full load. Typical a small single phase electric motor draws about 5 amps per hp at 240 volts.

    Unloading the compressor will reduce the amount of time the motor draws peak current but not do much to reduce the peak current. The peak current is mostly a function of the motor speed relative to the frequency of the supply as opposed to actual load (load slows the motor thus more current).

    A simple unloader would be a small air tank between the compressor outlet and the check valve.

    A VFD does not work with single phase motors. So you would have to install a three phase motor but it would then greatly reduce the inrush current.

    What is your source of power that you will be operating this from, a generator or a low current circuit from a utility?

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    I am not sure a hard starting kit, i.e. a capacitor will cut down on the inrush. The addition of a capacitor will cancel some of the inductive reactance which could actually increase the inrush. It will allow the motor to come to speed more quickly as it provides more phase shift for torque. More torque>more flux>more current.

    The issue is that at stall, the only factor limiting the inrush is the impedance of the motor. The rotor is stalled so there is no back emf. The connected load has no effect this time, it only effects the length of time that the motor needs to come to full speed.

    In order to reduce inrush, the voltage to the motor must be reduced for the starting period. This gets to be a bit dicey as the motor must still be allowed to reach the cutover speed where the starting switch opens. The ideal regulated current controller would be a magnetic amplifier or a solid state current limiting power supply where the current is dynamically controlled. Resistors might work but would have to be bypassed after starting.

    Tom

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    [QUOTE=TDegenhart;3396902]I am not sure a hard starting kit, i.e. a capacitor will cut down on the inrush. The addition of a capacitor will cancel some of the inductive reactance which could actually increase the inrush. It will allow the motor to come to speed more quickly as it provides more phase shift for torque. More torque>more flux>more current.

    /QUOTE]

    Exactly! A hard start kit (start cap and potential relay) was never intended to reduce inrush, it was intended to BOOST start torque on the motor that otherwise only has a run capacitor. I see HVAC guys doing it all the time without question, but I am NOT a fan and have personally witnessed them doing this when there was an obvious voltage issue at inrush due to wiring.

    I got into quite an argument with an HVAC 'guru' over current through a cap because he was convinced and telling people the cap would blow up from continuous starts. Not the case. The current through a cap is fixed by the uF, frequeny, and voltage.

    Anywho. It doesn't sound too promising. Yes, I realize a VFD would require a 3P motor, and no, this is NOT a china POS. It is a CH back when they made good stuff. I am aware the actual performance of the motor is NOT 5HP, and they all fib about that.

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    Have done this for A/C compressors. Autotransformer voltage reduction. That DOES reduce current two ways. It lowers the voltage, AND it reduces the line current by the autotransformer turns ratio.

    Usually no current sensor, just a timed changeover. It should be able to reduce the current to where you want, but you may have to dig around to find a transformer based reduced voltage start kit. We did our own design because it was for a stocked product, but I have seen commercial ones. Finding the size etc wanted may be a but harder.

    There are also units from Emerson, (probably Nidec now) that are solid state limiters. They work, but not as well in terms of current reduction vs start performance.

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    The problem is that this is a single phase motor, not a 3 phase. Solid State Soft Starters work GREAT on 3 phase compressors. But because of how a single phase motor starts, they are problematic. The reason is, to an SCR (the electronic power device in a Soft Starter), the motor starting capacitor charging current in the single phase motor looks just like a short circuit, so it has too high of a dI/dt (too rapid of a change in current), which can cause the SCRs to "self-commutate" (fire without being told to) and damage themselves. At the same time, the partial firing of the SCRs in each sine wave (which is how they function, if normally), creates high harmonics in the circuit, which get absorbed by the motor starting capacitor as heat, then BECAUSE you are lowering the current and torque, the capacitors stay in the circuit longer than they are designed to, WHILE absorbing more heat. So basically it becomes a race to see what fails first; the motor capacitors or the soft starter SCRs. In my experiences, the SCRs usually lose first. There were, for a few years back in the 80s and 90s, several soft starter mfrs who sold single phase soft starters, but they had SO MANY field failures and returns that so far as I know, they have all been discontinued. They were made only for Permanent Split Capacitor (PSC) motors, which, ironically, rarely needed them, but nobody read the fine print...

    An autotransformer start scheme would actually work to eliminate some of those problems, if there were such a thing for single phase motors. But it would STILL reduce the torque, which would increase the acceleration time, which leaves the cap in the circuit longer than it was designed for and shorten the life, probably considerably.

    As previously mentioned, make sure it has an unloader valve and that it works correctly, that's your best strategy.

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    Solid points have been made. Thanks guys! Things that I probably already knew but don't want to accept....lol Just hard to get around the start circuit of a 1P cap start motor. Lower voltage a bit and caps stay in too long a burn up. We have also had cases where a run cap welded up, causing high current through the aux winding and burning the whole motor out.....

