Potential Relay Issues and Replacment Selection - Page 2
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  1. #21
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    I don't know why people use potential relays. Initially I bought a phase converter recommended by a machinery dealer for the mill I bought from him. It had a two speed motor that the converter couldn't cope with. It was a simple static converter with a potential relay to switch out the start capacitors. I rewound a normal relay with only a few turns of 12ga enameled wire and adjusted the spring to drop out at the right time. The relay was in one of the lines, not the generated phase. Starting the motor on low speed would not pull the relay in and the motor started and ran on the run capacitors. Starting on high speed would close the relay until the current dropped off. Switching from running on low speed to high would also pull in the relay until it got up to speed. It worked that way for 9 years until I got real three phase. I did have to use a lot more spring tension to get the proper drop out. It had several of the regular springs hooked in parallel and took some fiddling, but it never needed any attention after that.

    Bill

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    Quote Originally Posted by TDegenhart View Post
    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
    I Have the same problem, though the Hendey will start as if connected to 3 phase but if I reverse it the 5HP will overcome the 3HP RPC and reverse it so that the Hendey will continue to run forward thought though it's in the reverse electrical connection. Not important to me at this time but I have solved the problem quite a while ago in a previous shop. I purchased from a Junk yard a large truck motor Fly wheel. Cheap and balanced! Made a hub and connected to the RPC shaft. However the RPC motor must be anchored to the floor or something!! Harder to start that way so I used a pony motor. On the other hand the motor on my P&W with 4 pole motor will not start with the 3HP, when I add the Hendey as an RPC the P&W motor will start but need to wind-up to speed.

  3. #23
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    That's why to have a larger idler.... saves the machines arguing about who is the RPC and who is the load machine...! Usually the bigger one or the one with more rotating mass (if of equal power) will win.

    Plug reversing is a hard case, and puts a premium on having the current to do the job available. that may mean an RPC that is more oversized than would otherwise be needed.

  4. #24
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    Yes I agree plug reverse is hard on the motor and why I don't do it as I one time did. Back then I had a few jobs that required using a large tap, also did Metric threading with no lead screw reverse. I didn't go into reverse until I thought the motor was going slow enough but too often I was wrong and reversed the RPC. Fly wheel solved that problem. I have a #1 Hendey, has no clutch or break. Clutch handle is connected to a Drum switch (Original as made by Hendey and not altered) the drum switch then operates a reversing contacter. I do use the reverse as a break but more like quick reverse pulses or it's 3 HP motor will reverse my 3HP RPC unless I have the other 2HP connected but depending on what I have in the spindle the 3HP will reverse the 2 RPCs because of the mass. Hendey #1 has a flat belt pulley on the motor driving another flat belt pulley with the shaft connected to Vee belt pulley driving another on the spindle. Spindle too is quite heavy, about 30" long with an 8" 3 jaw chuck.
    Though it has a drum switch, it's set-up so another relay is involved along with the reversing contacter so if power is shut down with the drum switch in forward or reverse motor will not restart until another start button is pushed but it will not activate the locking relay unless the drum switch is in the Off position.

  5. #25
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    The DC powered Monarch 10EEs have an antiplugging system. When you throw the switch from forward to reverse, it goes into braking mode and monitors the back EMF from the motor. When it gets low enough, indicating that the motor is almost stopped, it allows the reverse contactor to close, making a normal start. It works the same regardless of spindle speed or mass.

    Doing that with an AC motor would be tricky, but it should be possible.

    Bill

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    Quote Originally Posted by 9100 View Post
    I don't know why people use potential relays.
    Those free light-weight rpc circuit designs on the internet are why.

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  8. #27
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    Quote Originally Posted by JST View Post
    That's why to have a larger idler.... saves the machines arguing about who is the RPC and who is the load machine...! Usually the bigger one or the one with more rotating mass (if of equal power) will win.
    Wow, I didn't even realize that a high braking load could even cause reverse idler rotation. This obviously must be a setup without a potential relay, or at least one without a restart lockout on the PR.

    As far as "why do people use potential relays?", I have a couple answers:

    1) I bought the original 7.5HP RPC kit off of ebay in 2003, and finally built it, with upgrades for 10HP, in 2009. The kit included an auto-start PR. At the time, the better pre-built RPCs all had "auto start". IIRC, it came from JL in Niagra NY -- seemed like an auspicious location to get an RPC from. Amusingly, some of the current ebay ads for RPC kits look almost the same as they did 16 years ago:
    ebay-7.5hp-rpc-kit-2003.jpg

    2) In the garage shop (well, technically it's a garage, but a large out-building or 1/2 barn might be a better description), the RPC is out of the way in a corner and the on/off switch is ~30' away. With the noise from more than one person working in the shop, the instructions to "hold the on switch until you hear the idler motor get up to speed, but never more than 3 seconds" are prone to problem.

