regenerative braking

hello everyone,

i am looking at a project where i will be able to control the speed of a flywheel driven by a motor. i want to use a drive that can reduce the speed of the flywheel in a controlled manner using regenerative braking.

can anyone tell me of a suitable drive or give me any help in how id do this. i know of some drives that have built in regen braking but these are expensive.

thanks
benny

The Allen-Bradley PowerFlex series can do this. External dynamic braking resistors can be connected to dissipate the heat.

See this document:
http://literature.rockwellautomation.com/idc/groups/literature/documents/at/pflex-at001_-en-p.pdf

thanks very much john,

yes i see, i am aware of breaking resistors, however would it be possible to recover the power to a battery rather that chopping it across the resistor?

I am more familiar with Siemens products such as the s71200 plc. i know that Siemens have sinamics drives that have regen functions however these are expensive.

any idea of the AB power flex cost? Also i am looking at driving a small motor such as a 0.75 or 1 kw motor?

It won't recover power to a battery, but the most cost effective method would probably be to use a DC motor and a 4 quadrant drive. That will regen the power back to the line, so no overheat problems with resistors, and much more inexpensive than an AC drive.

There are AC drives that either use an active front end, or can directly regen power back to the line, but they are very costly and much more complicated than the DC solution.

Braking resistors should only be used for intermittent loads, not something that can continuously overhaul the motor/drive combo.

You have to be around 25+hp at 480VAC to make the cost of a regenerative drive pay off and then only if there is a lot of braking.

And, just to clarify the terms, regenerative braking recovers the braking energy and puts it back into the AC power network. Resistor braking often called snubber braking and erroneously called dynamic braking wastes the braking energy as heat in resistors. Dynamic braking is properly a DC braking term involving a resistor to waste the braking energy as heat.

There is also DC injection braking which puts DC thru two windings of an AC motor and also flux braking where the VFD uses some of the excess DC to saturate the motor. Flux braking will only give you about 10% of the motor hp in braking hp and the energy is wasted as heat in the motor. The motor will be spinning so the heat is easily dissipated.

DickDV said:

You have to be around 25+hp at 480VAC to make the cost of a regenerative drive pay off and then only if there is a lot of braking.

And, just to clarify the terms, regenerative braking recovers the braking energy and puts it back into the AC power network. Resistor braking often called snubber braking and erroneously called dynamic braking wastes the braking energy as heat in resistors. Dynamic braking is properly a DC braking term involving a resistor to waste the braking energy as heat.

There is also DC injection braking which puts DC thru two windings of an AC motor and also flux braking where the VFD uses some of the excess DC to saturate the motor. Flux braking will only give you about 10% of the motor hp in braking hp and the energy is wasted as heat in the motor. The motor will be spinning so the heat is easily dissipated.

Click to expand...

Dick
SO I have been wrongly using term dynamic braking for both AC and DC motor control.

To keep straight in my head

dynamic is most proper for DC motor where resistor is paralleled with armature when motor is generating under overhauled conditions.

Snubber braking is term for AC motor where resistor is paralleled to VFD DC bus to dissipate energy produced by motor when generating under overhauled condition. Is there another term that is also correct?

Dan Bentler

leitmotif said:

Dick


Snubber braking is term for AC motor where resistor is paralleled to VFD DC bus to dissipate energy produced by motor when generating under overhauled condition. Is there another term that is also correct?

Dan Bentler

Click to expand...

AB use "Dynamic Braking"for AC drives, can see it in all PowerFlex manuals, most of the VFD producers use that term for AC motors too.

mdim said:

AB use "Dynamic Braking"for AC drives, can see it in all PowerFlex manuals, most of the VFD producers use that term for AC motors too.

Click to expand...

Well if I am wrong then it appears I am in good company.

Maybe a good comparison is BLDC ie Brushless DC Motor = more properly termed Permanent Magnet Synchronous Motor (PMSM) since they are a 3 phase motor.

Dan Bentler

Dan, I'd say you've got it right. But, as I expected, you'll always find someone or some manufacturer that mixes up the terms. So, there are no hard and fast rules.

DC dynamic braking takes the energy produced by the internal generator in a DC motor and directly shunts it with resistors to waste the energy as heat. The voltage at which braking occurs is determined by the motor characteristics and the rotor speed. But the resistor is across the armature leads continuously while braking.

AC snubber braking is voltage sensitive at the VFD internal DC bus. At a typical 480VAC VFD DC bus voltage of 770VDC or so, a voltage sensing circuit called a "chopper" diverts energy to the resistors as long as the voltage remains at or above the snubber level of 770VDC. Below that bus voltage, the motor may continue to generate braking energy but however much appears at the DC bus is used for other auxiliary loads in the VFD, such as running the electronics and keypad. Flux braking also operates in that range under the chopper's snubber voltage.

Incidently, that snubber voltage can vary between different brands of drives. Drives that are designed for the world market and have an input voltage range up to 550VAC clearly will need a higher snubber voltage (typically about 840VDC). That's why when you buy a general purpose snubber like PowerOhm, you have to set the snubber voltage with DIP switches to match the requirements of the drive.

I agree, the term Snubber Braking more accurately describes the function of VFD power shedding to an external resistor.

In the old DC servo days, the term was 'Shunt Regulator', and described the transistor/resistor circuit more accurately, as utilized on the bus power supply.

The problem is that the manufacturers have standardized on 'Dynamic Braking' to describe the use of a shunt regulator circuit for the DC bus. While the end-effect is true, the function is misleading So, we continue to reinforce the 'DB resistor' as an option day-in and day-out, since that what they sell it as.

Back to the topic at hand, at this power level, line-regenerative AC drives are a very expensive option, but could be required by the system and environment circumstances. A properly sized resistor bank could be used, though, as this is not that much power. For winter operation, it would actually be more efficient for heating than pumping it back on the line and then using it in the HVAC system

Are there other loads on the machine where the energy can be used? If so, a common DC bus may be the ultimate answer... Share the power among several VFD's, so the recovered energy is put back into the machine!