PF70 drive

Hi, A specific question on the Powerflex 70 drive.
Parameter 148 is the "current limit value"
If this value is set to say 10 amps then the drive will limit to 10 amps when lifting a load.
My question is what happens when the load is being lowered and the load is actually driving through the gear box accelerating the motor. Does the drive limit itself to 10 amps to try and keep the motor at the required speed or does the drive output the maximum current that it can supply (ie 20 amps) to regulate the speed.
Regards Alan Case

Hi Alan.

I believe that it limits itself to the current value set with that parameter.
And I think it does so by dynamically adjusting the speed (actually limiting the decelleration).
Notice, that when the drive is decellerating the load, then the energy flows backwards towards the PF70. It will then result in DC bus voltage faults if you do not have a brake resistor installed.

Is the application some kind of crane or lift ?
I am not sure if you possibly have a problem when the load (on the drive) is not reduced by limiting the decelleration. I guess that lowering a suspended weight means that you have regenerating energy even at a constant speed. In that case the drive cannot reduce the current by regulating the speed. Maybe you need extra big brake resistors.

I think that maybe the elevator experts or DickDV can help you more.

That is a good question.

I suspect it limits it to 10 amps in either direction. 10 amps period.

I am curious to hear from someone that really knows though.

RSL

Alan, I suspect that the 10amp current limit is bi-directional but you need to get a definite statement on that from AB as I am not experienced enough on the PF70 to be sure.

If the current limit is bi-directional, you've got a problem with lowering an overweight object since I would expect the overcurrent detector to alter the speed reference to increase speed. That would be unacceptable on a hoist application.

Somewhere in the software should be the option to change the overcurrent limit action from altering the speed reference to "electronic shear pin" action. That would be preferred since an overload would immediately engage the safety brake by faulting the drive. ( I trust that there is a mechanical safety brake!)

The application is a hoist which lifts a bin full of product to tip into a hopper. The bin is driven by a 4 kw motor through a helical gearbox onto a chain drive. There is amechanical brake fitted which holds the load while the vFD is not driving. I have set the current limit to 8.7 amps on the drive (motor FLC) to limit any damage caused if the limit switches for end of travel fail.
I have no trouble lifting the load but if the operator makes a mistake and decides to lower a full bin without emptying the product then the load accelerates and overdrives the motor and the whole lot comes crashing down. I am not sure of the process that is happenning.
I assume

a) that the drive will output current to the motor to try and keep the motor at the required speed on the down section.

b)If the motor is being overdriven by the load then current/voltage is being delivered back to the drive which is being dissipated in the brake resistor (external)

The options as I understand them is
a) A higher ration gear box that will not allow the the load to drive back through the gear box. This is my preferred option as it means we do not rely on a mechanical brake to hold a load of a tonne at a height of 2 meters. (the area is guarded to cat 4 and access is usually only allowed when the bin is on the ground. Mainenance can access the area under signed permission if the bin is stuck up high) Problem is we did not design the motor gear box combo and the designers SEW say everything is OK on their side.
b) A massively oversized motor and VFD capable of holding a decelerating load. (Cost prohobitive)
c) A torque proving system ie a PF700 drive instead of the PF70 and an encoder on the bin chain. The drive will only release the brake fully when it proves the load is proven to be held by the motor current and if runaway happens (the encoder sees the speed of descent faster then the VFD commands) then the mechanical braske is applied. Unfortunately I think that with this option the motor and drive will also need to be massively upsized otherwise the descent will be a continual actuation and release of the mechanical brake.

It all comes back to what I think is a miscalcualtion by the mechanical designers, ie we need a better ratio in the gear box (we can double the ration and still keep the speeds required) and/or a different gear box drive. I am told a worm drive will be better suited than a helical drive.
Any comments.
Regards Alan Case

Alan, bear with me please thru a couple of thoughts on your application.

First, it seems to me that it should take equal or greater current to raise the load than to lower it. You state that you are not at current limit raising the load so it would seem that you are not at current limit to lower it.

I conclude that current limit issues are NOT involved here. Rather, I would look at your snubber braking system. You need the lowest ohmic value resistor for the size drive you are using. The brake chopper needs to be rated for the maximum current this resistor can draw at the highest DC bus voltage the snubber circuit will permit. Finally, and this may well be the problem here, you must change a parameter somewhere in the drive to make it stop altering the speed reference on high DC bus voltage. When this feature is turned off, the DC bus is free to rise to snubber levels without any change in speed reference. I wish I could tell you specifically which parameter that is but I don't have PF40 documentation to search thru.

Second, if the brake system is all working properly per the above, then you may have a second option rather than changing the gear ratio. How about changing the motor to one with the same kw but with twice as many poles and half the base speed. You effectively double the available braking torque at slower speeds. The disadvantage, of course, is that the motor is going to be physically larger and may not fit. Just a thought to help get you out of a jam.

Third, if none of this can work for you, how about monitoring the DC bus voltage on the drive display and report back on where the voltage goes when the load starts to fall. It would be useful to know at what voltage the snubber brake package is designed to operate, as well.

Looking forward to your response.

Hi Dick.
Logically you would think it is harder to raise a load then to lower the load but in practice I have always found the opposite to be true. Maybe because the load starts from rest and as the torque increases then the load can start to raise. If the load is held by a brake then if you release the brake then the load starts to drop before full torque is available to hold the losd at the required descent rate.
The PF70 uses an internal brake chopper and I have an external brake resistor fitted.
I found the parameter to limit the change of frequency. I had 5 choices
Disabled
Adjust Freq
Dynamic braking
Both - DB 1st
Both - Freq 1st

I had selected both with Dynamic Braking 1st. I will alter this to Dynamic braking only.
I also found another parameter 153 which is the maximum power limit allowed to transfer from the motor to the DC bus. Default is -50% but with an external resistor the recommendation is -800%.
I am still a bit dubious about the total system as I would rather see a gear box that can hold the load without relying on a mechanical brake. Also the way the system is now configured I have to torque up the motor before releasing the mechanical brake otherwise gravity takes over and everything heads towards TerraFirma.
Regards Alan Case

I think you are correct in setting the parameter to Dynamic Braking. It would be better to fault the drive than alter the speed reference.

On some drives, the output relays can be configured to operate at a determined level such as torque. This is especially handy with mechanical brakes since you can leave the brake engaged until the motor magnetizes and develops enough torque to hold the load. I also am familiar with some drives that have a pickup or release time delay on the relays. This also can be useful with holding brakes thru piloting the brake release with a drive run relay with enough pickup delay to eliminate the short drop when the brake releases. I don't know if the AB drive has these features or not.

It is important, as I mentioned before, that you have the absolute lowest value resistor specified for that drive. That will provide the maximum braking capacity. You should be able to develop the same torque motoring as braking. That parameter 153 setting of -50% sounds like trouble too. Now, it may be that AB's internal brake chopper is not rated for full capacity. In that case, I would put a full capacity brake chopper external to the drive with a proper resistor to handle the full capacity of the drive. Of course, the DC+ and DC- connections must be available externally to do this. I would change the parameter to Disable in that case.

Please let us know what develops with these changes. I'm interested in seeing this problem solved.

Hi Alan,
You need a braking resistor connected to the chopper. The value will be prescribed by the AB manual. However the wattage of the resistance will be calculated thus..

W = (V X V)/R where V = 760 Volts for most AB drives.

Also torque proving for a lifting object will only prevent the object from falling during the initial start. It is needed, but will not solve your problem of over speeding in the lowering direction.

You can also reduce the lowering speed and solve this problem.

Hope this helps.
Ron