Powerflex drive hoist application

Has anybody on the forum used powerflex 700 drives in hoist application? I visited a customer recently who had a 75 kw motor with mechanical brake, with an accel and decel time in the drive of 0.5 seconds. The ramp was set so low they said so as to catch a suspended load when starting and stopping. The brake was driven from the drive output relay which was set to parameter "at frequency". The result was a large current (drive limit of 250 amps)when starting/stopping and when changing to low speed.I was wondering how other members have achieved control of mechanical brakes with powerflex drives. In the past I have controlled the brake via the plc. I've set the drive reference to at or near zero, enabled the run signal released the brake then set the reference to the run value This has worked in the past albeit with d.c. drives. Any thoughts?

Pauly.

We use powerflex drives on hoist applications where there are no safety issues (auto palletisers). Our methodology is almost exactly the same as yours and we do not have any problems. The only thing I would add is that we fit considerably oversized external braking resistors as the AB parts are only rated for light duty use. Resistors are not expensive so we do not run close to the wind with them.
Andybr

Do you see the same large current varaiations on start and change of speed? I should add that the drive had been set up in sensorless vector mode.

I don't think the "At Frequency" output would affect the starting current that much with a .5 Accel. Normally when you hit the up button, the brake releases and the hoist does slightly drop till the load brake catches before the motor begins raising. Now I don't know how old the hoist is but if it has a busted load brake it would alleviate the hoist dropping several inches before beginning to raise which kind of sounds like there reasoning for the "At Frequency" output.

We do this quite frequently and like Andybr has mentioned, you definitley want to install external braking resistors to the drive or you will constantly "trip out" the drive due to the regenitive power when going down with the load.
We generally dont see very large variations on start and change of speed unless you are not energizing the brake contactor for too long a time while the motor is being energized (0.5 or less depending on load and ramp time).

Regards,
Chris

Pauly.

I have just had a look at one of our systems and the initial current is higher than normal but not excessively so.
Andybr

This is a trace of the motor current I took. The first big variation is a speed change from 50-15 hz. The second is a stop. Am I right in thinking the fast accel/decel is causing this?

Where's the graph?

Current trend for Powerflex

Sorry about that the previous file was too large!

To tell you the truth it looks good to me. Split second peak but not too high then looks like it purrs like a kitten.

Powerflex cntd!

I thought so too. The original reason I went there was that that part of the hoist movement has caught in the past and buckled some of the supporting steel work instead of tripping the drive so I was to look at the drive parameters to have the drive trip instead. I looked at the shear pin function beforehand and went to implement that. The only problem was the customer saw these traces and thought maybe he would be getting trips all the time when starting and stopping. I didn't get the chance to try any changes because production wouldn't release the machine.

Pauly, it sounds like you work alot with hoists so I may be preaching to the choir here but, at least in the US, you must have a certificate for hoist work to be legally working on them. And, in view of the usual hazards, it's certainly a good idea.

I do not have one of those certificates but there are several excellent hoist and elevator people on this BBS.

Having said that, when I do hoist work (with a certified tech by the side), I have found that smooth transition into and out from the mechanical brake is easily the most challenging part. Even a slight drop in the load is most unnerving to a crane operator and, to eliminate it, is most desireable.

I have found that using one of the drive relays set up to operate at an adjust torque level works the smoothest. On the drives I normally work with, I can set up level detection and pickup/release time delays which will almost always produce a nice smooth transition.

Regarding those current peaks, I would expect that .5 seconds accel on a hoist drive is way too short. There may be some production-dictated reason why it must be but, generally, that's too short. Consider also using S curve accel and decel. That will help to reduce the surges in torque and current.

The comments about braking above are right on. I doesn't hurt to have a drive sized one size larger as well if only to avoid nuisance faulting when something unusual happens.

Finally, for the benefit of those that don't do much hoist work, it absolutely must be that, when the drive faults, the mechanical safety brake is instantly engaged. I prefer to see the drive fault relay wired directly to the brake activation circuit---not sent thru any other control equipment. You also have to analyse carefully just what happens and in what order when there is a power failure or interruption.

Probably, this is all stuff you already know but, for some others, it hopefully might be useful.

Hi dickdv thanks for the replay we don't nesessarily work with hoists this is just one application of the drives/controls business we are in. Plus this is a job some one else has put in that we were asked to sort out.

Hi Pauly,

You have to use a `torque proving method' to operate the brake. This can be done by setting one relay to operate above 70 - 80% motor torque, interlocked with another relay which would operate at 4 - 8% motor speed.

Ron