Shock Relay

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ptonsparky

ptonsparky

Tom
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NE (9.06 miles @5.9 Degrees from Winged Horses)
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EC - retired
I have an application that could use a "shock relay". I have not installed one as yet but know they are available. I have PLC control of the load in question and am wondering if I could use a standared current relay and tweak the control via the PLC. Haven't gotten a price back yet on the Shock relay so I don't know the price difference, if any
 
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dicklaxt

Senior Member
It takes the place of the conventional overloads but not sure how.The operation description says that current flow from 2 of the 3 phase legs pass thru a CT in the same direction.

Now with that having been said,it just graduated into the lap of an engineer and I am waiting for an explanation,:confused::D.

I still confused by the name "SHOCK RELAY"?

I'm going to guess that with current flow in the same direction from any 2 phases it will keep the main contactor relay latched out holding the normally open contact(shelf position) 95/98 terminals open thus keeping it from re-energizing until the fault has been cleared and the reset button pushed.

dick
 
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dicklaxt

Senior Member
I'm going to guess that with current flow in the same direction from any 2 phases it will keep the main contactor relay latched out holding the normally open contact(shelf position) 95/98 terminals open thus keeping it from re-energizing until the fault has been cleared and the reset button pushed.

dick[/QUOTE]

Let me reword the above

I'm going to guess that with current flow in the same direction from any 2 phases(this would have to occur after fault,still guessing:D) it will keep the main contactor relay latched out holding the normally open contact(shelf position) 95/98 terminals open thus keeping it from re-energizing until the fault has been cleared and the reset button pushed.

dick
 
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Jraef

Jraef

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Electrical Engineer
All it is is a Current Sensing Relay (CSR), also sometimes called a Current Trip Relay or an "Electronic Shear Pin" because it works to sense when a shock load is dropped onto a motor suddenly that may damage mechanical components, and a "shear pin" is what would be used mechanically, providing an easily replaceable part that breaks first to save other more expensive parts. They are available from almost all of the major manufacturers and a few small vendors like the Tsubaki (which by the way is just a brand label of a device made in Korea made by a company called Samwah and available from dozens of sources). Tsubaki just coined the term "shock relay" in the US because here, it cannot be UL listed as an "Overload Relay" because it does not monitor all 3 legs, something required by the NEC. Everywhere else this thing is sold it is used as an OL relay.

But there are a couple of issues.
  1. Monitoring current will be subject to current fluctuations due to voltage changes and may be problematic. It's often better to monitor POWER, as in kW, because that is representative of true motor shaft load. There are many relays that do that, but they are more expensive that the simpler CSRs.
  2. If you think current is fine for what you want to do, there are very simple little Current Transducers that have a donut CT, some with split cores so you can add them to existing wires. Then you can take that signal, i.e. 4-20ma, back into the PLC since you already have it to let the PLC make the decision as to what to do if there is a shock load. But this has to be weighed against the cost of an analog input for your PLC if you don't already have one. There are versions of these little Current transducers that also have a threshold adjustment to cause a trip signal that can be sent to a digital input as well.
  3. You get what you pay for. Most of the Asian relays, like the Samwah / Tsubaki, are not up to the quality standards we tend to be used to here in the US. You can get similar devices from Siemens, ABB, A-B, Schneider, GE, Cutler hammer etc. etc. etc., you have to but ask. But don't call it a "shock relay" because that will confuse suppliers.
  4. Many of the newer Solid State Overload Relays available from the above suppliers now offer that Current Trip function as a separate threshold from the regular OL trip set point, specifically for this purpose. that may be a better way to go since you are going to add something anyway.
 
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gar

Senior Member
Location
Ann Arbor, Michigan
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EE
100630-1411 EST

The name for this device should maybe include the word breaker instead of relay. And instead of shock maybe impact so I do not get the idea it something to detect electrical shocks.

I think monitoring two leads to the load is used to shorten response time. Better yet would be a 3 phase watt transducer that would further reduce the response time to less than 3 MS. Obviously this is more expensive.

.
 
Jraef

Jraef

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Location
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Electrical Engineer
gar said:
100630-1411 EST

The name for this device should maybe include the word breaker instead of relay. And instead of shock maybe impact so I do not get the idea it something to detect electrical shocks.
Click to expand...
Tsubaki is primarily a mechanical power transmission component supplier, sold through PT houses, hence the use of terms we would not use in the electrical biz. This is the only electronic device they sell, they have no clue.
 
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BJ Conner

Senior Member
Location
97006
The newer solid state relays have a lot of funtions all of which don't get used in every application Eaton Cutler-Hammer, Basler, GE etc. have relays that have Jam/Stall funtions that can detect when something like a big bolt falling into a klinker grinder. The microprocessor in the relay can measure and analyze how fast the current increases and know it's a jam. If the grinder is just being overloaded it will trip but not as quickly.
If a motor trip you can go to the relay and see if you got a trip becasue of a jam, over current, etc.

The ANSI/IEEE device number for this function is 51R (51 being time overcurrent and R being Rate).
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
thunk thunk thunk

thunk thunk thunk

Could not sell this project. I think they had lost 3 sets of belts @ $2k per when I started this thread. Belts were installed each time & never checked again until they slipped when the load caught a bit. Normal. Manager raised hell until they tightend the belts after a break in period. Can anyone guesss....drumrolll.....bearings are out. Worst of it is they noticed the 200 hp motor jerking to a stop and then starting again, several times. $6000 motor plus down time. Manager is thinking they may grind the shear pin down a bit so it shears quicker. ????

Maybe I need to hire a salesperson.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
ptonsparky said:
Could not sell this project. I think they had lost 3 sets of belts @ $2k per when I started this thread. Belts were installed each time & never checked again until they slipped when the load caught a bit. Normal. Manager raised hell until they tightend the belts after a break in period. Can anyone guesss....drumrolll.....bearings are out. Worst of it is they noticed the 200 hp motor jerking to a stop and then starting again, several times. $6000 motor plus down time. Manager is thinking they may grind the shear pin down a bit so it shears quicker. ????

Maybe I need to hire a salesperson.
Click to expand...

To quote comedian Ron White...

"You can't fix stupid..."
 
R

rcwilson

Senior Member
Location
Redmond, WA
Passing two of the three motor leads through a donut CT or current sensor is a way of reading the average current of the two phases. For a three phase load Ia + Ib + Ic = 0. (vector addition).

Two phases through one sensor means the sensor is monitoring (Ia + IB) = -Ic. Its a cheap way of monitoring motor curent for overload.

Refer to the other recent threads on two versus three overloads.
 
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