    On other stuff, I now have a replacement PR on order (Mars 66), plus the parts to make a remote low-voltage start/stop panel (240-to-24V transformer, a couple relays, and some green/red pushbuttons). I'll use the normal pushbutton start to energize and latch the main contactor, and a pushbutton stop to break the main contactor coil. I'm going to use a double-pole start pushbutton and run the PR-to-start-cap-contactor-coil wire through it. This should allow push to start without the 3second rule -- the instructions would be to push the green button for at least 5 seconds, and the PR will cut off well before that. As soon as the green button is released, the PR is locked out.

    As to my original questions, I've been doing some digging and will post what I've found tomorrow.

  9. #28
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    I connect my RPC through a contacter that locks itself on, if power is lost by pushing the stop button or line loss the contacter drops out and system needs to be restarted. The RPC needs to be restarted if I get a slight line loss, Light blink or flicker usually require restart. If I had a potential relay it probably will reconnect the cap. that is now connected to the machine motor that is changing the phase relationship that's causing the RPC motor to reverse which in both motors is opposite the direction the cap will start either motor and probably damage the cap. The reversal is so fast it's not noticeable, a machine light connected to a transformer in the control cabinet on the machine connected to the same single phase input as the motor and not the created leg don't even dim let alone blink. When it happens I simply shut the machine motor off, wait longer and restart now in forward and being the RPC motor is running backwards will result is the reverse rotation I wanted and continue working on what I was doing knowing that my forward/reverse switch is backwards. When I have the time I will stop the RPC wait for rotation to stop so as not to create problems for the cap. and restart the RPC. Being I have a 1200 Sync speed motor the slower speed is not objectionable noise wise. I have a few Tach generators from other machines I salvaged, I could connect to the RPC motor, being DC I could use that to determine motor speed and direction if I wanted to locate the RPC remotely but I'm lazy, have to many other projects on hand and as I said the reversal is not a problem and less a problem then having to stop what I'm doing to restart the RPC.

  10. #29
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    After the replies here, and lots more reading, my best understanding of how to select a potential relay for a RPC is as follows. I'm still very new at this, with limited experience, so make your own decisions. Please post any suggestions/corrections and I'll update this post.

    I think of a Potential Relay as just a standard relay with a couple special features. A potential relay has a single set of NC contacts, with one of the contact connections being shared with one of the coil connections. By convention, the pins on a potential relay are numbered 1, 2, and 5. Pin 1 goes to the coil, pin 2 goes to both the coil and one side of the NC contact, and pin 5 goes to the other NC contact.

    While a standard relay just opens and closes when rated voltage is applied to the coil, a Potential Relay has a carefully built and adjusted coil and actuator such that it activates and deactivates at specific coil voltages. Starting with no power and the contacts closed, as the coil voltage increases, the actuator activates when the coil voltage crosses the "Pick-up" voltage. This is one of the key values for the potential relay, and is specified as a narrow range of minimum and maximum Pick-up voltages. Once the pick-up voltage is reached, the contacts open to disengage the start capacitor.

    The potential relay will stay energized, with the contacts open, as long as the coil voltage stays above the "Drop-out" voltage. The Drop-out voltage is always much much less than the Pick-up Voltage. Sometimes, a min/max Drop-out voltage is specified; other times, just the max is specified.

    OK so far? Here's a couple tables for potential relays, one from Grainger and one from GE Mars:
    potential-relay-specs.jpg

    There are a couple other key values for a potential relay: the rated current for the contacts, and the rated coil voltage (called "continuous coil voltage" or coil "max hold" voltage) . If the potential relay is directly connected to the start capacitors, then the rated current must be high enough for the switched capacitive currents. The equation for calculating this is somewhere in the sticky'd RPC Design thread. In my case, a secondary contactor is used to connect the start capacitors and the potential relay just needs to drive the contactor's coil. In the above tables, the Grainger potential relays are listed as 35A. I couldn't find an official spec for the Mars relays, but other posts here say that they're rated for 30A, and that the NLA Steveco 90-66 is rated for 50A. The rated coil voltage must be at least the nominal leg-to-leg mains voltage, or higher. 240V for USA.

    At a high level, to start an RPC, something needs to nudge the idler motor to get it rotating. Start caps do this by briefly storing and releasing enough energy on the 3rd leg to get the motor rotating. Without the start caps, or an initial manual spin, the motor would just vibrate in place.

    Start caps are only rated for intermittent use, which allows much higher capacitance in the same sized package as compared to run caps. A common start cap duty cycle rating "is 1.67% or 1/60th full time and corresponds to a maximum duty of 20 starts, each of three seconds duration per hour."

    Start caps are like heating your coffee in the microwave. 2 minutes is fine, but 4 minutes makes a mess. If you do 2 minutes, forget about it for an hour, and then do another 2 minutes, you're fine. If you forget and come back after only 10 minutes, then punch in another 2 minute, you'll likely be cleaning up a mess.

    For an RPC, it's desirable to get the idler motor up to speed quickly to prevent over-stressing the start caps and for reliable starts. A manual pushbutton switch can be used to briefly engage the start caps, or a potential relay can be used. With a pushbutton, the button's pushed and then released based on experience or when the idler sounds like it's mostly up to speed. With a potential relay, the generated voltage on the 3rd leg is monitored and the relay contacts open when the voltage reaches the Pick-up voltage.

