"... AC motors can not regenerate. ..."

This is just plain incorrect and misleading. Please do not feel compelled to post if you do not know what you are posting about. ALL AC motors are capable of regenerating. All they need it to be spinning faster than the applied synchronous frequency, and to have excitation. A VFD, because it is capable of VARYING THE FREQUENCY, can create a relative synchronous frequency that is ALWAYS lower than the motor rotational speed, thus keeping it in regeneration all the way down to near zero speed.

No, you only need a VFD with braking functionality matched to the braking cycle duty.

The motor generates back to the DC intermediate circuit, then the 'excess' is either switched across a resistor bank or fed back into the supply. Whether you discharge this regenerated energy into a resistor bank or back to the supply is a separate question and issue.

Very high workloads often mean a regenerative supply side is beneficial but you can do the same by discharge too.

If long times and high duty cycle etc, ensure you have adequately rated braking components if you are just discharging - essentially the switching device (sometimes, these are thermally limited in some manufacturer's designs as often only short duty times and %)and also the resistors.

With centrifuges, the operative issue is production time. If you have a batch process that must be accelerated and then decelerated to allow for the next batch, then braking is paramount. Most of the time the coast-to-stop time on a centrifuge is extremely long, sometimes measured in hours! If you have to wait that long to start the next batch, then that is all non-productive time. You have an expensive machine that is not making money for you.

If on the other hand you have a continuous process that only needs to shut down at the end of a week or the end of a shift and doesn't need to be cleaned out, then braking is less important because you can just turn the centrifuge off and go home.

If you decide that you need braking, then you must decide on the TYPE of braking. Of all of the available methods, Line Regeneration wins hands-down on centrifuges, no contest. An AFE (Active Front End) VFD with line regen* has essentially another inverter on the front end (for those who seem to think VFDs are incapable of this) that will take the excess energy from the DC bus as it comes back off of the motor, and fire the front-end inverter back into the line source as an energy feed. This not only allows for virtually unlimited braking time at the full capacity of the motor, it also recaptures the kinetic energy from the spinning load, reducing your overall energy costs.

Your other choices all come with relatively severe restrictions. Dynamic Braking, i.e. to burn off the excess energy from the DC bus as heat using Braking Resistors, has the limitation of the capability of the resistors to avoid catching on fire. They work fine for many other types of loads, but for centrifuges, they have to be very big and often need external cooling, which INCREASES your energy consumption.

You cannot use DC Injection Braking at all because that forces the MOTOR to absorb the kinetic energy of the spinning load into itself, which causes motor damage.

Mechanical Brakes, the method traditionally used, involves wear and tear on brake pads / drums / rotors etc., plus it also involves considerable heat from the friction.

Line regenerative braking using a VFD is probably the highest initial capital expenditure but accomplishes the lowest possible operating costs. Considering that operating costs for an AC motor systems are typically 5X the initial investment in control equipment, the decision is usually a no-brainer.

I should note however that there are companies that make Line Regeneration units for AC motors that do NOT need a VFD to work. So if you are never changing the speed of the centrifuge, you may want to consider that option.

*(Line Regen is not always included with AFE just so you know, some mfrs offer an AFE only for harmonic mitigation, so they are not fully rated for the line power and therefor cannot regen the full motor power. make sure you specify Line Regeneration when discussing this with suppliers.)

Just one more thought - if you have 10 centrifuges (I missed this nuance in your post), you can possibly load share when you have more than 1 VFD. Then, you can use the energy recovery from a stopping machine to drive or accelerate other(s) by sequence control. This is achieved by commoning DC buses. You could still have a braking circuit as well which would then be mostly redundant and you save energy.

Am I correct in thinking that you cannot feed in regenerated energy (Anonymous?)