Potential Relay Issues and Replacment Selection
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  1. #1
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    Default Potential Relay Issues and Replacment Selection

    I've been having problems with blown start caps in a 10 year old RPC.

    [Sorry in advance if this is too long of a post, I didn't want to miss anything important. I hope the pictures keep it more interesting. If you're bored, please skip to the questions at the end.]

    Almost 10 years ago, I put together a 10HP RPC for my brother using an ebay kit, plus some upgraded parts and added caps for better balance. The RPC runs a woodworking shop with a 5HP table saw, a 5HP shaper, a bigger 9HP saw, plus a couple other similar machines. Mostly one machine at a time.



    For the last 9 1/2 years, it's been working fine with zero maintenance. A few months ago, a surplus 5HP dust collector (high starting load) was added and it didn't want to run right. Within maybe 15 minutes?, one of the start caps (200uF 220VAC) smoked.

    At the time, it was setup with (see above picture for details):
    - Start caps: 900uF total
    - Primary Run caps: 240uF total
    - Seconary Run caps: 102uF total
    - PFC: 25uF total

    The original set of 3x 200uF 220VAC start caps, or should I say 2x caps and one blown cap, was swapped for new higher voltage surplus 200uF 330VAC start caps, and the RPC was working again, but without trying to use the new 5HP dust collector. This lasted a couple months and then another start cap blew, this time, a 100uF 330VAC one that had been in use for 10 years. When it blew, the RPC had been on for ~10 minutes and table saw was cutting 1/4" ply, so not a big load.





    The remaining original 3x 100uF 330VAC start caps were then replaced with a couple more new surplus 200uF 330VAC start caps, giving 5x 200uF or ~1000uF of total start capacitance (a bit high for a 10HP idler, but I wanted a fast startup to help eliminate issues). It's all running again but I'm worried that the potential relay is occasionally sticking and causing the start caps to blow up.

    The measured voltages VAC are (L3 is generated leg):
    L1-L2 L1-L3 L2-L3
    Idler Only 242.9 255.1 254.6
    Idler + Idle Table Saw 242.2 246.8 244.4
    Idler + Sawing w/ fast feeder 240 239 243


    Here's oscilloscope graphs of the current setup running just the idler motor and running the 5HP table saw with the power feeder on high rate (but soft wood). Seems reasonable to me, but I'm far from a RPC expert. Note: L1,L2 (blue,magenta) are 50V/div scale while L3 (green) is 100V/div scale.





    The potential relay is mis-mounted with "This Side Up" to the left.



    I couldn't find an original datasheet for the White Rodgers #128306-2965AB potential relay, but one of the surplus sites listed it as:
    PICK-UP:245-275V. DROP-OUT:140V CONTINUOUS
    COIL VOLTAGE: 420V. 10,000 OHMS NOM.
    RESISTANCE 5-VA NOMINAL; 10 VA INRUSH

    Oh, and here's a graph of the generated leg at initial startup (10HP idler). Mighty fast, and maybe too fast?



    Questions:
    1) How often do potential relays "stick"?
    2) Will mounting the potential relay in the wrong orientation cause it to "stick"?
    3) How do I select a replacement potential relay? Is there something I can measure when trying to start the problematic dust collector to help select good pick-up and drop-out voltages?
    4) If the potential relay "sticks", how long until the start capacitors blow? Seconds? Minutes? Hours?
    5) Does anything seem off in the RPC setup or measurements?

    Thanks,
    Bob

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    I hope you are starting the RPC independent of any loads, then starting application motors. By that time the RPC starting circuits are out of the picture. Does the dust collector pull the RPC down enough to cause the voltage relay to drop out?

    Tom

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    The 10HP RPC is always started up first, maybe 10 or 20 seconds before walking over and starting up a machine.

    I'll need to verify what happened originally, but I think the new used 5HP dust collector was turned on, didn't quite sound right, and smoke from the RPC box was noticed a few minutes later (one of the 10 year old 220VAC start caps had vented).

    After replacing half of the original set of start caps, with 330VAC ones, we tried the dust collector and yes it kept tripping the start circuits (start contactor chattered and start light turned on). We immediately shut it down. I don't have my notes here, but we did try some quick phase-to-phase measurements and they were weird: 230V / 308V / 197V IIRC. Some, or all, of these may have been with start caps engaged.

