[Question] 3 Phase AC Variable Speed Drives

[ACEsterhuizen]

I need some advice please.

I have 5 of these small .37kw 3 ph ac motors driving one continues metal grid conveyer via a gearbox, installed at various places along the conveyer each feeding from their own contactor, also installed at different locations.

These little motors all work in tandem to drive the belt.

I need to control these 5 motor's speed very accurately and from one location (at the end of the conveyor). The speeds of the motors must be exactly the same or it will damage either the conveyor or the motor or the gears. Can it be done and how. Any help or advice would be appreciated. Thanks.

ps. I phoned Big Dutchman (their installation and plant) but not much help coming from there.

I am looking at these currently.

[AndyD]

You can use a single VFD in V/f control mode and with a pre-selected curve to run all 3 motors, there's also the option of using an encoder if very accurate speed is required. If you need to limit torque to prevent conveyer damage in the event of overload then vector control would be the best option.

It's highly application critical and there's several ways to skin the cat so without getting too deep into it I'd suggest you speak with the technical department at one of the bigger drive companies local to you. They should be easily able to assist you with the design and a final spec.

[ACEsterhuizen]

Thanks Andy I have contacted various VFD suppliers including Mitsubishi. Your suggestion of one VFD seem to be the simplest and most cost effective one, but would distances be a problem? Some motors more than 100m away from the controller?

[AndyD]

Yeah, 100 meters may be an issue, how old are the motors and what insulation class are they? Again I'd consult with the drive suppliers or agents.

I remember in the early days of VFD's even 20 meters used to be an issue but nowadays the cable lengths can be much longer although I've never seen 100m. The problem comes with the IGBT rise times which are very fast and tend to set up a standing wave with long cabling, this tends to cause premature motor winding failure effectively due to simple overvoltage. I usually draw the line at 20 meters for a standard VFD but I know there are units designed with different PWM switching transistors and they also play around with the carrier frequencies as well to allow for longer transmission distances. There are also impedance reactors you can fit which lengthen the rise time and prevent motor damage. Running shielded and fine stranded cabling can also help sometimes.

In Cape Town I've always found companies like Varispeed, Weg and Electromechanica very helpful when it comes to assistance with design. Not sure about Jhb area though.

[Justloadit]

I am sure you can have individual drives, provided there is a master digital control which synchronized to each drive, and provided all motors are the same including the gearbox. If the control says that the frequency is 25Hz, then all drives will drive at the same RPM simultaneously, however all fault trips must be in series to all act together when a fault does occur.

I would speak ABB as well as a drive manufacturer to get their advice.
Synchronized drives is used in many applications.

[AndyD]

I'm interest to find out if you managed to find suitable drives for the 100 meter distance or did you go for individual synchronised drives at each motor?

[ACEsterhuizen]

The only 3 who actually responded to my 11 enquiries. Gator the only one with a solution (sort of) but no quotation to date.

Mitsubishi recommended a single drive and dismissed the distance issue instantly, but that was it. No quote, no further info. See ABB an Gator responses.


*** The customer called in the designers of the system and they will redesign and recommission the plant to the customers specs and the redesign vfds of choice is Kmise (KCLY). Total estimate +- R275K.


Aug 19 (13 days ago)
Please note that we are busy with compiling the solution and quote for you and one of the Sales Engineers will send you a mail during the day.
Regards
Kim van der Merwe

Product Manager: VSD and LVS
Adroit Technologies
Mitsubishi Electric
Factory Automation

Tel: +27 (0)11 658 8100
Fax: +27 (0)11 658 8101
Mobile: +27 (0)82 802 3474
Website: www.adroit.co.za



Good day
Thank you for your enquiry, We have forwarded your enquiry to Dany Kabeya
he will respond in due course.
Regards


AUGUST 18 (ABB)

|Gwen Mkhonza
|CCM Team Leader - ABB South Africa
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On Wed, Aug 17, 2016 at 2:55 PM, Alister Chalmers - Gator <AlisterC@gator.co.za> wrote:

I refer to your enquiry into multiple drive synchronization and provide some feedback in this regard.

Option 1 – Single drive with multiple motors
The simplest solution is to select a single drive which is large enough to power all the motors that are operating in tandem in the system. The frequency which the drive is set too is provided to all of the motors simultaneously. As your motors are spread over a large distance and cabling between the motors and the drives may be longer than the typically accepted norm of 40 to 50m you may experience problems and as such this may not be an acceptable solution to follow.

Option 2 – Master drive cascade system using rotary shaft encoders.
Equip each motor with its own variable speed drive. Equip motors 1 to 4 with analogue (preferably 4 to 20mA output) output rotary shaft encoders on the motor shafts. Connect the output of the encoder on the first motor to the analogue input of the variable speed drive fitted to the second motor. Connect the output of the encoder on the second motor to the analogue input of the variable speed drive fitted to the third motor. Connect the output of the encoder on the third motor to the analogue input of the variable speed drive fitted to the forth motor. Lastly, connect the output of the encoder on the forth motor to the analogue input of the variable speed drive fitted to the fifth and last motor.

The first motor will be the master (leader) motor. A common start/stop signal can be provide to all the drives simultaneously. Each motor will synchronise its speed to the actual speed of the motor directly ahead of it in the line. You could also interlink the drives to fault if any one drive in the system should pick up an fault / trip error providing additional system protection.

Option 3 – Master drive cascade system using drive analogue outputs and inputs
Equip each motor with its own variable speed drive. Connect an analogue output which is providing actual motor speed on the master (leader) motors drive to an analogue input which is set to run on speed on the second motors drive. Connect an analogue output which is providing actual motor speed on the second motors drive to an analogue input which is set to run on speed on the third motors drive. Connect an analogue output which is providing actual motor speed on the third motors drive to an analogue input which is set to run on speed on the forth motors drive. Lastly, connect an analogue output which is providing actual motor speed on the forth motors drive to an analogue input which is set to run on speed on the last motors drive.

The first motor will be the leader (master) motor. A common start/stop signal can be provide to all the drives simultaneously. Each motor will synchronise its speed to the speed provided by the drive of the motor directly ahead of it in the line. You could also interlink the drives to fault if any one drive in the system should pick up an fault / trip error providing additional system protection. This would be your cheapest option but the system may suffer some delays.

As you are working with conveyors that have a fairly high start up torque requirement we would suggest you use a suitable variable speed drive that can deliver acceptable torque at start up and through the operating range.

I hope my explanation has been of some help and feel free to call me if you have any questions.

Best Regards,


Alister Chalmers

[AndyD]

I like option 2 with the separate drives for each motor and cascaded 4-20mA rotary encoders, there's a certain elegance to it. I'm sure you could throw that little lot in for less than half what the customer got quoted.

[Justloadit]

I tend to agree with AndyD.
However my concern is the frequency to current deviation of each encoder, may cause hunting in the system. This is about the accuracy of the frequency to current conversion process, and the smaller the percentage deviation the better the system operation. ALso look carefully at the specifications of the error where temperature is involved, after all the rotation of the shaft is converted to a current, which is usually done with op amps, which have a time based drift and affected by temperature as well as part of the drift which may affect the size of the error.

A digital output is far more accurate as it is an absolute number as opposed to a frequency to current convertor which may have an operating error. X frequency on the shaft compared to a reference frequency number, rather then using voltage as a reference (Current loops are converted to voltage via a shunt resistor, which are also rated at an error percentage. EG Metal Film is available from 0.1% to 1%, wire wound is 5% error in the resistor value)