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Home » Motor control » Soft Starter/VFD with Star Delta Bypass System
Soft Starter/VFD with Star Delta Bypass System
Case:
I'm designing a VFD starter panel for chilling pump along with a VFD bypass system of Star Delta starter for when VFD is out for maintenance. VFD rating is 15 KW. What would be the standard (if any) rating to be chosen for the incoming breaker, as both VFD and Star Delta starter have to be fed by that breaker and while VFD shall run on the system for most of the time.
The VFD is being used for driving a chilling water pump and is part of the integrated HVAC system. The Star Delta starter would only be used while the VFD is in maintenance as a backup system. The speed control feature is disabled while in Star Delta but that wouldn't affect much the overall system working.
We thought of using a soft starter but as it turns out the soft starter costs significantly more than the Star Delta system in our part of the world. The Star Delta shall be used only to start the pump and still would not affect the system greatly.
The installation is critical and is a part of manufacturing unit that produces pharmaceutical products and the temperature of the system always has to be between certain given parameters.
And, please care to elaborate on how semiconducting devices and be made to endure for longer time as it seems with my experience that VFDs and Soft starters develop a problem, usually thyristor modules burn out, commensurate with other mechanical system although there are no moving parts in VFDs and soft starters.
Answer:
A variable frequency drive when no speed variation is required is not worth the money when they have also stated that the Soft Starter is too expensive. The standby VFD is therefore going to be way too expensive.
The VFD has continuous losses of about 5 to 2.5% 750 to 375w based on 15kW. The Soft Starter has just the contact resistance of the built in by-pass contactor. Watts or Milliwatts. The VFD will add complexity where it is not required.
If the chiller is in a critical Pharmaceutical environment, I feel that the Star Delta should have electronic protection. Not to necessarily give it better protection but to give you qualified feedback when it trips. If you then add the earth fault protection it will be more expensive than a soft starter. The protection is the “planned point of failure.” When it fails (trips) exactly where and when you want it to, you will need to know why to get it back up and running as fast as you can. Depending on the setup, you can use on electronic overload for both Star Delta but I would not recommend it. Again based on the parallel reliability factor and that the installation is deemed critical.
It is sad that a critical installation comes down to cutting back on the engineering side and not on the fancy car the boss arrives in. The difference could be just one meal out with the client and you gain all the added features.
Both VFD and Soft Starter should be protected by fuses. If you wish to grade it to a break you may but this adds cost. The definition of a VFD is an electrical device that solves all mechanical problems.
Selection could be due to process requirement (norm for chiller), energy saving as a bonus. If you want to control of the production temp (close loop system, motor speed controlled via temperature output gauge) and that would be the main reason for motor starter (VFD) selection, then go for it, forget about Star Delta (adjust configuration at the motor terminal box), foregone 3 cable. In term of protection, VFD has its own protection. You may maintain existing TOR, OCEF protection...then you would have double protection system. Normally, for VFD we don't defeat its protection as it meant to protect the VFD itself. Thus requirement for VFD warranty. Such short cable length, small motor rating, VFD would not cost you much and normally on shelve available.
I'm designing a VFD starter panel for chilling pump along with a VFD bypass system of Star Delta starter for when VFD is out for maintenance. VFD rating is 15 KW. What would be the standard (if any) rating to be chosen for the incoming breaker, as both VFD and Star Delta starter have to be fed by that breaker and while VFD shall run on the system for most of the time.
The VFD is being used for driving a chilling water pump and is part of the integrated HVAC system. The Star Delta starter would only be used while the VFD is in maintenance as a backup system. The speed control feature is disabled while in Star Delta but that wouldn't affect much the overall system working.
We thought of using a soft starter but as it turns out the soft starter costs significantly more than the Star Delta system in our part of the world. The Star Delta shall be used only to start the pump and still would not affect the system greatly.
The installation is critical and is a part of manufacturing unit that produces pharmaceutical products and the temperature of the system always has to be between certain given parameters.
And, please care to elaborate on how semiconducting devices and be made to endure for longer time as it seems with my experience that VFDs and Soft starters develop a problem, usually thyristor modules burn out, commensurate with other mechanical system although there are no moving parts in VFDs and soft starters.
Answer:
A variable frequency drive when no speed variation is required is not worth the money when they have also stated that the Soft Starter is too expensive. The standby VFD is therefore going to be way too expensive.
The VFD has continuous losses of about 5 to 2.5% 750 to 375w based on 15kW. The Soft Starter has just the contact resistance of the built in by-pass contactor. Watts or Milliwatts. The VFD will add complexity where it is not required.
If the chiller is in a critical Pharmaceutical environment, I feel that the Star Delta should have electronic protection. Not to necessarily give it better protection but to give you qualified feedback when it trips. If you then add the earth fault protection it will be more expensive than a soft starter. The protection is the “planned point of failure.” When it fails (trips) exactly where and when you want it to, you will need to know why to get it back up and running as fast as you can. Depending on the setup, you can use on electronic overload for both Star Delta but I would not recommend it. Again based on the parallel reliability factor and that the installation is deemed critical.
It is sad that a critical installation comes down to cutting back on the engineering side and not on the fancy car the boss arrives in. The difference could be just one meal out with the client and you gain all the added features.
Both VFD and Soft Starter should be protected by fuses. If you wish to grade it to a break you may but this adds cost. The definition of a VFD is an electrical device that solves all mechanical problems.
Selection could be due to process requirement (norm for chiller), energy saving as a bonus. If you want to control of the production temp (close loop system, motor speed controlled via temperature output gauge) and that would be the main reason for motor starter (VFD) selection, then go for it, forget about Star Delta (adjust configuration at the motor terminal box), foregone 3 cable. In term of protection, VFD has its own protection. You may maintain existing TOR, OCEF protection...then you would have double protection system. Normally, for VFD we don't defeat its protection as it meant to protect the VFD itself. Thus requirement for VFD warranty. Such short cable length, small motor rating, VFD would not cost you much and normally on shelve available.
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