RPC, 7.5HP in rush current and the big green box in my yard
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  1. #1
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    Default RPC, 7.5HP in rush current and the big green box in my yard

    Hi all,

    I am spec'ing out what I would like to do with an RPC, and I'm hoping for some general guidance here regarding the start up amperage from a duplex compressor with 7.5HP 3-phase motors and how well my residential service will support it.

    My shop is at my residence, where I have a big green humming box from the power co (a pad mounted transformer). This transformer feeds only my house, and it consists of two-200AMP services (one is general house, one is house HVAC system). My shop is fed from a 100AMP sub-panel from the house feed.

    Does having my own transformer (which must be at least 400AMPS) suggest that I am going to have enough power to my site to not cause problems at the house/neighbors with the in rush from a 7.5HP motor? I know that running both will be stressing my 100AMP service, but I'm more concerned about brownouts, etc. I'm feeling like it should be plenty.

    If it isn't sufficient, should I be considering a commercial RPC which has the additional boost/hard starting capacity which lowers the current inrush to load motors via electronic wizardy (note this is not soft-start of the idler, but boost of the load starts)? I would be all for one of these, but in general they are a lot more expensive and they are not expandable. I'd like to start simple and alter as I change me mind, not to mention keep more $ in my pocket.

    My current plan is to go with a commercial 10HP soft-start system and add idlers to support what is needed over time.

    Thanks for any thoughts,

    Don

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    If that transformer is only feeding your two service drops, then nothing you do is likely to affect your neighbors. It may affect YOU because you are on the secondary (LV) side of that transformer, but it will not likely have any noticeable effect on the primary (HV) side, unless you are at the far end of a very very long run from the nearest substation. Even then, the full 400A on a 240V secondary only amounts to about 8A on the 12.47kV primary side. Your 7.5HP motor, assuming single phase to the RPC, is only going to represent about 270A of starting current, so you are only looking at 5A on the primary side. A pittance.

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    Nominally, the 400A service would be a very significant power. 240V x 400A - 96 kVA.

    Almost certainly the pad transformer is smaller than that, the powerco routinely undersizes transformers on the basis that nobody pulls full power for long. In multi-house setups, one 25 kVA unit may be used to service 3 x 200A services. And the lines to and from the transformer are often undersized the same way. The "200A service" that the city requires is often fed with a 3 wire drop using 8ga wires over 100' long. (the new spiral drops are larger wire, but it is aluminum).

    So, if you ever did have a reasonably long draw at the power nominally provided, I would expect a transformer fire, or at least a shutdown, to follow They take a while to heat up, so it could take over an hour. Such things are reasonably common in urban areas as homes add central air. Some of the homes around me had units that dimmed the lights of the whole block when they came on. That has calmed down after some outages and both transformer replacement, and installation of newer A/C units.

    Point being that the undersizing tends to come from a smaller core, and more turns of smaller sized wire, which is figured to stay good enough with the usual duty. That adds up to higher series impedance, and the potential for significant local-to-your-facility voltage drops in the case of large surges. On that service, 270A would be a large surge, and I would expect dimming of lights.

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    multiple VFDs was my solution to RPC browning out the neighborhood.

    I'll not go back to a RPC.

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    Quote Originally Posted by dschad View Post
    Hi all,

    I am spec'ing out what I would like to do with an RPC, and I'm hoping for some general guidance here regarding the start up amperage from a duplex compressor with 7.5HP 3-phase motors and how well my residential service will support it.

    My shop is at my residence, where I have a big green humming box from the power co (a pad mounted transformer). This transformer feeds only my house, and it consists of two-200AMP services (one is general house, one is house HVAC system). My shop is fed from a 100AMP sub-panel from the house feed.

    Does having my own transformer (which must be at least 400AMPS) suggest that I am going to have enough power to my site to not cause problems at the house/neighbors with the in rush from a 7.5HP motor? I know that running both will be stressing my 100AMP service, but I'm more concerned about brownouts, etc. I'm feeling like it should be plenty.

    If it isn't sufficient, should I be considering a commercial RPC which has the additional boost/hard starting capacity which lowers the current inrush to load motors via electronic wizardy (note this is not soft-start of the idler, but boost of the load starts)? I would be all for one of these, but in general they are a lot more expensive and they are not expandable. I'd like to start simple and alter as I change me mind, not to mention keep more $ in my pocket.

    My current plan is to go with a commercial 10HP soft-start system and add idlers to support what is needed over time.

    Thanks for any thoughts,

    Don
    As far as disturbing your neighbors, if you have your own transformer there is no problem. I do find it unusual that you have two 200 amp services. Is your HVAC all electric including heating?

