Rated HP of a VFD relative to that of the Driven Motor?

[ali.abbas]

Guys,

We are in process of evaluation of proposals for Gas Turbine driven Centrifugal Compressor to be installed at our plant. The start system of the GT comprises of a Starter Motor that is itself driven through a variable frequency drive. The purpose of the VFD is of course to control the rpm of the motor and therefore the GT during start-up.
The Starter Motor is rated at 75 HP whereas the VFD is rated at 150 HP. Can anyone tell me what the design basis of a VFD should be? I mean, the VFD here is rated twice as much as the motor it is supposed to drive. Is that in line with, or against, standard electrical design practice?

[wasifmehmood]

Normally VFD's are sized according to the rating of the motor, but this may vary depending upon your vendors design basis ( service factor, etc). Post the complete specs of your motor and VFD (Name Plate Data, model, datasheet(If any)) so that basis for this design can be determined.

[ali.abbas]

Specs of the motor are,

380 VAC, 4000 RPM, 3.5-133 Hz, 3-phase, 75 HP (56 kW) Rated

[Black Onyx]

Ali,

For a 56 kW only motor on non-continuous service (surely operation will be less than 2000 hrs / year), why do we need a VFD ? Have an enquiry with the vendor because you may not be needing this and it will reduce cost as well as space in area/switchgear.
In case there is a very specific reason, kindly share with us.

regards,

[ali.abbas]

Yes, I guess I should have shared the reason earlier.

The motor is to be used as a starter motor for a gas turbine. As per the start-up of the machine, the motor has 2 different speed set-points to operate at, high speed SP and low speed SP. The reason for the VFD is obviously to control the speed based on the control signal from the PLC.

And yes, as far as the operation hours goes, I am quite sure they would be much less than 2000 hours per year. This is merely a starter motor and once the turbine lights off and acquires a minimum speed, the motor free-wheels and finally drops out.

[wasifmehmood]

One query that you can forward to the vendor regarding the utility of the VFD is that since you need to run the system at 2 predefined speeds only, you can utilize a 2-speed motor instead. See how he justifies the use of this VFD and probably that'l answer your question

[Black Onyx]

Ali,

There are no standards for choosing a VFD for a specific wattage of motor to be operated. Normally different companies have different design practices to incorporate safety margins (usually more than 1.1 times motor rating) when sizing / selecting a VFD. Factors such as motor rating, voltage etc are considered, however, this does not justify using a VFD of twice the size of that of the corresponding motor.

Sometimes, like many other equipments, when selecting VFD the designer has to make a selection from one of the product lines available i.e. they do not produce VFDs for every size and in your case the motor is so small (75 HP only) that the nearest VFD available from the supplier is 150 HP and they have chosen it. In my opinion this is the reason for selection made by vendor. Just to site an example, we have a 3.2 MW rated Motor (blower for primary reformer) with a VFD of 3.8 MW only.

regards,

[ali.abbas]

Thanks Nouman, the point youve made seems quite logical.