# Dash pot O/L relay



## joe-nwt (Mar 28, 2019)

For those that have never seen one....


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## John Valdes (May 17, 2007)

When I worked for the railroad they were everywhere. I even forget what they were for? I mean I know what a relay is for. I saw them in passenger car electrical panels.
And those backboards were very common as well. Some on stand offs with circuit traces behind them.


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## joe-nwt (Mar 28, 2019)

Dash pots were old school time delay. Plunger with a small port moved through dash pot oil to provide the time delay.


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## ATSman48 (Feb 12, 2021)

From what I remember back in the '70's they were used in MCC's to provide overload protection to motors. Chiller motors in particular. They were then replaced by thermo links called "OL heaters" that would trigger a bi-metalic lever to open the contactor coil and shut the motor down. Now it is all done with solid state circuitry.
I guess it could be used as a TD relay also.


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## SWDweller (Dec 9, 2020)

Testing them was a pain in the ass. As the oil heated up the values would change. We had lock outs for 3 starts an hour on all of our chillers, >1000 hp. The boss would always have me do the tests on the dash pots as I would wait 20 or so minutes between tests. Our paper work for the yearly test was to test each dash pot 3 times on each phase. Need clean oil as well. 

Thanks for the blast from the past.


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## paulengr (Oct 8, 2017)

ATSman48 said:


> From what I remember back in the '70's they were used in MCC's to provide overload protection to motors. Chiller motors in particular. They were then replaced by thermo links called "OL heaters" that would trigger a bi-metalic lever to open the contactor coil and shut the motor down. Now it is all done with solid state circuitry.
> I guess it could be used as a TD relay also.


Almost. I think you are mixing up technologies here.

Eutectic overload relays have a spring loaded pawl held in place by a solder pot. When the solder melts the pawl spins releasing the spring loaded relay. Great technology. Two problems. You had to adjust by swapping heaters and lead is in the EU idiot list (RoHS) so effectively it’s banned technology. These were called either “heaters” or “eutectic” overloads depending on the manufacturer.

Bimetallic relays are similar but have a strip of two metals with different expansion rates. As it changes temperature the strip bends triggering the relay trip lever. Used a lot in old thermostats too where it was a big coil. This is NOT outdated technology. Most of the IEC motor circuit protectors on the market still use bimetallic relays. They always have the word “bimetallic” in them. By moving the mechanism you can easily adjust the trip point over a 2:1 range. The originals had replaceable elements but these days mostly all I see is that the entire relay is replaceable. Very convenient because the old eutectic overloads have such a narrow trip range that swapping motors could mean swapping eutectic heaters for a slightly smaller or larger size,

Electronic microprocessor trip units are more expensive but basically fixed cost so you see them either in plants with way too much money that buy Allen Bradley MCCs or replacements with NEMA contactors, or in special applications like motors over 200 HP where the rotor is the thermal limit instead of the stator, or where inventory is an issue. One relay works over basically the entire range of a frame size so with the typical 3 IEC relay sizes you only need 3 microprocessor trip relays. Cheap ones are around $200. Mid range like the Toshiba RX are $300-400. Expensive like SEL 710-5 and GE 369 are in the thousands. I started buying them when the price for eutectics was higher than the microprocessor relays but now with the combined disconnect/short circuit/overload relays there’s no need until you get up over about 150 HP.


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## paulengr (Oct 8, 2017)

SWDweller said:


> Testing them was a pain in the ass. As the oil heated up the values would change. We had lock outs for 3 starts an hour on all of our chillers, >1000 hp. The boss would always have me do the tests on the dash pots as I would wait 20 or so minutes between tests. Our paper work for the yearly test was to test each dash pot 3 times on each phase. Need clean oil as well.
> 
> Thanks for the blast from the past.


Over Memorial Day weekend most likely I will be testing a couple dozen old ITE breakers. Some still have dash pots. The rest were converted to URC. Any guesses how many I will end up condemning?


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## brian john (Mar 11, 2007)

Some of the first-generation Ground Fault systems utilized Dash-Pots for the operation of the shunt. Most were never tested or set to an effective value. These dash-pots were operated dry which resulted in a less than a second delay. 

Also had them in chillers for overloads.

Tested quite a few dash-pots hardly see them anymore.


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## ATSman48 (Feb 12, 2021)

Anybody remember the large air compressor motors that used dash-pots for OL protection, the manufacturer was Joy?
Although this may have just been in California.


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## Mbit (Feb 28, 2020)

paulengr said:


> Almost. I think you are mixing up technologies here.
> 
> Eutectic overload relays have a spring loaded pawl held in place by a solder pot. When the solder melts the pawl spins releasing the spring loaded relay. Great technology. Two problems. You had to adjust by swapping heaters and lead is in the EU idiot list (RoHS) so effectively it’s banned technology. These were called either “heaters” or “eutectic” overloads depending on the manufacturer.
> 
> Bimetallic relays are similar but have a strip of two metals with different expansion rates. As it changes temperature the strip bends triggering the relay trip lever. Used a lot in old thermostats too where it was a big coil.


GE made bimetal OLs with heaters. I just had to order some new heaters after changing out a motor. You can only go up or down 10% of FLA before you have to change heaters.


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## mburtis (Sep 1, 2018)

Those are really cool. I really like the old tech. Some of it was just flat ingenious.


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## paulengr (Oct 8, 2017)

To me the crazy MG set schemes are pure genius. Ever work on old elevators? Huge relay walls. DC motors are the main motors. Driven by a DC generator which gets its mechanical power from a synchronous AC motor. This is a Ward Leonard loop. Over on the relay wall we have diode rectifiers to make DC excitation fed through rheostats and capacitors with relays to create an analog voltage to make the excitation ramp up and down smoothly to the desired speed. This is the kind of basic servo controls we build today except that it was done starting in the 1930s with NO electronics at all. Transistors were known (put two cat whiskers on a single crystal set) but were not “officially” invented until Bell Labs came out with bipolars in the 1950s. At that time the Don II regulator came out, a completely analog compact industrial computer capable of vast improvements in for instance generating excitation for Ward Leonard loops.

What is scary is for me these are not just in history books. The NC State legislature still has one and there are two on Duke University campus and I’ve even seen one in a Doubletree hotel in Chapel Hill. And the Dom IIs and even older tech like Amplydines and Amplistats (Google them) are on dragline excavators. Imagine a 3500 ton robot run by an ancient magnetic amplifier system.

The only reason DC fell out of favor is that the motors and generators are maintenance pigs. Cleaning and inspection is required on the rotating equipment or the carbon dust and worn brushes eventually takes it out of service. Test tools include oscilloscopes and far more knowledge than you even need for VFDs. And the motors are crazy…rewind costs are normally 10 times more than the same size AC and the AC will last almost twice as long. DC fell out of favor because even though it lasts virtually forever even outright replacing AC parts is so much cheaper.


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## SWDweller (Dec 9, 2020)

I have done projects where we had AC and DC controls side by side. Like the controls for the gates in the CAP. AC did the big portion of the work. The DC was there to prevent the 40 ton gate from crashing into the concrete ditch. ( Central Arizona Project big water ditch from the Colorado River to Tucson)
I asked the techs from P&H once about AC vs DC controls on the shovels. Again back came the answer "fine control". If you want a new shovel nowadays I believe the DC ones have gone the way of the dinosaur and all you can purchase is an AC one.


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## just the cowboy (Sep 4, 2013)

I alway wanted to strangle the person that build panels that way. We had panels like that and would get a PM to change oil. When you unscrewed the pot to dump it the stem would leak onto the contactor below no matter how you put a rag in.


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