    You might be being mislead about inrush reduction for a mechanical RPC. Standard RPC units that use a motor to generate the 3 phase system do not reduce the inrush current at startup. To reduce inrush would require the use of a transformer or primary resistor with associated controls to remove them from the circuit once the motor is started. Where you could get into trouble is the 100 amp breaker. A 10hp 230 volt single phase motor will draw approximately 420 amp for about 100 msec. Depending on the breaker, you might get trips. One area I would be concerned with is will the pulsations of the compressor cause the house light to flicker?

    As for a hard starting controller, these don't DECREASE the starting current, they INCREASE it. More current is needed for more torque.

    Tom

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    I start a 220v 10hp Rotary phase converter and then a 5 hp (7500w) gear head lathe in high gear off a 50 amp breaker with no issues.


    Sent from my iPhone using Tapatalk Pro

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    Quote Originally Posted by Jraef View Post
    If that transformer is only feeding your two service drops, then nothing you do is likely to affect your neighbors. It may affect YOU because you are on the secondary (LV) side of that transformer, but it will not likely have any noticeable effect on the primary (HV) side, unless you are at the far end of a very very long run from the nearest substation.
    This is generally what I thought, I just need to deal with my wife on this...


    Quote Originally Posted by JST View Post
    So, if you ever did have a reasonably long draw at the power nominally provided, I would expect a transformer fire, or at least a shutdown, to follow They take a while to heat up, so it could take over an hour.
    This is very comforting...

    Quote Originally Posted by CalG View Post
    multiple VFDs was my solution to RPC browning out the neighborhood.
    I looked into those, but I like the flexibility of an RPC. If it were my only machine the cost is competitive, but I can forsee additional motors down the road.

    Quote Originally Posted by TDegenhart View Post
    I do find it unusual that you have two 200 amp services. Is your HVAC all electric including heating?
    The HVAC is a geothermal system - well pump, heat pump plus water heater and some odds and ends. On the inside the panel there are 180 amps of breakers right now.

    Quote Originally Posted by TDegenhart View Post
    You might be being mislead about inrush reduction for a mechanical RPC. Standard RPC units that use a motor to generate the 3 phase system do not reduce the inrush current at startup. To reduce inrush would require the use of a transformer or primary resistor with associated controls to remove them from the circuit once the motor is started. Where you could get into trouble is the 100 amp breaker. A 10hp 230 volt single phase motor will draw approximately 420 amp for about 100 msec. Depending on the breaker, you might get trips. One area I would be concerned with is will the pulsations of the compressor cause the house light to flicker?
    I was referring to a commercial RPC with the soft-start, as described by the manufactures. Custom idler motor which I believe are designed to greatly reduce the in rush current (I'm awaiting a reply as to how much the inrush actually is). If I don't go with one of those options I'd make a pony-start idler(or plan on one after trying it out).

    Thus far, I can run a single, single-phase 5hp compressor plus some additional equipment without interfering with the house lights. My concern is upgrading to 10 or 15hp total draw (two 7.5s). The new compressor does have continuous run as a hedge so I can control the cycling if need be. It will also be running at around 550 rpm, so maybe that helps with pulsations?

    Quote Originally Posted by TDegenhart View Post
    As for a hard starting controller, these don't DECREASE the starting current, they INCREASE it. More current is needed for more torque.
    This is a very interesting point - my understanding is that the hard-start feature is similar to the starting capacitors. From an email from the tech support, the booster will provide 300% extra current. I interpret that to mean that the inrush would be lowered. But your comment I could interpret it as the inrush is the same as a normal RPC, but there is "free boost", so it is of shorter duration?


    Quote Originally Posted by ripperj View Post
    I start a 220v 10hp Rotary phase converter and then a 5 hp (7500w) gear head lathe in high gear off a 50 amp breaker with no issues.
    I tend to think I'm over-worried, but I don't want to spend a lot of time and/or money only to find that I need to redo things and spend more time and money. If I had a pile of 3-phase motors I'd give it a try and see what happened.

    Actually, now that I think of it I have consistently run a 3hp RPC driving a 2hp motor with my 5hp single phase compressor cycling. Sometimes with the induction heater running too. No flickering reported. Maybe I don't need the fancy booster after all.

    Thanks for all the replies.

    Don

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    One issue with an RPC is that it draws extra current for itself, normally around 30-40% of the nominal full load current. Because the usage is different from a "motor" (input on two wires only), that may not be 100% true, but there is extra current. (the POWER is not much, it's current at a low power factor).

    So if you are low on supply current capability, then an RPC imposes an additional load, current-wise. You may want to consider that when deciding on the type of converter.

    A VFD normally is over 95% efficient, and draws less extra current (althugh the power factor coming into it may be lower than desirable).

    A phase perfect has a power factor correcting circuit built in, and will draw the least current of all options, at any given output load.

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    A few years back, I built up an RPC with a 7.5 hp Baldor AO (Air over) motor that I got from my local second hand shop. ($30, LOL) I was concerned about a possible inrush issue as well due to the size of the motor.

