PowerFlex700

Hi All

One more question:
One 15 HP motor controlled by PF700, there is a external brake.
when the brake engaged, it will hold the shaft on, and the motor current will go up more than 50%, my question is if I change to a 30 hp motor, how much percent the current supporse to go up?

thanks

When the brake is engaged, the VFD should not be enabled.
If the VFD is controlling the brake, there are parameters than can be used to control the timing of the brake signal so that the load is always under control by either the brake or the VFD.

To add to OkiePC's post, look in the PF700 manual for a section on 'torque proving'. Those are the parameters that should help you.

Keith

Hi Angi,
It really doesn’t matter if you are using 15HP or 30HP motor. The PF700 VFD is intelligent enough to control the entire motor operation. I don’t understand why you need to know the current, torque is more important here . If your application is using hoisting or lifting you can program the drive in “FVC Vector” or “Torque Proving”. You just have to take note that Vector Control or “Torque Proving” can operate in one of two configurations:
1. Encoderless
Not to be confused with Sensorless Vector above, Encoderless Vector based on Allen Bradley’s patented Field Oriented Control technology means that a feedback device is not required. Torque control can be achieved across a significant speed range without feedback.
2. Closed Loop (with encoder)
Vector Control with encoder feedback utilizes Allen-Bradley’s Force Technology™. This industry leading technology allows the drive to control torque over the entire speed range, including zero speed. For those applications that require smooth torque regulation at very low speeds or full torque at zero speed, Closed Loop Vector Control is the answer.
To program the “Torque Proving” you need to set PAR 600 to 611. Here’s how Torque
Proving Application works:
1.) Providing a Start command. The drive does a Transistor Diagnostic test to check for phase to phase and phase to ground. If a fault occurs the brake will maintain a holding state. The drive will drop back to a stopped state with the appropriate fault displayed.
2a.) 1st time Start. If no fault is generated during the Transistor Diagnostic test then the drive produces - Output voltage and Flux current - as always the drive is checking the current through the current transducers on each phase U, V and W. If no issues/ faults with the output current – then the drive is ready and waiting for the brake to be released and produce the required Torque to hold the load. This happens the very 1st time a Start is commanded. A preload may need to be utilized for the 1st time Start.
2b.) Additional Start commands. If no fault is generated during the Transistor Diagnostic test then the drive produces-required Torque to hold the load–provided by the last Torque measurement – which the drive does automatically, Output voltage and Flux current - as always the drive is checking the current through the current transducers on each phase U, V and W. If no issues/ faults with the output current – then the drive is ready and waiting for the brake to be released.
3.) Once the brake is released the drive will control the load after the programmed Time has expired. – [P604 [Brk Release Time] typically is set to the actual time it takes for the mechanics of the brake to release.
When the drive receives a Stop command the following will occur:
1.) The brake will close when the speed of the motor reaches zero - After the time programmed in P607 [Brk Set Time].
2.) After the programmed time has expired in P607 [Brk Set Time] – The drive verifies if the brake can hold the load. The drive does this by ramping the Torque down at the rate set in – P608 [TorqLimSlewRate]
3.) The drive performs a Brake Slip Test while the torque is ramping down. If the brake does slip and it exceeds the value set in P609 [ BrkSlip Count]. Then an Alarm is set and the drive will start its built in Brake Slip Procedure. The drive will allow the Motor/Brake to move the distance set in P610 [Brk Alarm Travel]. If there is a mechanical brake failure the drive will automatically start and repeat over and over again the Built in Brake Slip Procedure until the following scenarios have occurred: a) the Load/brake stops slipping. B) The load is controlled by the drives brake slip procedure until the load is at rest. Typically to the end of the downward travel.
Sorry for this much long. Hope the info helps.
Don

Thanks guys!
Sorry for misunderstanding. the reason for me to monitor the current change is to make sure the brake works (the brake is not controlled by PF700) , when the brake engaged, the motor still try to run, so in a short time, the current will go up, if the brake did not work, the current should be same.
the current program just monitor the current up to 50%, it will give us an alarm if it less than 50%.

