Holding Torque with a VFD and AC Induction Motor 4

[450x]

Can I produce holding torque with an AC induction motor with a Powerflex700 VFD

 

[ozmosis]

I think you need to explain a little more on what you are tring to do, what you are trying to hold and what torque is needed.

 

[jraef]

Not only that, but I would think that would be a question to ask of A-B. In general a Closed Loop Flux Vector drive on an Inverter Rated AC induction motor can produce up to maximum locked rotor torque of the motor at zero speed. There are a lot of "ifs" in that statement however, both for the VFD and the motor, and only the manufacturer of the drive (or someone deeply versed in it if you're lucky), can really answer that definitively.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[450x]

I am not sure of the required holding torque at this time since it is early on in the design. I was mainly curiuos about using VFD's and 3ph induction motors in a point to point motion application. I have read alot of correspondance regarding the similarities/differences of servos systems and vfd's, none of which touched on maintaining position with holding torque. I guess I can always use a brake, but they are prone to mehanical wear with extensive use.

Thanks gents

 

[waross]

By holdingtorque, do you mean resistance to an overhauling load, resistance to turning, or holding the shaft motionless as with a mechanical brake?
yours

 

[ozmosis]

If you are looking at the difference between a VFD and a servo for point-point motion control then it comes down to your required accuracy and repeatability. Of course a servo control is going to be better but you pay more for it. Look at your application requirements first before your try and fit a product into it otherwise the 'perceived' savings will not happen. The investment in the system right for the job will always pay dividends.

 

[450x]

I ment holding the shaft motionless, acting as a mechanical brake.

Thanks

 

[itsmoked]

You are still way short on useful info 450x.. Notably waross's questions.

You are possibly describing a vector drive as compared to a scalar drive the "standard" VFD. They can hold loads all day long. With a bunch of conditions which we cannot guess about with your meager info.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

Sensorless vector systems are not good at developing torque at zero speed for more than a second or two. If you add an encoder to the motor shaft and shift to Flux Vector control, nameplate rated torque at zero speed is possible for extended periods of time. That's assuming you have provided cooling for the motor under these difficult conditions.

Generally, AC drives are not good at positioning in the sense that they have no way of keeping track of where they are or where you are telling the load to move. Servos do that routinely. Having said that, it often possible to get acceptable servo-like action with a precision AC drive following a positioning controller in a PLC or stand-alone. Some would call that a single axis controller.

Hope that helps

 

[jraef]

An important issue to also consider is that even if you did design a system with a Closed Loop Flux Vector drive and a properly cooled motor to hold a load motionless, it's never a good idea from a safety standpoint to rely upon that if the nature of your application involves risk of injury or damage should the power fail. The "perfect" combination is to use the VFD/motor for normal operational braking, and then have a mechanical safety brake anyway in case of a power failure. Crane hoist designs require that of course, but an often overlooked aspect is to consider the mass in movement at the exact moment of a power failure. Is it going to coast and crash into something or someone? Is gravity involved? Is inertia of a dangerously spinning load such as a blade involved?

This is why we have been asking you to give more details. As I said early on, lots of "ifs".

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Hackerjack7]

The Yaskawa G7 drive with an encoder has a feature called "zero servo". I have had a chance to play with this drive a bit, not the zero sevor, but Yaskawa says it can hold the motor like it had a machanical brake. The G7 after i have played with it shows that it has some good control in the slow speed area(open vector 2)

 

[jraef]

...but Yaskawa says it can hold the motor like it had a machanical brake.

But not without power it can't!

 

[Skogsgurra]

That's a very good and important point!

If you tell a machine designer that this drive can hold the load like a mechanical brake - and he takes your words for it - you are both in for a very unpleasant surprise the day (or night) you get a power outage.

Gunnar Englund

http://www.gke.org

 

[itsmoked]

Yeah one of those..
"I wish I'd
t a k e n
T H E
S T A I R S !!!!!!!!!!!!!!!!!!!!!!!"

Nites.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Hackerjack7]

Yes I understand that. It will depend on application. He asked I had answer. That is for him to take a look at not me. Im just giving options.

 

[digitrex]

I had done that successfully with ABB ACS600 series inverter.

I had a motor driving a cryogenic(liquid nitrogen) pump at 4600rpm and we need in inverter to run the motor at 77Hz (This had saved a mechanical gearbox).

The pump is required to operate at -150deg.C and it is only required to be started intermittently (once in a few days). Before the pump can be started, we need to cool down the pump from atmospheric temperature to -120deg.C. Before the pump is cooled down to -120deg.C, we don't want the pump rotor to rotate or else the pump seals will be damaged. Therefore we applied brake on the motor through DC current from the inverter. This braking function is available in the inverter control menu.

 

[digitrex]

Our VFD driven motor has a separate motor driven cooling fan. Therefore, when the motor is under DC current braking condition, it is still being cooled by the fan.

However, the DC holding torque (current) level is adjustable as you wish. Base on our experience, the DC holding current level we set is not high and I hardly feel very much warmth on the motor body during DC hold, I don't think the winding insulation will fail in the event of cooling fan breaking down.

I don't have a Powerflex 700 drive but I believe electrical braking should be a standard feature in that drive.

If you download Powerflex 700 User Manual from Allen Bradley website:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/20b-um002_-en-p.pdf

I am sure you can find more details about Stop/Brake Stop/Brake mode.

 

[DickDV]

From the last couple of posts, it seems that the subject has shifted to DC Injection Braking. That type of braking will tend to hold an induction motor at zero speed but not with very much torque since it takes rotor slip to magnetize the rotor. So, for DC injection braking, you will get a little drift speed as holding torque rises.

It is also true, as the poster above mentions, that the ABB ACS600 in DTC mode, can hold a load at zero speed. But you must be careful since this is only true for a couple of seconds. After that, the drive looses control of the motor torque and will release the load. Addition of an encoder will fix that but, as has been stated many times already, this is not a suitable brake technique for safety purposes. For that, trust only a spring-set mechanical brake.

 

[jraef]

Thanks DickDV.
That's the first honest explanation of the holding torque issue with DTC drives I have heard from either ABB or competitors, and now I understand. A number of ABB marketing people leave out the "couple of seconds" issue, and customers have experienced some disastrous failures as a result. On the flip side, a lot of their competitors only mention the part about it losing the model, allowing people to believe that it can't do it at all. Your explanation puts the two incongruous stories together and makes much more sense.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

A very easy "demonstration" to do at trade shows is to grip the shaft (there used to be a hand-wheel) of a machine that is being kept still by a DTC system. Make a mark on shaft and end-bell. Then hold the torque for a while and you will see how the shaft creeps away from the mark.

I did that when the DTC was introduced many years ago. The "stand soldiers" immediatley switched the drive off. They told me that it was too risky - the motor could start spinning when someone was holding on to the wheel and someone could get hurt.

I refrained from asking them if they didn't trust their system - running away just like that doesn't sound very good. I think that it would have been too tough on them.

Gunnar Englund

http://www.gke.org