Ac Drive Will Not Trip

I have a application with a powerflex 700s phase 2 drive running an oven chain.

Fla for the motor is 37 amps and we have a situation where the line will stall during running and sometimes startup.

Drive is powering the motor but nothing is moving i watched it pull full fla 37 amps on the drive him and the drive never tripped?

What could cause this? I have verified all motor data plate info in the drive.

Drive has mostly default settings from quickstart/autotune.

I would think at fla for a minute or so would trip?

You need to check the FLA of the drive, if it is much larger than the motor then it might well sit at full load motor amps if none of the protection functions have been set correctly.

The Plc Kid said:

I would think at fla for a minute or so would trip?

Click to expand...

No, you can run at the motor's full load amp rating forever. If anything, it should trip out on a speed regulator or speed feedback fault. That is, if there is any feedback.

A motor on a vector drive may be drawing anywhere from 25% to 100% of it's full load current rating at any time, even stopped. You need to look at the calculated torque being delivered by the motor. That 'filters out' the magnetizing current sent to the windings.

Also, it is possible that it is simply stalling, but limited to only 100% output. Perhaps you need to allow 125 or 150% overload output? That will trip on a time-derated curve if the motor is exceeding 100% rating.

Kid
Is your amperage reading correct?
Where did you get your amp reading -- from the drive display or
clamp on and was that on drive output or drive input? If on output you may get an error because that is PWM and meter is not setup for that.

I recall you are in a food plant - are you getting burnt product?
Is management running around all paniced?

Your motor and drive may be just fine. Are you sure you are not encountering a mechanical jam in the drive system - I envision torn up chains gearbox couplings etc etc. I would go thru mechanicals real close and look for loose set screws on couplings froze bearing etc etc.

Is this the same 30 HP motor that you posted where it has oil inside from leaky gearbox seal? If so I would check that real close - did they put the key in??

I think I would be concerned with the motor stalled and overheating and may want to set the overcurrent trip on the drive a little lower.

Dan Bentler

rdrast said:

No, you can run at the motor's full load amp rating forever. If anything, it should trip out on a speed regulator or speed feedback fault. That is, if there is any feedback.

A motor on a vector drive may be drawing anywhere from 25% to 100% of it's full load current rating at any time, even stopped. You need to look at the calculated torque being delivered by the motor. That 'filters out' the magnetizing current sent to the windings.

Also, it is possible that it is simply stalling, but limited to only 100% output. Perhaps you need to allow 125 or 150% overload output? That will trip on a time-derated curve if the motor is exceeding 100% rating.

Click to expand...

Is this true on a scalar drive also? I am having trouble understanding the magnetizing currents.

Can someone explain magnetizing currents? Is this the same as flux current parameters that are in the vector drives?

No. On a pure V/Hz mode drive, there are no magnetizing currents generated as there are in flux-vector modes.
In short (though not completely by any means), in vector mode, an AC Drive will keep current flowing continuously through the stator in order to keep the rotor magnetically fluxed up.

If you made an analogy to a DC machine, the AC magnetizing current would be equivalent to the shunt field of the DC motor. By keeping the rotor magnetically alive, it is possible to generate full torque all the way down to zero speed, and even hold zero speed.

It took a long time for AC drives to get to that point, mainly because they need quite a bit of raw computational horsepower to do the calculations fast enough.

That is also the reason it is generally required to do a "motor identification run" before an AC drive can be enabled in Vector mode.

I'm speculating here, but I imagine the "Vector" part of the name comes from the fact that by doing the computational heavy lifting, the vector drive's CPU can separate out the torque generating component from the speed control component as separate "Vectors", and deal with them individually.

The drive can be programmed to allow as much as 200% FLA through the motor or the drive output maximum, which ever is lower.

The motor can pull above FLA for a variable length of time which will depend on how far above the FLA it. The more amps, the sooner it will trip. The drive will "remember" the duration and level of these overload conditions so that successive attempts will result in a shorter trip time... (I^2*t)

With that fact in mind, and to get the machinery control we often need, We normally try to monitor the "At Speed" status (with a drive relay output or via a comm channel) with a PLC and program accordingly on drives which run for long periods at the same command reference.

