50HP DC motor to AC !!!

ArikBY, sorry about the lecture. I didn't mean it for anything other than useful information among us techies.

I can only speculate about the manufacturers' reasons for the oversize and I would guess that there is a comfort level that comes from overdoing a design especially when the technology is new. These manufacturers have their reputation to protect just like we do and most are being very conservative. I predict that, as they get used to the technology, they will begin reducing their systems back down to the real load demands. Also, engineers seem to be very reluctant to embrace AC motor overspeed operation. The manufacturers publish overspeed info in their motor manuals but that doesn't seem to convince many of them.

On the other hand, I trust the numbers and am willing to take moderately more risk. This is possible for me because I have been associated with ABB for over ten years and have become very comfortable with their published specs. I have pushed their drives to the end of reasonable limits and have not been dissappointed especially with their DTC drives, ACS600 and now ACS800.

Using DTC, you can expect full motor torque at zero speed in speed regulation mode. I have done it and have seen it demonstrated with hanging loads on a hoist. Torque control mode does not require an encoder and, again, full rated torque is available at zero speed.

For those extruders, A DTC controlled drive can resolve out about 90% of motor slip from no load to full load. Extruder loads typically don't change much so speed regulation is rarely an issue.

As for rotor/load intertia matching, I am not a servo tech and wouldn't claim to be. However, as I understand it, interia matching is primarily a tool to improve dynamic response as in step-change torque inputs, etc. AC drives and most commodity grade DC drives are not intended for such applications and, I assume, for that reason, I don't run into those kinds of issues.

Dick

I did not mean "lecture" in the cynical way I really appreciate your opinion about motion control.
I did not mean to offend.
So if I understend you correctly I can take one of this VFDs ACS600/800 for hoist and hold the load up when the controler in run and reference is 0V with regular AC motor?

I will get one of this VFDs at soon and I will try it.

Thanks for the information.

ArikBY, my description of DTC performance is accurate and you will find it so, but, I don't mean to say that a hoist system should be designed this way.

First, a hoist should always have a mechanical brake for holding purposes even if the drive could hold the load as I claimed. Power failures, blown fuses etc would be too great a hazard if the result were a dropped load.

Second, DTC depends upon an accurate motor model which the drive builds into its memory during a motor ID sequence that is run at commissioning. The hoist motor could conceivably heat up while holding the load to the point where the motor model becomes invalid. At that point, the load could also be dropped.

So, for demonstration and testing purposes, yes, the drive will hold the load for short periods of time (until the motor overheats). But don't be tempted to rely on that solely. Redundancy in hoist braking is essential for safe operation. Typically, the drive stops and holds the load while the brake is applied. It also builds torque before releasing the brake when motion begins again for a smooth restart.

That same torque at zero speed can be very useful for machine applications other than hoists and elevators. Conveyors with rollback tendencies, machines with eliptycal loading characteristics, and ordinary hard start applications all benefit nicely.

And now I'm starting to sound like a salesman and may be in trouble with the moderator. I hope not.

Dick

You did not mention the name of the company you work for even once in this post.So dont worry.
I have BIG BALLS to try new things but to rely on VFDS stall option
only ,not in my school .
When it come to hoist my first choice is conical motor then the other brake motors.
What more interesting for me that the option to hold the motor shaft
in convyors winding machine ect...
I going to try it at soon,maybe then I will bother you again.

Thank you very much for the useful info.

We're using 6 pole motors at 117 Hz on ACS800's (and competitors') drives on hoist applications without major problems.

We have external cooling blowers to take away excess motor heat. We have emergency/parking brakes, but the operators just call for zero speed during shift changes. So far, we've had about 3 years of this without any real problems.

The ABB drives haven't yet been out but about a year. We now use the ACS800's for this purpose, but we started with different drives and manufacturers.

The hoist is rated for about 500 tons, with sheaves/pulleys giving the mechanical advantage. We often run up to 150% rated torque, even at zero speed.

As an aside: The motor runs at synchronous/excitation speed minus slip. The slip gives it torque, therefore, the drive is running the motor at slip speed (excitation frequency) in order to get zero actual speed. It is NOT running at zero volts at zero speed; more like 0.6 Hz, or similar, depending on the motor and load.

Don, that bit at the end about the motor not being at 0 volts at zero speed is a very important point. I guess it ought to be obvious that the motor cannot make torque without some incoming power.

That is exactly why I try not to set up industrial machines to operate at zero speed on the drive. A motor at zero speed looks like a motor that is off but it most definitely is not off. 650 volt pulses are as much a hazard at .6Hz as they are at 30+ hz. Further, even tho a motor running at zero speed looks stable, a simple noise pulse on the speed reference can cause the motor to move suddenly and at full torque--very hazardous.

Of course, there are some applications like we mentioned earlier that need torque at zero speed. For those, you have to hope that the proper safety precautions are in place and enforced.

We use our PLC's to massage the drive command speed. We also set up a slight offset to the speed command so that a small error from the analog/user system isn't seen as a speed signal to the drive.

I guess it all comes down to the engineering. You anticipate as many problems as you can, and prevent them... then find more fixes for the ones you didn't anticipate and have to fix, now, anyway.

My comment about the excitation at zero speed was in response to ArikBY's mention of 0V, even though I think he was thinking about that as a speed reference. I know when I first started with AC motors, I couldn't understand how you excited a motor at zero volts to get zero RPM. Isn't zero volts a DC voltage, anyway? At least, that's how I used to think.

Position

Pierre,

Another thing to consider with most offset press's is you will have to
feed back a position or angle of the press.

Some use the encoder/tach from the motor and some use a stand alone encoder mounted to the first unit.

Oh yeah, this is needed because when you hang/change plates, the clamps use 0 to 360 angle or degrees for position, lead clamps open at 220, tail clamps open at 85 etc...

Re: Position

Mike Williams said:

...this is needed because when you hang/change plates, the clamps use 0 to 360 angle or degrees for position, lead clamps open at 220, tail clamps open at 85 etc...

Click to expand...

Will have to verify this 'cause I was under the impression that the mechanica-shaft linking every offset to the main system was "mechanicaly" lniking them together hence no need to measur where we install the plates but in there usual position.

Pierre
Mike is probably talking about systems with the feature automatically indexes the units to the position to remove/hang plates. Not all presses have this but all newer ones that I've seen do. Also remeber that the feed board eyes(registration) are controlled off of the encoder, as are variable ink ductor systems on some presses. But then agian, none of this may apply to you. Just something to remember. I know that Komori has the encoder under the first unit as do Meihle that I have seen no idea about KBA, Miller or Planetta.

Pierre, I just spoke with a tech guy from the manufacturer of our printing presses. Our conversation was about the main motor brake air gaps. This is our only press that has an ac motor, the rest has dc. I asked why this particular motor has an ac and the rest dc. His response was that they are now going to all ac motors due to alot of motor brush problems and easier to maintain an ac motor. I also asked him if the HP rating changed and he stated that as far as he knew, no. Just thought you may be interested.

dale

With all this talk off presses, I am now a little curios.

Has anyone worked with Sanden Presses, and if you have what is your opinion of them?