Soft Starter Vs Vsd

[candy]

Hi all...

 

A 270 KW(preliminary fig. from Mechanical Calculation) Blower Motor is to be started from a 11kV Switchboard (50 Hz) system.The Substation also has a 525V MCC.There is a min & max flow rate requirement for the blower.There are 2 vendor offers for this :

 

Vendor 1 : Instead of one blower (with MV Motor), the vendor has suggested two 50% rated blowers (with LV motors to be supplied from the 525V MCC).One machine will remain on while the other off to cater to min flow conditions.The motor rating calculated by the Vendor is 185 KW(2-pole) considering max. flow rate. The vendor has also suggested the use of electronic soft starters to accelerate the motors smoothly to full speed, minimizing effect on power system.

 

Vendor 2 : The vendor from his flow-rate based calculation has suggested a 290 KW (6-pole) blower motor to be supplied from the 525V MCC using VSD.

 

- It is evident from the vendor offers that both are avoiding to supply the motor from the MV board. Please guide which will be a better offer technically for starting a blower motor, especially from Soft-starter vs VSD point of view.

[marke]

Hello candy

 

Welcome to the forum.

 

11KV controllers are very expensive and only available from a few vendors. This will be why the 525 volt option has been recommended.

 

You do not mention what the flow rates required are, but a single large machine would normally be very capable of achieving your requirements.

Two machines, one with a soft starter and the other with a VFD is another option, provided that the VFD has control of both in a cascade control system.

Two machines with soft starters will give you three flow options only, no flow, part flow and full flow. If the two machines are of different sizes then you will have two part flow levels.

 

I would suggest that as you specify variable flow, you will need to look at one of the VFD options, or add shutters in the air flow.

 

Best regards,

Mark.

[candy]

Hi Mark,

 

Thanks for such a prompt response.

I am specifying the flow conditions & absorbed KW rating calculated by the vendors.

 

Vendor 1 :

 

Required Capacity per Blower at 50% Capacity each

Volume Flow : 13,882 kg/hr & Absorbed power at shaft : 164.29 kW

Required Capacity per Blower at min. flow

Volume Flow : 5556 kg/hr & Absorbed power at shaft : 88.98 kW

Drive Details

Required Motor rated output : 185 kW & Required Motor full load speed : 2975 rpm

 

Vendor 2 :

 

Required Capacity of Blower at max. flow @ 973 rpm

Volume Flow : 27763 kg/hr & Absorbed power at shaft : 289.8 kW

Required Capacity of Blower at min. flow @ 242 rpm

Volume Flow : 5553 kg/hr & Absorbed power at shaft : 64.2 kW

Drive Details

Required Motor rated output : 330 kW, 6-pole

 

No other flow conditions are available as of now from process point of view.

Also, I would like to know your opinion regarding the no. of poles(with regards to rpm) recommended by vendor.

[marke]

So for option 1, you run one motor only when you want minimum airflow, but you have twice the airflow requirements. Are they going to use shutter or dampers to regulate the flow?

 

Best regards,

Mark.

[candy]

Yeah Mark, you have pointed out rightly for option 1.

I presume there will be usage of dampers/shutters.....actually the project is at a very early stage & these are the initial bid offers.

 

Presently, it seems Client would like to supply the Blower Motor from the 11 kV Switchboard only with suitable drive.