    I think I will try removing all the air from the compressor or opening the compressor output pipe and doing another inrush test. I know the compressor does not have an unloader so that will at least show me what it could do with an unloader.

    However, the inrush is showing about 5x of FLA, which, IMO, is probably about right. My guess is I might be able to reduce the amount of 'time' its at high current, but probably still see close to 5x.....

    I will certainly do the test and report back! If nothing else, if it proves advantageous, it could help others that may fight with this!

    I am also looking at the advantages I would have with a VFD though. I just love them because I can protect that motor in so many ways. under voltage, over amp, remote start/stop switching, soft start, over/under speed, etc.

    To be right honest, the digital age is here! We are seeing more and more digital motors in HVAC, water well pumps, etc. I know it brings more complexity and cost, but I think time has shown that VFD controlled systems can be very highly reliable.

    It still floors me that so many air conditioner condensers were kicked out without some very simple protections. Protections through monitoring current and voltage that could save thousands in compressor failures, not to mention time. I have seen someone have an Acoil failure/loss of refrigerant, and that compressor just slugs away until it dies. Avoidable!!!

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    Both the trasformer AND the solid state soft starters I referenced were for single phase motors, 3 and 5 HP air conditioning pumps. They worked fine, and the transformer type really did a good job of reducing the line current.

    The issue is that the starter needs to be rated and designed for the size motor it is used with. If so, the start cap will not be an issue, because it (they) are selected to be OK for the usage. In out case, the transformer system was supplied with a replacement start cap due to the possibility of a problem on start.

    But, the A/C system also had a timer to prevent another attempt at a start for 5 minutes, because A/C pumps have nothing like an unloader, and if started too soon may be trying to start against full head pressure. The unloader is of course a thing to be checked if the system includes one, but you say yours does not. I think a 5 HP compressor ought to have an unloader.

    The 5x FLA is actually a fairly low start inrush. Did you check that with a peak reading meter? If it is short enough, there may be no issue, breakers (and even fuses) have a high current capability if the duration is short. And motors and start caps do not overheat if the duration is short. The issue arises if the time is longer.

    The whole point of the unloader is to make sure the duration of high current is short The inrush itself is a given, unless some sort of soft start is used, it is the time that is important. A lower current may take longer to start the motor, but remember that all the heating is proportional to current squared. So the total energy can be much lower. If a soft start doubles the time, but uses half the current, then the total energy dumped into the motor as heat during start is half that at full current. Same for the capacitor. And even if it is 4x longer, the energy is no worse than it was for the original setup.

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    Quote Originally Posted by huleo View Post
    It still floors me that so many air conditioner condensers were kicked out without some very simple protections. Protections through monitoring current and voltage that could save thousands in compressor failures, not to mention time. I have seen someone have an Acoil failure/loss of refrigerant, and that compressor just slugs away until it dies. Avoidable!!!
    LOL! I parts-bin "designed" then installed my own, called in another guy to make two sets of braze joints at the ends only 'coz with but 30% vision and only from one eye, I was prone to misjudge depth with the torch badly enough to set my own arm afire!

    Annnnd had both over and under pressure cut-outs added.

    Downside? Those connections that he made for the protective switches are where too much of the precious-as-gold R22 escaped! Net-net, I'd have been better-off without those non-welded/brazed mechanical connections, and most experienced HVAC techs already know that. At least he had talked me out of built-in pressure METERS. They fail, too, given time.

    I knew ahead of time those connections were risky. "Ahead of time". Not after the slow leaks.

    Optimists. God must ha' loved them 'coz she made so many of them.

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    Actually, 5X FLA is about right for small single phase motors. I was surprised going from GE industrial devices (6X) to Eaton Appliance where lab tests showed 5X to be much closer to actual.

    Tom

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    Yes, my reference to "low" is due to there being designs that will draw more. A lot of motors are made in the 5 to 6x FLA range. But there are some that are considerably higher. Point being that 5x is not bad. You will not find a lot of motors with less than that (dual cage being one type that can be less)

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    I have been working on a somewhat similar problem, a magnetic base drill that had only clockwise rotation to be used for drilling. The customer wants to use it for power tapping in big steel plates. The tap will be run in and immediately reversed. I managed to stuff a DPDT switch in a space intended for a SPST one, but if you throw the switch from forward to reverse instantly, you get a big blue flash from the brushes. Telling the operator to wait until the motor stops before reversing is a waste of breath. A capacitor would need a relay to switch it out after the motor gets going. I am looking at a saturable reactor to act as a current limiter. I make them, so I can tailor one to the job, but it is looking to be a lot larger than I would like. Still, it would just be a box that you plug into the wall and plug the drill into it. The nice thing about these reactors is that you can set a current limit with the control current and it will almost seem like a short until you reach the current limit, then it won't let the current go higher. I wonder if there would be a market for a box with an ammeter and a control that you could set to any limit?

    I'll keep you posted.

    Bill


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