    We were able to get the dust collector running briefly without tripping the start relay if a table saw (5HP) was started up first, but I didn't want to risk it and didn't take any measurements. The dust collector hasn't been used since.

    This last weekend, after replacing the 2nd blown start cap (from just running the table saw), we did notice that maybe 1 in 20 table saw starts was jerky? and caused the start relay to re-engage for a fraction of a second.

    Is the start relay re-engaging always a bad thing, or is it OK if it only trips briefly when spinning up a new motor?

    -Bob

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    Popped Starter Spring Repair

    Are you now "For Real"

    Or is this a joke too ?

    Credibility does count.

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    Start caps DO "wear out". They are good for a limited number of "standard starts", but the actual number of starts depends on how long start current flows, and how much flows, per start.

    So a cap may last a long time, or not so long, depending.

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    As to your dust collector:
    If there is a damper or some other way to choke off / close the air duct, do so. In a centrifugal blower, air flow = load. So no air flow, no load. Starting a blower with the air flow enabled puts a huge load burden on the motor, making it pull very high current for a long time, which may be what blew your caps. Once you get the RPC fixed, start that blower with the air choked off, then open the damper after it is at full speed.

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    Quote Originally Posted by Jraef View Post
    As to your dust collector:
    If there is a damper or some other way to choke off / close the air duct, do so. In a centrifugal blower, air flow = load. So no air flow, no load. Starting a blower with the air flow enabled puts a huge load burden on the motor, making it pull very high current for a long time, which may be what blew your caps. Once you get the RPC fixed, start that blower with the air choked off, then open the damper after it is at full speed.
    It can be done with blast gates at each machine drop, from the mainline trunk, or a manual damper on the exhaust side of the fan.

    There are available manual and automatic blast gates. The automatic variety are interlocked with the machine controls. Machine starts and blast gate opens.

    SAF Ω

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    To pop the start caps with a start of a heavy load, it would need to drop the generated leg far enough to re-engage the start relay. The spec given for that relay was "PICK-UP:245-275V. DROP-OUT:140V CONTINUOUS".f
    That's a bit high on pick-up, which I assume was adjusted down somewhat, but also pretty low on drop-out at 140V. If the generated leg drops that far, it may be begging for a larger RPC.

    Of course, it is somewhat common to adjust the voltages with external resistance. If there is an external resistance added to the start relay circuit, then the drop-out voltage may be high enough to risk a drop-out with a heavy load. If the start relay drops out, and there is no protection, the start caps may re-engage, which is pretty likely to blow them with a slow stating load.

    Some RPC designs have added protection relays that lock out a re-start unless the power has legitimately been cut off, and those will not let the start caps cut in again..

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    ^^^ Thanks JST, that's very helpful. I'm certainly learning more about how this is all supposed to work.

    There is no added external resistance, so pick-up is the un-modified 245-275V. I was looking at the commonly recommended Steveco 90-66 (or actually a GE Mars 90-66 cross to the Steveco), with specs of:
    Pick-up: 215min / 225max
    Drop-out: 120max

    For starting the 10HP idler, the lower pick-up seems fine, and the lower dropout (120V vs 140V) should give a little more margin when starting heavier loads, correct?

    Could I go even a little lower on the potential relay, with the ~1000uF of start capacitance, since the idler seems to startup nice and fast?

    I think I'm chasing two issues now:
    1) High load from starting the new used dust collector (I'm not convinced that there's not something wrong with it - no shorts to the chassis, but I didn't undo all 9 wires and measure the coils separately).
    2) Second blown start cap when using a table saw that's been trouble free for 10 years.

    For #2, all the start caps have now been replaced, the original PR is now pointed Up, and I'll buy a new PR just in case the original one has degraded.

    For #1, I'll try to take some additional measurements this weekend. I talked with my brother yesterday, and there were additional problems when trying the dust collector for the first time. Apparently, it caused a 50A breaker to trip in the main panel, and also blew a 30A? fuse in the disconnect.

    Thanks again for the insight

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    Why not add a simple lock out? Start relay locks itself out after start. Then if load drops the voltage the start relay will not connect the caps. Odd that the low amperage saw was able to destroy the caps, they should have been out of the circuit when the saw was started. Lock-up for the start can be made to reset when the stop switch is pushed/thrown for the RPC.