Basically, you are measuring locked rotor current and, if it rises to a set level, that is interpreted as acceptable braking torque.

First, while this technique may be adequate for your purposes, using the drive's torque signal would be a better measure since current can vary all over the place especially at stall for any number of reasons not related to torque.

Second, the locked rotor current is highly dependent on the voltage presented to the motor at stall. This voltage is highly dependent on several drive setup parameters and also by the selection of open loop scalar (V/Hz) or closed loop vector operation.

If the verification of brake torque is critical, I would rethink how you are doing it and, maybe, make some changes. If not critical, you may be fine right where you are.

Hi Dick

I need keep monitoring the motor current, my job is just do some modification, not allowed to change to monitor the torque.

Angi

What does the motor and brake unit drive? That is the major question. The LOAD is KING all else (motor gearbox brake etc) must be able to meet the demands of the load.
Sounds to me you are
1. starting the motor with the brake set
2. When motor current rises to some point
3. The brake is released
4. Motor moves the load without any "reversal" of load.

Is management making you do this because that is what they want REGARDLESS of good practice?
OR
is it that critical that load never reverses when motor starts.

If brake is between gearbox and motor then you have slop in the gearbox. All slack is taken up by the load and gears are "reverse loaded". When you start motor and release brake the motor will drive the gears, take out the slack and you could hear some banging in the gearbox. This may not be the case in your situation but for sure would occur when reversing.

If non revesal of load is that critical then you should have a worm drive gearbox - properly sized - worms CAN be driven backwards with enough load on output shaft. You should also consider a brake on the load - as example large hoisting winch drums have a brake right on them.

Dan Bentler

Your insistence on monitoring motor current makes me wonder if you are trying to measure the motor current directly as with current transformers on the motor leads. If you are doing that, you are hopelessly buried in error. CT's will not read the high frequency pulses that the motor is running on and will give you a reading that is totally false.

Hopefully, you are using one of the drive's 4-20ma analog outputs to read output current. At least, this is accurate for current.

If that's what you are using, then a simple parameter change in the drive will convert the 4-20ma signal from current to torque. Then you have all the improvements I mentioned above.

If someone is standing over you and insisting that you keep using CT's to directly measure motor current, I guess all you can do is suggest a better way. IF that is rejected, then you've done your part.

Good luck!

Angi

Reread all and picking up on a few points.

I would read the VFD manual and see if:
1. I could control the electro mechanical brake with an output signal from VFD
2. I could make the signal in #1 proportional to current ie at 20 or 30% FLA turn brake off.
3. If on loss of power or any VFD fault the brake is set automatically. I assume it is energize magnet to remove brake and spring to set.

As Dick said it is difficult to use VFD to motor current as control measurement. You can with some better result use an analog type meter than a digital. You can monitor current on line to VFD maybe but you may want to put scope on line and check for peak currents that may throw off controls / sensors.

FOOD FOR THOUGHT you bought a VFD with many control options. You bought good equipment and paid a good price. Why not make best use of the invested dollar and let the VFD do much of your work for you.

If this is all new to you and you (and your management ??) and are still learning but scared of new things then get a small 1 HP VFD and motor set them up in shop and try the features out. Even if you do blow something up it is only a couple hundred bucks. Education is a lot cheaper than ignorance.

Dan Bentler

When the break is applied, are you measuring current at 0rpm?

Thank you very much Dan and Dick, your informations are very usefull for me.
I just check the setting of PF700, it set the analog output#1 as Amp output from 0-5VDC, and the PLC just use this voltage convert to current, and monitor the current changing (50% difference) when brake is engaged.
So far I donot want to change the way the PF works, if replace the motor by a 30 HP one, could I still keep the 50% setting?

If you change the motor, you will have to enter all new motor data, current limits, overload fault points, etc. to the drive. If this is done correctly, then the analog output will span from 0 to 100% of the new motor's current rating.