    The RPC is set up with a Steveco 90-66 that has had the coil separated from the points as recommended by Peter Haas for motors larger than 5 hp.

    Anyway, I tried to test for inrush current, but the unit starts up so fast that I couldn't get a reading with my Craftsman Clamp Meter. The unit starts instantaneously with no noticeable issues. I did get a reading of approximately 14 amps with the 3 hp mill motor running but no other load. The unit is on a 30 amp line and had a power factor capacitor attached.

    It's been running for 7 years off and on so no problems.

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    Quote Originally Posted by dschad View Post
    This is generally what I thought, I just need to deal with my wife on this...



    I was referring to a commercial RPC with the soft-start, as described by the manufactures. Custom idler motor which I believe are designed to greatly reduce the in rush current (I'm awaiting a reply as to how much the inrush actually is). If I don't go with one of those options I'd make a pony-start idler(or plan on one after trying it out). I would be interested as to how they do that and by how much they reduce inrush.

    Thus far, I can run a single, single-phase 5hp compressor plus some additional equipment without interfering with the house lights. My concern is upgrading to 10 or 15hp total draw (two 7.5s). The new compressor does have continuous run as a hedge so I can control the cycling if need be. It will also be running at around 550 rpm, so maybe that helps with pulsations? The proof of the pudding is in the eating.



    This is a very interesting point - my understanding is that the hard-start feature is similar to the starting capacitors. From an email from the tech support, the booster will provide 300% extra current. I interpret that to mean that the inrush would be lowered. But your comment I could interpret it as the inrush is the same as a normal RPC, but there is "free boost", so it is of shorter duration? Stop and think about it. As noted, hard starting mean more current is needed. That means MORE inrush not less. "Hard starting" needs to be defined. It could mean just very short period of time as say a compressor that just needs a pulse of current to get it over the hump, or it could an extended period of time such as a loaded conveyor belt to a high speed lathe gear train. Each is handled differently.





    Thanks for all the replies.

    Don
    Tom (and padding for 10 characters)

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    There are transformer based starters that provude a good reduced voltage start with less line current... the transformer step down ratio is the current reduction. Not a "hard start" kit, but a "line current reducer".

    Reduction to roughly 60% voltage is a common amount. That reduces the current due to lower voltage, plus also the line current is reduced as well, also to about 60% of the already smaller current.

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    I heard back from the RPC manufacture who uses custom idlers. The startup current for the 20HP soft start is 100 amps. I believe this is on the order of 50% (I'm thinking startup is 4x FLA?).

    Lots of good ideas mentioned here, so I have some more ideas to think about. I think the only definite decision is that I'm not going to go with a big phase converter, if I go with one. Perhaps a small multi-idler.

    The VFD approach seems interesting, but I'm just getting into it. Perhaps costly - 2x15HP VFD (I am assuming that a single 15HP VFD will be required for each motor, and they cannot share).

    [Edit: The cost of VFD might not be as high as I was initially thinking. I was under the impression that you need to 2x the size of a VFD, but I believe this to only be true if you are derating a 3-ph input for use with 1-ph input. High HP single phase VFDs are very pricey; the 3ph versions are much cheaper but appear to require the 2x oversizing. At the end of the day 2x15HP VFD is comparable to a commercial RPC of similar size.]

    Until you know the secret of "derating", shopping online will produce some big numbers for 1 ph input, 3 ph output

    And since we are talking about it, I've been reading some threads but it isn't clear to me if there could be an advantage to over-driving the compressor motors? Is overdriving going to be any different than if I just swapped out the pulleys? Soft-start aside is there an advantage in that? The one place where I imagine I could see a benefit is during continuous run when the compressor is running unloaded. But then again, maybe not since the min RPM is 400, and it is going to be running pretty close to that anyway (400rpm @ 5hp, 550 or something @ 7.5hp).


    Don
    Last edited by dschad; 08-31-2019 at 08:14 AM. Reason: Learned more about VFD sizing

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    Nobody's yet mentioned it here but the standard approach to minimizing the inrush for rotary converts is to
    start the idler motor's rotor spinning before energizing it. A small single phase motor, belted or otherwise
    coupled to the idler will bring it up to near syncronous speed. The inrush is minimal then.

    Obviously cannot be done with a commercial idler without a shaft.

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    Quote Originally Posted by jim rozen View Post
    Nobody's yet mentioned it here but the standard approach to minimizing the inrush for rotary converts is to
    start the idler motor's rotor spinning before energizing it. A small single phase motor, belted or otherwise
    coupled to the idler will bring it up to near syncronous speed. The inrush is minimal then.

    Obviously cannot be done with a commercial idler without a shaft.
    Thanks, I was aware of this and will do that if I end up with a large RPC. That said, for my compressor application I am starting to think that a VFD might be the way to go.

    Don


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