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    The lower cut-in is better, but the lock out beats any low drop-out numbers, IMO

    Added resistance would raise both voltages. The Steveco , or old ones, anyway, had a mechanical setting for the cut-in, a spring tension. They were set up in various ways to get the range between the two to the numbers wanted.

    The lock-out is set up to break the circuit to the caps separately from the PR, so that even if it cuts in, the caps are not connected. But the relay for the lock-out is NC, and closes again if power to the RPC is disconnected (when the RPC is shut off).

    The PR is normally closed when power is off, connecting start caps, and opens when the generated leg comes up to the "pick-up" voltage. If voltage drops below the "dropout" voltage, it closes again, which you normally do not want.

    If you have motor run caps used to make up the start capacitance, they can be connected at any time with no cap failures, but the motor start winding may be the next point of failure, so you still don't want the start circuit re-connected.

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    For trial purposes, put a switch in the start circuit so that after the RPC comes up to speed, just open the switch. That will definitely prevent the start circuit from re-energizing. setup a voltmeter and an ammeter to record the inrush current and voltage of the wild leg. On a side note, your oscillogram of the starting of the RPC are quite normal. The ramp up time is normally in the 6-10 electrical cycles as shown.

    Tom

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    Thanks all for the quality info.

    I'd certainly like to add a restart lockout so that I can hopefully go another 10 years without a service call.

    I flipped briefly through the posted plans and didn't see a good example of what I'd like. Anyone have a link to a restart lockout circuit that works with: no neutral in RPC box, and the existing RPC uses just a remote light switch to turn on the RPC (simply applies power to main contactor coil). It doesn't need to be a RPC-specific example (I can probably translate it to what I need). I know I could set it up with start/stop buttons on the RPC cabinet, or remote start/stop buttons if I run more conductors through the conduit, but an additional relay, or two?, inside the RPC cabinet would be ideal. (There is a spare pole on the main 240v single phase contactor if that helps.)

    Tom - yes, adding a temporary switch to prevent restart was exactly what I was planning on doing this weekend. I'll certainly feel much better working with the open cabinet without having visions of little black bazookas pointing right at my head. I think I can add in a spare discharge resistor to the PR contact so that I can still trigger the oscilloscope on a now-blocked restart attempt. I'm curious to see what it looks like.
    Thanks too for the starting time confirmation - all I'd seen previously said that the idler would be up-to-speed and the start caps disengaged in 1 to 2 seconds.

    -Bob

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    There is another option that I use in my RPC. Replace the voltage relay with a timing relay. Set the time for one or two seconds or whatever works the best. The relay is time delay on energization. Once the relay times out, the start circuit is disabled until the relay is reset which is when the power is shutoff.

    Tom

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    Not sure how the shop is wired, I have mine wired so that the Red wire that is used in most SO type power cords is the "created leg" Everything connected is set up so that the "created" leg feeds only the one leg of the motor. Everything like lights, relays, transformers and what ever is connected to the single phase line. Wire in conduit is the same, red wire is used to mark the "created leg". If motor reversal is required the other 2 lines are reversed.

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    Quote Originally Posted by Froneck View Post
    Not sure how the shop is wired, I have mine wired so that the Red wire that is used in most SO type power cords is the "created leg" Everything connected is set up so that the "created" leg feeds only the one leg of the motor. Everything like lights, relays, transformers and what ever is connected to the single phase line. Wire in conduit is the same, red wire is used to mark the "created leg". If motor reversal is required the other 2 lines are reversed.
    What your are saying without saying it is don't run anything except the motor from the generated leg and the reason is the generated leg is unstable and will wander all over the place especially during start. The voltage drop is often so much that control wired to it will not function.

    Tom

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    Yes Tom but if as in most 3 phase machines a transformer is connected to get low voltage it too will be connected by all machines having them (in my situation all have a single phase transformer)if connected to the "created" leg they will hinder start and use energy created by the 3rd leg of the RPC and will add to the voltage drop. I'm using a Delta wound RPC motor that has been working great. Haven't had the time to put a scope on it or do much testing but 3rd leg seems to be quite high when I have measured it.(215 to 230VAC on 245VAC line)

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    Quote Originally Posted by Froneck View Post
    Yes Tom but if as in most 3 phase machines a transformer is connected to get low voltage it too will be connected by all machines having them (in my situation all have a single phase transformer)if connected to the "created" leg they will hinder start and use energy created by the 3rd leg of the RPC and will add to the voltage drop. I'm using a Delta wound RPC motor that has been working great. Haven't had the time to put a scope on it or do much testing but 3rd leg seems to be quite high when I have measured it.(215 to 230VAC on 245VAC line)
    Why connect the transformers to the wild leg? As you have said, that causes problems with voltage drop. When you measured the wild leg voltage, was that during starting of a load motor, or running?

