Pumps, motors and VFDs.

Overall: What are the conditions in which you need a VFD or a motor starter?

I have a Centrifugal pump and a Diaphragm pump running separately. I am confident that I need a VFD for my Centrifugal pump, however, based on how a diaphragm pump works with air, does the Diaphragm pump need a VFD or even a motor starter?
If a VFD is needed, how do I determine the size of my VFD? Is it based on the size of my pump's motor?

I am new to the field and would love your knowledge and support.

If you are going to run the electrical motor at nameplate motor speed all the time, you will need a motor contactor with some kind of overload protection to turn the motor on and off as well as protect the motor and wiring. If you want to very flow or pressure, you will have to do so with valving.



If you wish the vary the speed of the motor, you can use a drive instead. So instead of using a bypass valve to bleed off excess pressure, you can now slow the pump down, saving energy. It all depends on the type of pump and your application. If you are running a pump to fill up a tank, a drive does nothing to help you. If you are constantly trying to hold a header at a precise pressure, a drive can be very beneficial.


You size your contactor/overloads or drive by the full load amps of the motor.