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### VFD with Regenerative Function

06/10/2012 10:22 PM

We are planning to replace our centrifugal basket with a vfd control from a conventional. my main concern is do we need vfd with regerative function (having a 10 centrifugal baskets) and for the time being we have to test one basket with this control. any comments? thanks and much appreciated

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#1

### Re: VFD with Regenerative Function

06/11/2012 1:15 AM

If you wish to decelerate the centrifuges more rapidly than their natural coast-down rate, I think you need regeneration in the VFD, so that the rotational energy can be returned to the grid or other power source.

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#3

### Re: VFD with Regenerative Function

06/11/2012 1:34 AM

what if the rotational energy return is not possible this time, meaning only one vfd for the moment without utilizing the regenerative power?

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#4

### Re: VFD with Regenerative Function

06/11/2012 3:29 AM

Then, logically, the regenerative function is unneccessary.

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#2

### Re: VFD with Regenerative Function

06/11/2012 1:27 AM

A VFD converts 3 phase or single phase AC to DC using simple high voltage bridge rectifiers; then this High Voltage DC is converted to 3 phase with variable Frequency. AC motors can not regenerate. Some VFDs braking facility apply DC to one of the phase through a "Breaking High Wattage Resistor". Hence regeneration is not possible with AC motors. Yes it is possible with DC motors and many industries like Coal mines, Rail ways, Steel plant, etc are already using this technique.

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#6

### Re: VFD with Regenerative Function

06/11/2012 11:18 AM

"... 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.

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#10

### Re: VFD with Regenerative Function

06/12/2012 2:23 AM

Thanks for correction. I mean that comparing to DC motors. I have, during 1976 had practical experience during Bargi Dam Project. Once 25HP welding generator (2930 RPM) working and a small 3 phase 2 HP concrete vibrator running on the same line. Welding generator is a 3 phase motor directly coupled to a DC welding generator. And then suddenly on phase failed. The small vibrator motor was working normal but welding generator though running it was not giving enough current to do welding. When we checked current on 3 phases of small motor it was equal but welding generator current in failed phase was very low.

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#5

### Re: VFD with Regenerative Function

06/11/2012 6:28 AM

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.

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#7

### Re: VFD with Regenerative Function

06/11/2012 11:53 AM

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.)

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#15

### Re: VFD with Regenerative Function

06/12/2012 7:44 PM

Thanks.....this is practically the better explanation and much appreciated.

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#8

### Re: VFD with Regenerative Function

06/11/2012 5:04 PM

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?)

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#9

### Re: VFD with Regenerative Function

06/11/2012 10:32 PM

what are you planning to do with the power regenerated?

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#12

### Re: VFD with Regenerative Function

06/12/2012 11:17 AM

?

It is banked by the power grid.

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#13

### Re: VFD with Regenerative Function

06/12/2012 12:26 PM

is it?

It could be used to off-set consumption, if no feed in tariff is available.

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#14

### Re: VFD with Regenerative Function

06/12/2012 1:20 PM

Very few installations have net energy production.

The net result is a "sloshing around" of the consumed / produced power.

Seldom do you need to specifically determine what you are going to "do" with the energy of braking a section for a few seconds.

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#11

### Re: VFD with Regenerative Function

06/12/2012 7:11 AM

Since you probably need some hot water in your process, you could also use an electric water heater to act as the dynamic braking resistor. There are some very powerful in line heaters that could be installed before your main water heater. The energy from braking would then be used to pre-heat the water. No wasted power! . The main heater runs as needed to reach the desired temperature.

This may be the least expensive setup. You will need a good ground on this installation (on the heater) as the dynamic breaking chopper combined with the leakage capacitance will produce some leakage current in the pipes.

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