    Tom

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    As I said If Connected to the "created" leg will hinder the start and add to the voltage drop. What I'm saying is be sure that because most 3 phase machines that use lower voltage especially 120VAC will have a transformer that's connected to the line and active before the machine start button is pushed is NOT connected to the created line.
    My measurement was taken before starting any thing connected to the RPC which is everything requiring 3 phase. And in my shop there is nothing connected to the "created" leg other than the motors winding that is not on the same line as any controls or transformers. In addition quite a few motors are 6 pole and will start as if connected to 3 phase! I have been watching places like HGR and grab low speed motors simply to get 6 or more pole motors. I just got a 5HP 8 pole motor! That was just rewound and never used. Looks like new, not sure if it Delta or Wye wound. I addition all twistlock type connections are all wired the same so if I move a machine connection is simple. When I wanted to use the machines I had and didn't have a motor large enough I connected a 3 HP Delta 6 pole motor to the start circuit then added a 2HP Wye. But many machines such as a Vert Tool Master Mill I don't fire up the 2 HP. Yet when using a Hendey 14" with 5HP 6 pole motor (didn't wire it so not sure is Delta or Wye) that 3HP will start the 5HP in the Hendey that's connected to the machine with belts driving a pulley with clutch assembly inside as if it's connected to 3 phase so I often don't bother with starting the 2HP RPC. In addition I like the older American made machines because I like taking as heavy a cut the machine is capable of and have not yet been able to stall the Hendey! Over that last year I was able to gather up quite a few slow speed motors but don't have enough time to experiment! Without any data it does seem that higher pole motors make better RPCs and motors to run on them!

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    Quote Originally Posted by Froneck View Post
    As I said If Connected to the "created" leg will hinder the start and add to the voltage drop. What I'm saying is be sure that because most 3 phase machines that use lower voltage especially 120VAC will have a transformer that's connected to the line and active before the machine start button is pushed is NOT connected to the created line.
    My measurement was taken before starting any thing connected to the RPC which is everything requiring 3 phase. And in my shop there is nothing connected to the "created" leg other than the motors winding that is not on the same line as any controls or transformers. In addition quite a few motors are 6 pole and will start as if connected to 3 phase! I have been watching places like HGR and grab low speed motors simply to get 6 or more pole motors. I just got a 5HP 8 pole motor! That was just rewound and never used. Looks like new, not sure if it Delta or Wye wound. I addition all twistlock type connections are all wired the same so if I move a machine connection is simple. When I wanted to use the machines I had and didn't have a motor large enough I connected a 3 HP Delta 6 pole motor to the start circuit then added a 2HP Wye. But many machines such as a Vert Tool Master Mill I don't fire up the 2 HP. Yet when using a Hendey 14" with 5HP 6 pole motor (didn't wire it so not sure is Delta or Wye) that 3HP will start the 5HP in the Hendey that's connected to the machine with belts driving a pulley with clutch assembly inside as if it's connected to 3 phase so I often don't bother with starting the 2HP RPC. In addition I like the older American made machines because I like taking as heavy a cut the machine is capable of and have not yet been able to stall the Hendey! Over that last year I was able to gather up quite a few slow speed motors but don't have enough time to experiment! Without any data it does seem that higher pole motors make better RPCs and motors to run on them!
    I must have misread your statement about the transformers. I agree with you. I was hoping that you could measure the wild leg voltage during the startup of machine motor, not will its running. My experience with my RPC mirrors yours in that I have a 3 hp RPC driving a monarch 12ck with a 3 hp motor. The monarch always starts, acts like its on a reduced voltage starter. New have been able to stall it. However, if I snap it from forward to reverse without allowing the motor starter to drop out, it will overcome the RPC and reverse it.

    Tom


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