    I couldn't find any detailed recommendations for selecting the min Pick-up voltage for a RPC. The rule of thumb seems to be 80% to 90% of the line voltage, e.g. around 192VAC to 216VAC for 240VAC. The max Pick-up voltage should be less than the line voltage, but there's some leeway because the generated leg on an idle idler at startup will be slightly higher.
    ^^^ Can someone confirm this is OK? I'd think that starting an unloaded idler motor would be pretty forgiving compared to starting an AC compressor, and that the min. pick-up voltage choice is pretty flexible.

    Once the pick-up voltage is selected, there's little or no choice in drop-out voltages. This is fine for setups that don't try to stall the RPC when turning on heavy loads. For setups that do have heavy loads, locking out the potential relay after initial startup is needed (or manual pushbutton start only).

    Here's some example potential relay choices for USA with 240V mains:

    The commonly recommended but NLA Steveco 90-66 is spec'd as:
    Pick-up 208min/239max, Drop-out 130max, 395V hold, 50A
    This gives a min pick-up voltage of 87%. The 240V line voltage is well below the max allowable 395V coil hold voltage. Contacts are good for 50A.

    A Mars 66 is spec'd as: (These are available on ebay NOS for ~$20. Try "Mars 66 relay" or "Mars 19005 relay")
    Pick-up 215min/225max, Drop-out 120max, 395V continuous coil voltage, 30A
    So min pick-up is 90%, continuous coil voltage is fine, 30A contacts

    A Grainger 5MLZ8 is spec'd as:
    Pick-up 189min/205max, Drop-out 130max, 450V Max hold, 35A
    So min pick-up is 79%, continuous coil voltage (aka max hold) is fine, 35A contacts

    A Grainger 5MLZ2 is spec'd as:
    Pick-up 212min/235max, Drop-out 121max, 420V Max hold, 35A
    So min pick-up is 88%, max hold is fine, 35A contacts.
    The max pick-up of 235VAC is getting close to the 240V mains voltage, so I'm not sure if this is enough margin with an unloaded RPC idler startup. I'd think it would be fine, but don't take my word for it.

    I ordered a NOS Mars 66 off ebay because they're a known brand. I have no experience with the Grainger potential relays, but the $20 price is about the same as the ebay Mars. I don't know if any of these can be opened up and tweaked / rewired for special setups.

  11. #30
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    All relays have a pull in and drop out voltage. Usually it listed in the specs. The potential relay is more defined so as to allow it to operate at the voltage required. Your statements as to what a capacitor does is incorrect in a way. What happens is it shifts the phase. And inducter can also be used but a capacitor is easier. The reason why the time limit is because the shift is dependent on the current and as the motor get up to speed the current lowers. Usually the RPC motor will come up to speed in the time frame given to remove the capacitor. The shift now is incorrect so something like a potential relay removes the Capacitor from the circuit. The problem comes when the load lowers the "created" 3rd leg voltage the potential relay will dropout connecting the Capacitor to both the RPC and the driven motor. Granted I'm giving a rather simplistic description of what is happening but the capacitor at that time is total incorrect! In reality you are devoting too much into the potential relay. I can make a better one using a zener diode. As some one else said why are potential relays used, check out answer!

  12. #31
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    I can only think of one reason to use potential relays instead of current sensors. You can use the same potential relay on a 1 hp or 100 hp system as long as they are both running on the same voltage. Current relays need to be adjusted to the load. In my experience, the time to do that is well spent.

    In reality, the whole debate is just a rough hack at the problem. What is really required is a variable capacitor adjusted to the varying loads. No one is likely to make one that big in the near future. There is a way, though. I have connected a capacitor to a Variac which transforms it into a variable one. Watching an oscilloscope, I could optimize the phase shift across the range of the motor. It would be easy to make a phase angle detector controlling a servo to do that automatically. The only problem is that it would be moving constantly and soon wear the Variac out. Another possibility would be to have a lot of capacitors and relays that would switch them in and out. It wouldn't take that many if you arranged them in a binary progression- 1 mfd, 2 mfd, 4 mfd, etc. The minimum value would be permanently connected and not need a relay. 5 more relays would give 32 steps, which should be plenty. You wouldn't need an idler at all.

    It could also be done with an inductor. Since it would need to have an air gap in the core, you could use E/I laminations and pivot the I ones to rotate off the ends of the Es.

    Bill

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    The OP has different motors. Thought he said they Mostly run one at a time that's not always. Seems everything worked fine until the High Starting Load dust collector was added. Not sure why it's high starting load simply a blower but might have 2 pole motor. Since that motor don't seem to run right the first thing I would check is that motor. To get back to basics I would remove the run and balance Caps. First because of different motors neither will be right! Second because I have seen more problems created by adding balance caps then they solve and the wrong run cap can cause overheating. I would forget looking for another potential relay, if he wants remote start I would simply lock it out after start-up. If it has only one set of contacts then add another relay and use those contacts to control it.


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