Converting from DC to AC drives

From my understanding it seems the industry is moving away from DC to AC motors. The thought of no brushes/maintenance wise is appealing and the cost is very lucrative now.

I started a little project using some Schneider Altivar 32 drives. My big issue is how to handle the regen and braking. With DC this is a none issue but with the AC drives I see some real draw backs now. These drives are capable of sharing the DC bus which will help dissipate some of it. Is it normal practice to share the DC bus to help with braking or should I just get a braking resistor for each drive?

If for example each drive can disapate 30% (without a braking resistor) of the load would they in fact dissipate 60% as long as each drive is not braking at the same time?

I also have to size another breaking resistor for a 15 HP drive with a cycle time of about a minute and 10 second braking time to 0 speed.

Anyone start doing a swap over from DC to AC?

DC buses of VFD are tied together to let the energy of a braking motor be transferred to the bus of a driving motor. IF both motors are braking I think you would still need a braking resistor.

Be a bit careful of replacing DC with AC on a HP to HP basis. One reason is load startup ie breakout torque.
KNOW THY LOAD WELL
- use a torque wrench to measure breakout torque.

Evaluate load to ensure you have chosen the correct motor to drive it.
THEN choose the VFD
THEN choose the braking rssistor recommended by VFD maker.

15 HP with one minute on and a 10 second braking time to stop
COULD be done by just turning motor off and coasting.
ASSUMING a "high torque" load.
Low torque (low friction may be more appropriate term) load like a fan would more than likely need a braking resistor.
Overhauling load ie crane hoist will almost always need braking resistor

Get some manufacturer reps (engineering types not sales) in there and have them help you with evaluation and selection process.

Dan Bentler

Use a Regen rectifier feeding a common DC bus and eliminate braking resistors.

I have worked with one system which utilized a common dc bus. This system had it's own regenerative power supply connected to the DC bus of all the drives. In that very large web handling material system, by physical design, half the drives would be in torque mode (regenerating and keeping tension on the web) while the other half of the drives were pulling or feeding material and consuming power. The common dc bus allowed us to decelerate a 4000lb roll of stock from 30rpm to 0 in about 3 seconds without any complaints.

So, if the system is similar in that one drive is always in regen while another is running, then a common DC bus might be applicable. If all the drives need to brake at the same time, of course the regenerative supply will have to be sized to handle that worst case scenario.

I agree with Dan's comments about getting expert help. The low end torque issue when switching from DC to AC is less of a problem nowadays than it was a few years ago, but it is definitely something to investigate closely and carefully.

There are probably very few applications left that still require DC and aren't suitable for modern AC drives, but each application needs to be analyzed carefully. Hopefully DickDV can add to this thread with his excellent experiences in this field.

L D[AR2 said:

Use a Regen rectifier feeding a common DC bus and eliminate braking resistors.

Click to expand...

Depending on the drive setup you may still need a resistor.


If you have more regen power coming back on the Bus from deceleration or torque / holdback loads than the other drives can consume it has to go somewhere or else the bus voltage rises and drives start tripping on OV.

In most cases it is good practice to use a chopper module connected to large central braking resistor.

If you are running a active front end then this is not needed.

When changing from DC to AC technology, the same basic rule applies and that is to size the drive and motor based on torque, rather than hp. And, that is two different torques: continuous torque and short term torque (less than one minute for accel and sudden load changes).

It is often thought that somehow DC motor/drive systems automatically develop large slow and stall torques but, if the DC drive is sized for only 10% or 50% short term overload current, that's all the short term overload torque you're going to get too regardless of how much torque the motor COULD develop with an infinite current source.

My approach is to mostly ignore the old drive system and use load data only. Design the new system on what the load needs, not what the old drive/motor could have developed.

Of course, there are always applications out there that seem to have no load data of any sort and, for those nasty rascals, the old DC drive can be a handy source of data. Keeping in mind that amps and torque are in direct relationship to each other in DC motors, you can monitor the fluctuation of amps over the whole range of short term and continuous operation and get the torque data you need from the old system. It's necessary that the old system still be operational, of course!

As to braking, it surely is true that it is easier and more cheaply done in DC but not all DC applications require braking. Again, it comes down to understanding your load, this time from a braking rather than from a motoring perspective.

The question of resistor braking, regen braking, common busing, or DC injection braking are application specific and often are chosen based on the VALUE of the recovered energy. It takes large amounts of recovered energy to justify a regen system. (Actually, this also comes down to understanding your load)

I do DC to AC conversions alot and almost never have to increase the hp. Using AC overspeed wisely usually takes care of the low speed and starting torque issues for free and improves the motor cooling too. Incidently, just because the DC motor has an auxiliary blower does NOT mean that the AC motor needs one. It could be, for example, that the DC blower was needed to keep the DC motor field cool while the motor was stationary.

Bottom line, no generalizations. These conversions require study and economic analysis before proceeding. None of us can do this on a BBS for you. If you aren't sure how to proceed, hire a consultant that does.