1 VFD, 2 motors

[jraef]

It is also possible to protect motors with correct overload relay and contactor after drive.

Only if the VFD can run in V/Hz mode without needing to be tuned to the motor. This is rapidly becoming less and less possible with newer low HP drives, as the newest generation of IIGBTs are now turning on so fast that without being tuned to the motor, the motor runs really rough and hot. That's the benefit of having a VFD that can hold two separate motor profiles in its memory, in which case you then don't need the OL relays.

But again, if you lose the VFD, you lose both motors; that's still the big down side. I've priced this out with large drives, it made some economic sense at 500HP and up, but in every case where I went through the exercise, the end customer freaked out about that risk when I pointed it out and bought separate drives.

 

[DickDV]

Standard overload relays in the motor leads of VFD's are often used in multiple motor situations where the motors are all connected at the same time. The drive thinks there is one large motor out there and has no way to protect multiple small motors.

But, to describe the arrangement as "proper" is a bit of a stretch. Overload relays estimate motor temperature making the assumption that the motor is turning at 60hz (or 50hz outside North America) speed. Since motors running on a VFD can be running at many speeds both higher and lower than 60hz, the thermal calculation is not accurate. This is particularly true as the motor power goes below 30hz.

So, while there is no other convenient way to protect multiple motors on VFD's, the protection is less than ideal. Another reason to use individual drive/motor pairings whenever possible.

 

[Gene Bond]

It's a common *old* way to save money, at the expense of complexity. Much like a 3 contactor bypass circuit replaces 3 wire nuts

Just this week, I saw a spec for 2 drives, 2 motors and 5 contactors. This way, either drive could control either motor. What a mess!

Even though it can work, it's not always the best thing to do.