ThePunisher
Electrical
- Nov 7, 2009
- 384
HI folks, happy new year!
We have couple of series booster pump motors in the past that are controlled by a single VFD. Since the pumps will be running at the same speed set-point, it was practical to connect them into one large VFD to save costs on space allocations (and building), auxiliary power supply demand and circuits, operation and maintenance costs (OPEX). The current design indicates that the motors are connected to a supplier designed MV distribution equipment, installed as part of the VFD assembly, with MV switches and motor protection relays (MPRs). The CTs are located in each switch outputs and is wired to these individual MPRs along with the motor RTD cables. These MPRs interfaces with the VFD controller. The only DRAWBACK we see is that when one motor is at fault or at over-temperature, the entire VFD shuts down thereby stopping all the series motors and hence disrupting the process operations.
TO ELIMINATE THIS DRAWBACK, We are intending to consider and explore the option to have a standard MV MCC at the VFD output instead using MV switch-contactor combination. The MV contactor will be vacuum interrupting type and will be controlled from a separate 60 Hz, 120 V circuit from a UPS instead of deriving the control power via individual MCC feeder CPTs. The MPRs will be powered from a separate 125 Vdc power source. In this idea, the MPR and contactor will "race" or trip instantaneously by the MPR and hence isolate the faulty circuit or motor before the VFD decides to shutdown the entire process train. Our motors will be the 16 pole types (without gear boxes) and hence, we can operate the continuous process speeds within 30~60 Hz.
My concern would be the following:
a) Is there an issue operating the 60 Hz CTs on non-rated frequencies and if there is a continuous frequency range where they can operate properly?
b) Can I operate the contactor 120 V circuit from a CPT connected to a supply that is in variable frequency operation?
c) Is our design option sound hoping that others here did this in the past and if so, what are the lessons learned?
We will be contacting the VFD and MCC suppliers for guidance later but I am hoping this option is not that new and folks here already did it. I ATTTACHED AN OVER-SIMPLIFIED SKETCH TO SUPPLEMENT MY QUERY.
Regards and thank you in advance.
We have couple of series booster pump motors in the past that are controlled by a single VFD. Since the pumps will be running at the same speed set-point, it was practical to connect them into one large VFD to save costs on space allocations (and building), auxiliary power supply demand and circuits, operation and maintenance costs (OPEX). The current design indicates that the motors are connected to a supplier designed MV distribution equipment, installed as part of the VFD assembly, with MV switches and motor protection relays (MPRs). The CTs are located in each switch outputs and is wired to these individual MPRs along with the motor RTD cables. These MPRs interfaces with the VFD controller. The only DRAWBACK we see is that when one motor is at fault or at over-temperature, the entire VFD shuts down thereby stopping all the series motors and hence disrupting the process operations.
TO ELIMINATE THIS DRAWBACK, We are intending to consider and explore the option to have a standard MV MCC at the VFD output instead using MV switch-contactor combination. The MV contactor will be vacuum interrupting type and will be controlled from a separate 60 Hz, 120 V circuit from a UPS instead of deriving the control power via individual MCC feeder CPTs. The MPRs will be powered from a separate 125 Vdc power source. In this idea, the MPR and contactor will "race" or trip instantaneously by the MPR and hence isolate the faulty circuit or motor before the VFD decides to shutdown the entire process train. Our motors will be the 16 pole types (without gear boxes) and hence, we can operate the continuous process speeds within 30~60 Hz.
My concern would be the following:
a) Is there an issue operating the 60 Hz CTs on non-rated frequencies and if there is a continuous frequency range where they can operate properly?
b) Can I operate the contactor 120 V circuit from a CPT connected to a supply that is in variable frequency operation?
c) Is our design option sound hoping that others here did this in the past and if so, what are the lessons learned?
We will be contacting the VFD and MCC suppliers for guidance later but I am hoping this option is not that new and folks here already did it. I ATTTACHED AN OVER-SIMPLIFIED SKETCH TO SUPPLEMENT MY QUERY.
Regards and thank you in advance.