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Mpcb Vs. Microcontroller Based Relay


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Hi all,

 

For my new project I am considering using of microcontroller based ct operated relays in place of motor protection circuit breakers. I will be employing an MCB in the motor incomer for short circuit protection. I would like to avoid using a bi-metallic relay. The CT operated microcontroller provides me with several protection like phase reverse, dry run protection, highly reliable single phasing protection and seems better option. My project uses motors from 3HP to almost 400HP.

 

Any experience on such a replacement?

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It is done all the time. I personally prefer many of the good quality solid state over load (SSOL) relays over any electro-mechanical means for the simple fact that it is more reliable and repeatable. Bimetal OL relays tend to build up a "thermal memory" meaning that the bimetal strips never return to their original shape and over time and trips, eventually begin to trip sooner and sooner. While this mode of "failure" is not inherently dangerous to the motor because it errs on the side of early protection, this can be costly nonetheless because of increased downtime from nuisance tripping.

 

I too like the additional protection available from most SSOL relays as well. The biggest benefit in that area is from protection against unbalanced currents. Unbalance causes negative sequence currents to circulate in the motor windings which cause the motor to heat up disproportionately to the overall phase currents. So a motor who's current is well below the tripping threshold can still theoretically overheat without a bimetal OL relay even knowing there was a problem. While it may not burn up, the motor life can be severely compromised from that additional heat. Many of the better SSOL relays have a method of biasing the phase current to compensate for unbalance percentage and skew the OL trip curve to provide better protection.

 

If you look around, there are a lot of very good SSOL products out there now which offer far superior protection than older electro-mechanical versions, be they in OL relays or Motor Protection Switches.

"He's not dead, he's just pinin' for the fjords!"
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Thanks

 

With all the benefits mentioned by you which I agree, why is that all project consultants continue to specify the bi-metallic technology. The SSOL has been around for years and costs have come down where downtime costs substantially justify using them..............still I find convincing my consultant a bit difficult. He says we have little experience.......He can gain experience at my cost but he is frustrating me as the project manager.............Of course we cannot change what the consulatnt specifies!!

 

Also I find lot of the manufacturers are not very pushy about this technology and continue to send salesmen with convetional OLR and now MPCB!!

 

 

 

SGK2007

:excl:

 

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Hello SGK2007

 

Like all things, there are the good and the bad. Some of the low cost SSOL provide less protection than the thermal overload relay with some only measuring the average current.

 

Good overload protection models the thermal characteristics of the motor with an allowance for the iron loss and the copper loss and differential cooling dependent on the operation cycle of the motor.

 

Like jraef, I firmly believe that a good SSOL is a much better proposition that an electromechanical overload relay.

 

I also know of a number of engineers who will not use solid state because the older technology has always worked in the past. Some people just will not move with the times!!

 

Best regards,

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Hi

 

I for one would like to use them but after convincing myself about the features.

 

There is this feature in SSOL of tripping the motor on underload. I am given to understand that this feature can be used for preventing dry running of pumps- by setting the underload value to eg. about 20% of the FLA and the device trips when the motor draws less than 20% current. How reliable this concept is of tripping on low value of current for dry run protection ? If it is, I must say it is extremely useful feature as dry running of pumps is a major cuse o downtime in process industry.

 

 

Regards

 

 

 

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Undercurrent protection is quite common on solid state protection relays and soft starters and certainly can be used as a means of dry run protection on pumps, however there are some potential issues.

If you run the pump dry, there will be a low shaft torque load and so the current that you measure will be the magnetizing current of the motor plus a small amount of work current.

The value of the magnetizing current is dependent on the pump design and can vary from as low as 20% to as high as 60%. Magnetizing current is also very voltage dependent, so a small change in voltage can lead to quite a large change in magnetizing current.

 

If the minimum allowable pumping current is significantly higher than the magnetizing current, then under current protection will work well provided that it is set up correctly. If the magnetizing current is significant relative to the minimum allowable pumping current, then there will be problems with the reliability of this protection and potential missed trips or false trips.

Submersible pumps have a very high magnetizing current. If the submersible pump is required to operate at less than 50% flow, you may have difficulty with under current protection working correctly.

 

An alternative dry run protection is provided by calculating the shaft torque produced by the motor. This is independent of supply voltage and magnetizing current and will work very well with submersible pumps. Suitable relays are manufactured by Emotron and other manufacturers. see http://www.motor-control.co.nz/content/view/25/54/

 

Best regards,

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Hi Guys,

 

In start delta configuration, the OLR is connected in phase? Why?

 

Do we have to connect the SSOLR also in phase. Actually this one SSOL I am planning to use displays 3 phase currents and it makes sense to put it in line to see line currents. But I want to know whether it has to be put in phase or we can adjust the setting and put it in line?

 

Regards

 

SGK2007

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Hello SGK2007

 

The overload relay can be connected in series with the windings, or in series with the supply. In a star/delta starter, it is far more convenient to connect it in series with the windings because it clips on to the main contactor. If you used a larger contactor and used that to control the total current to the motor, you would also have to put in a larger overload relay. The normal configuration reduces the size of the contactor and overload and saves cost.

If you connect the overload relay in the supply, it must be set to the full motor current. If you connect the overload relay in series with the windings, it must be set to 1/rt3 of the motor ratings.

 

Best regards,

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Hi

 

Can we use the microcontroller based motor protection devices (MPD) meant for LT application for HT motors. by using HT CT- secondary outputs as input to the relay or there are separate norms for HT relays?

 

The relay o/p for this MPD is 5A continuous at 230V.

 

Can you recommend a good SSOL for HT motors

 

Thanks

 

SGK2007

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