Should I use a Regenerative Drive?

A belt conveyor transports material 900 Tons/hr from a higher level to lower (130 meters). It is equipped with 2 motors of 132 kW 400V each. In the beginning motors drive the load and when material starts going down hill, the load is driving motors and the Motors RPM measured are 1509 against rated 1485 RPM.

Theoretically in this condition the motor is acting as generator and feeding power back to system. Someone suggested installing a regenerative drive for energy saving. However I think we will get no benefit as we are already getting power back in to system. Am I right? Or is there any advantage of using a pair of regenerative drives?
We have the budget but should we do it?

it is reducing power feed to the plant, so you are currently benefiting with no extra expense.

You could hire a (3 phase) power meter and find out exactly how much energy you are saving and /or regenerating from the existing setup; so you can make an informed decision as to whether a regenerating system is better and economic.

You have stated:
"In the beginning motors drive the load and when material starts going down hill, the load is driving motors"

So if I understand that right, it's rotating 1 direction and applying torque in the opposite direction, basically holding back the load since it is sloping down with gravity.

If accurate, the energy is being absorbed by the drive in its capacitor bank. If this energy level is too high, the VFD will fault out on overload or dc bus voltage. The life of the Drive can be impacted from constant exposure to overhauling load energy.

There are 2 common ways to dissipate that energy-

1. dynamic brake resistors. switch the excess energy to a resistor bank and dump it as heat.

2. load regen module (as your colleague suggested). this is basically a second drive module capable of taking that excess energy and switching it back to the line.

Using a line regen method is more efficient than wasting energy as heat. It will handle a higher amount of energy and can run at 100% duty cycle. Resistors are often limited in duty cycle due to heat. Line regen costs more and is more complicated to install. There is probably some useful reading on the topic at Bonitron's website: www.bonitron.com

Hope that helps- good luck!

Without regeneration capability you are not realizing any savings. Any "braking" energy is dissipated as heat in the drive capacitors or dynamic braking resistors, not injected back into the power line. You will have to determine the amount of energy which is being dissipated when the drives are "braking", and what your electric utility cost is in order to calculate the savings/payback if you install regenerative capability into the drives.

Presently we are using standard squirrel cage motors without drives. DOL starting and for soft start we have mechanical fluid coupling. There is no VFD presently. We also have eddy current brakes and mechanical disk break for stopping in emergency. In normal operation belt is first emptied and then stopped.

I am planing to install 4 quadrant energy meter to verify energy fed back to system.