Example: If the oven is not at speed for two seconds, stop the upstream equipment. If the oven is not at speed for ten seconds, stop the oven drive and set a fault bit for the HMI...

I believe even in V/Hz mode the "at speed" status is still a pretty reliable indicator of a stall condition. Note that the "At speed" status is not true during accel and decel. If the oven speed is varied frequently, it may not be the way to go for you because the At Speed status may go off briefly every time the speed reference is changed. In cases like that, I normally subtract the speed feedback from the speed command and call it speed_error. When the speed error exceeds my programmed limits for a certain period of time, the appropriate action can be taken. Doing that requires digital comms or an analog speed feedback signal from the drive.

This has worked well with V/Hz drive which can seem to detect a stall condition and will report something like 10-16Hz when commanded to run at 60Hz. I don't know how the drive "knows" the motor is stalled, and frankly don't need to know how it works, but it has not failed me with A/B drives.

Of course with vector (even sensorless) I would expect much much better accuracy from these status values.

Hope this helps...

Never got response on accuracy or validity of current reading
SO ASSUMING
valid reading and the motor is at 100% power and still stalled

I realize this is a conveyer chain and they can have high breakout torque.

I think I would check mechanical for worn out components binding etc
OR
I would double check and ensure I have the right size motor to drive the load ie motor torque higher than torque demanded by load.

Dan Bentler

Just in case......

Be sure that there are no other drive components in play. I had this situation as well a while back. Turned out that there was a variable sheave system located out of sight that had a bad belt, causing slippage.

rdrast said:

No. On a pure V/Hz mode drive, there are no magnetizing currents generated as there are in flux-vector modes.
In short (though not completely by any means), in vector mode, an AC Drive will keep current flowing continuously through the stator in order to keep the rotor magnetically fluxed up.

If you made an analogy to a DC machine, the AC magnetizing current would be equivalent to the shunt field of the DC motor. By keeping the rotor magnetically alive, it is possible to generate full torque all the way down to zero speed, and even hold zero speed.

It took a long time for AC drives to get to that point, mainly because they need quite a bit of raw computational horsepower to do the calculations fast enough.

That is also the reason it is generally required to do a "motor identification run" before an AC drive can be enabled in Vector mode.

I'm speculating here, but I imagine the "Vector" part of the name comes from the fact that by doing the computational heavy lifting, the vector drive's CPU can separate out the torque generating component from the speed control component as separate "Vectors", and deal with them individually.

Click to expand...

Bob

Just to cure my curiosity in what application would you use full torque at zero speed for any length of time? Or are you talking about starting loaded conveyors or something to that degree?

Dan

Current readings are from the drive HIM.

Why not set the drive current limit to whatever motor torque level you want to limit to. When the motor loads to the set point, it simply slows down or stalls. That will keep the motor from tearing up the belt and conveyor system.

I am not very familiar with the PF700 but it may be that the current limit can be programmed to operate as an "electronic shear pin". With that, when the current limit level is reached, the drive faults out and everything stops. This is an even better way of protecting the conveyor.

You can do the same kind of thing by using an output relay and setting it up to operate on torque level detection. Connect the relay N/C relay contact to an external fault loop. When the motor hits the torque limit, the relay picks up and faults the drive.

The Plc Kid said:

Bob

Just to cure my curiosity in what application would you use full torque at zero speed for any length of time? Or are you talking about starting loaded conveyors or something to that degree?

Click to expand...

Well, for one, we have very large accumulators on several lines. They basically sit in one spot for hours (or days) at a time, just holding tension on the cable.

For another, we have many lines that are payoff-process-rewind lines, and about half the time the payoff's and rewind's are sitting holding tension, but not actively moving.

Where else... Hrm.. DC Drive, but same concept, we have a couple drive loaded dancers. That is, the DC motor, with just a set armature current provides a torque on a band attached to the dancer wheel carriage.

There are lots of applications where holding position or torque at zero speed is an effective control strategy.

Almost the same as how you use the lockup / torque feature on a servo drive to hold a load in place.

Makes sense.

Thanks.