single phase overload relay

rpoet · Sep 24, 2019

I find myself needing to protect a large batch (16+) of small single phase 120V motors from overload; I've been told by the client their FLA is 4.9A. Is there a more elegant way of doing it than wiring all three phases of a 3-phase overload in series? I was hoping for something more compact than 50mm wide each.

Thanks,

rpoet

Gene Bond · Sep 24, 2019

They used to make them, but the volume is likely the reason most don't anymore.

Like this: https://www.supplyhouse.com/Furnas-...PTIJE25fOQiElFpsQSP9XsrHl5kH5icUaAqziEALw_wcB

That appears to be an old-style heater-type overload, rather than the IEC style used by most manufacturers today... Looking at the price, it's easy to see why they no longer make them!

duckman · Sep 24, 2019

It might have been due to all the shocks people got from the IP 00 terminals

jraef · Sep 24, 2019

You can just use a toggle switch style Manual Motor Starter. Not smaller than what you describe, but cheaper and simpler to wire.

Siemens and GE still make and sell 1 pole bimetal OL relays. they are smaller, but more expensive than the other options, I've seen some for around $125, plus you have to buy the heater elements. The Siemens is a part number 48DA18AA4. The GE is part #CR124K028. Personally, I have stopped using GE control parts since they have been bought by ABB, their future is probably tentative.

rpoet · Sep 25, 2019

jraef said:
You can just use a toggle switch style Manual Motor Starter. Not smaller than what you describe, but cheaper and simpler to wire.

Siemens and GE still make and sell 1 pole bimetal OL relays. they are smaller, but more expensive than the other options, I've seen some for around $125, plus you have to buy the heater elements. The Siemens is a part number 48DA18AA4. The GE is part #CR124K028. Personally, I have stopped using GE control parts since they have been bought by ABB, their future is probably tentative.

They need to be auto-resetting. The operators involved won't have the technical chops to manually reset anything. Since they're going to be PLC-controlled, I'll plan on using these:

https://www.automationdirect.com/ad...ange_ac_current_operated_switches/acs200-cd-f

I can set the O/L trip to just above normal running current for each motor (they'll all be a bit different), so I can get very sensitive overload trips if I want.

Thanks for the help!

-rpoet

Aabeck · Sep 25, 2019

PLC controlled Auto-Resetting?

Just put a current transformer on the motor lead, wire it to the PLC analog and have the PLC monitor the current. If too much shut off a safety contactor in series with the main contactor. Keep it off for 30 seconds then turn it back on.

If it trips a second time too quickly then leave it off until the maintenance tech resets it after replacing the motor.

OkiePC · Sep 25, 2019

Aabeck said:
PLC controlled Auto-Resetting?

Just put a current transformer on the motor lead, wire it to the PLC analog and have the PLC monitor the current. If too much shut off a safety contactor in series with the main contactor. Keep it off for 30 seconds then turn it back on.

If it trips a second time too quickly then leave it off until the maintenance tech resets it after replacing the motor.

16 CTs and 16 analog input channels? No hardwired motor protection? Are you sure about that?

tragically1969 · Sep 25, 2019

OkiePC said:
16 CTs and 16 analog input channels? No hardwired motor protection? Are you sure about that?

I agree, that's a weird conclusion to come to, the OP already used the word "protect" in his post, if it's protection use a protective device.

Looks like he's chosen to use some sort of CT with a fault contact on it, don't roll your own with a CT, forgetting overload how are are you going to remove power in an instantaneous fault with a CT wired to a PLC ?

jimtech67 · Sep 25, 2019

the O/L should not be tripping...
if they are there is an issue...

auto resetting will mask the problem until something else goes wrong....

If the motors are tripping having it a hassle to reset is good..
This will force someone deal with and solve the issue causing the tripping..

Plus when it to comes to safety or protection hardwired is the way to go...

Just my 2 cents.....

JesperMP · Sep 25, 2019

Auto-resetting does not sound right. I doubt it can be legal. The reason that you must have an OL is to protect against fire, protecting the motor or the machine is of secondary importance.
If OLs are too big or expensive, how about fuses. Fuses are still a good solution in many cases.

rpoet · Sep 25, 2019

Aabeck said:
PLC controlled Auto-Resetting?

Just put a current transformer on the motor lead, wire it to the PLC analog and have the PLC monitor the current. If too much shut off a safety contactor in series with the main contactor. Keep it off for 30 seconds then turn it back on.

If it trips a second time too quickly then leave it off until the maintenance tech resets it after replacing the motor.

There's no way the project can support the cost of (16) analog inputs for CT's. The sensor I linked to is a CT with an integrated solid state N/C contact, that opens at a setpoint.

PLC will shut off power to the motor if it sees the sensor trip for more time than it takes to account for starting inrush. These are fractional horsepower shade roller motors; nothing large.

There's no safety issue here; I've been asked to try to prevent damage if the shade snags on something. The PLC will flag an O/L condition on the HMI, and force an operator to enter a password to reset. The control panel will not be readily accessible, so I don't like the idea of an operator having to open it to press a reset button on an O/L device.

I like the idea of monitoring motor current with analog inputs, but I don't think it's the right solution for this project.

-rpoet

tragically1969 · Sep 25, 2019

rpoet said:
There's no way the project can support the cost of (16) analog inputs for CT's. The sensor I linked to is a CT with an integrated solid state N/C contact, that opens at a setpoint.

PLC will shut off power to the motor if it sees the sensor trip for more time than it takes to account for starting inrush. These are fractional horsepower shade roller motors; nothing large.

There's no safety issue here; I've been asked to try to prevent damage if the shade snags on something. The PLC will flag an O/L condition on the HMI, and force an operator to enter a password to reset. The control panel will not be readily accessible, so I don't like the idea of an operator having to open it to press a reset button on an O/L device.

I like the idea of monitoring motor current with analog inputs, but I don't think it's the right solution for this project.

-rpoet

OK, so trying to counter bad mechanical design with a PLC, sound's sensible ;-)

rpoet · Sep 25, 2019

tragically1969 said:
OK, so trying to counter bad mechanical design with a PLC, sound's sensible ;-)

Welcome to my life.

-rpoet

Snyd1 · Sep 25, 2019

Look at Siemens 3RM1 series motor starters for a tiny foot print, have overload built in (I think). Not sure availability for single phase, but the size is hard to beat regardless. Also only had the opportunity to use these guys in one application and it was 3 phase but imagine they would work fine for single, would just want to do some digging.

edit: brochure

https://assets.new.siemens.com/siem...0060-01-br-sirius-motorstarter-3rm1144dpi.pdf

jaden · Sep 25, 2019

How about a panel mount circuit breaker?

single phase overload relay

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