Does a VFD Driven Motor Require Separate Motor Protection Relay?

it really depends on the model and make of your VFD. Back in the olden days (1980s and up), motor protection furnished by VFDs was primitive, especially when considering unbalanced phase currents, an easy way to fry any motor quickly. VFD manufacturers always recommended replica type overload relays at the VFD output.

Modern PWM VFDs are better designed, and will protect for single phasing a motor, in most cases.

It always pays to read the VFD manufacturer’s installation instructions, especially with a large expensive motor such as you have.

The addition of redundant overload protection at the output of a VFD requires some coordination with the VFD controls, so that the maintenance people who need to keep it running don’t encounter something unusual from their plant standard, leading to an unsafe condition.

Your statement about bypass and starting is unclear. If you have a running contractor that bypasses the VFD, then of course you need separate replica type overload protection, or a comprehensive motor protection relay. This separate protection needs to be integrated into the VFD controls.

If your VFD fails and you have to run the VFD in bypass mode in DOL then you need a motor protection relay. A motor protection relay will also protect for phase failure, earth fault, underload and overload should the VFD become erratic. All the units used in the coal handling plant also had a motor protection relay as well as the VFD.