Regenerative Drives

craigsimon

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Join Date
Apr 2002
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Minnesota
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I am looking at replacing out the controls on an old machine. This machine has a mixing motor that is controlled from a regenerative AC drive. I have used many VFDs, but never a regenerative drive. From what I can tell a regenerative drive is used for energy savings. That is, as the motor slows it acts like a generator. Am I way off? The mixing motor in question is a fractional HP motor running at speeds from 5000-7000rpm or 70Hz-120Hz. My question is, Why a regenerative drive? Any explaination is appreciated.
Thanks
 
Regenerative usually refers to braking. It isn't usually done for energy savings, but as a convenient method for controlled motor stopping. The power from the motor back to the line is controlled by the VFD, or it may be dissipated as heat in the VFD. For high inertia loads or fast stoppong you usually need a braking resistor for "dynamic braking". If you use regenerative braking with fast deceleration time on a high inertia load you will probably trip out on high DC bus voltage.

Consult your VFD manual.
 
Regenerative Drives (AC or DC) have the capability of taking DC power produced by the motor during braking, converting it to AC power and putting it back into the power lines. Most VFDs have the ability of converting AC power from the motor into DC power. This is usually dissipated with braking resistors (Dynamic Braking). Regenerative VFDs can also invert the DC into AC.

Regenerative VFDs are usually found on higher HP drives, which must brake for a large percentage of their operating time. The braking energy is not wasted as heat as it would be with dynamic braking. I have never heard of a fractional HP Regen VFD. Who is the Manufacturer?
 
Regenerative Drive

The drive is a Regadrive FVR-G5S. The energy saving can not be to much, that's why I was looking at other reasons for using a regenerative drive.

Tom, what you are saying is that with a regenerative drive the motor will brake quicker? Or is it a way to dissipate heat back into the AC lines instead of out the heat sinks? There is no braking resisters for these drives. The drives manuals do not discuss why you would use a regenerative drive and this application does not look like it is needed. Also as a side note, the manufacture installed three other motors with non-regenerative VFDs in the same control cabinet.

Thanks for the feedback.
 
A regen drive does essentially pump the braking energy back up the power lines. It will stop quicker than a standard VFD, but not as fast as a resistor setup. Of course this is one area where the technology is changing fast. Maybe the regen drives to stop faster now.
 
I visited Mitsubushi Electrics no so long ago, as I was changing from their inverters to siemens inverters as a site standard and wanted to give them one last chance to maintain us as a customer (they lost out cos their service was useless).

But anyway they had a test rig in which a inverter driven motor was linked to a servo motor which was then linked to a regenerative drive. The servo motor was used as the load to the inverter driven motor and provided a certain amount of usefull energy back into the national grid (a bit pointless in terms of creating energy)

But if you have a application where you have a spindle turning and you have the ability of coupling a drive to this (maybe 10-20kW) then you could harness the energy produced.

You could also simply use a drive as a break and rather waste the energy gained turn it into electricity.

Regards

Barry
 
"Tom, what you are saying is that with a regenerative drive the motor will brake quicker?"

Yes, but "it depends" on your point of reference - in other words, "quicker than what"?

Regenerative braking will brake faster than a coast to stop. As Rick says, it will not brake as rapidly as dynamic braking with a resistor, and a mechanical brake is usually fastest of all. In our typical application we have a high inertia load at 3,600 rpm, and it takes 5 minutes to coast to a stop. With regenerative braking we can come to a stop in around 30 to 45 seconds.

One other motor stopping aspect is the use of DC injection braking. This is intended to bring a motor to full stop once the speed is down to a very low point - like 1% or 2% - without coasting. Regenerative and DC injection braking are usually used in tandem. Regenerative braking can't achieve 0 rpm, DC injection can't bring a motor down from high rpm in a controlled speed ramp.
 
I do not agree that dynamic braking with a resistor will stop a motor faster than regenerative braking. They are both doing the same thing so why would similarly sized braking units provide different braking torques?

The stopping rate is determined by the decel ramp rate in the drive. This should be set to keep the motor current below 150% FL. The motor, when braking, generates power which charges up the DC Bus in the drive to a higher than normal voltage. Higher braking torque produces higher bus voltages. If the voltage is allowed to continue to increase, the drive will trip to protect itself. The dynamic braking resistors or the regen braking unit removes this excess voltage from the bus by dissipating the excess energy in the resistors or the power line. As long as the braking resistors or the regen unit are sized to handle the power, the motor can produce 150% of FL braking torque.

The regen unit can be built into the drive or it can be an add-on unit, which connects to the drive in place of the usual dynamic braking chopper transistor and resistor. Either way, it performs the same function as the dynamic brake.

Most VFDs also have DC injection Braking capabilities. This type of braking can only be used when stopping, not for slowing down or holding back overhauling loads. DC is applied to the stator windings of the motor causing current to flow in the rotating rotor bars of the motor. Generating this rotor current produces braking action in the motor. The amount of braking torque produced depends on the DC current and the speed of the motor. Higher speeds produce more braking torque (higher rotor current), lower speeds produce little braking torque (very low rotor current).

I hope this has cleared up some of the confusion on this subject.
 
Thanks

Thanks for all the info. The original manufacture must have used a regen drive for the braking aspect.

I will need to look at either using a regen drive or a braking resistor kit.
Thanks again
 

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