Star / Delta Switch on VFD Output

A correction

The reason is to get maximum torque from the motor (briefly) in Star mode then switch to Delta after it reaches the star torque turnover point.

The starting torque is 3 times higher in delta mode compared to star.

The problem is that this system is limited by controller current. So starting out in Star mode reduces the current requirement. Sorry I did not make that clearer.

If I had a 90kW controller I wouldn't need the Star/Delta complication.

VFD applies 0V-415V step by step during acceleration time. The starting current would be very low (not be 5 times of FLC).So there would be no need for star/delta conversion.

Simply increase the acceleration time (10 to 15sec). You dont need to compromise with starting torque also.

I will be operating the motor at it's absolute MAXIMUM rating - right at the point of breakdown torque. In Delta mode it would draw much more current than the controller can provide.

I am looking for the absolute minimum acceleration time possible. It must come up under FULL load. Using ramp times to reduce current will not be possible.

This is not installed in a factory - it is a traction motor.

Try to reduce load, or increase motor size. And use the VFD the way it was designed to be used.

I may have mislead here. When I say acceleration I really meant power under full load - just below breakdown torque point. The acceleration refers to the application - an electric vehicle.

never put any isolation switch between VFD and Induction motor.You can contact VFD supplier.They will guide you in this regard.

Wot about lock-off isolation for maintenance, Guv? <cough>

lock out the mains of course

Having any type of changeover / mechanical changeover at the o/p of VFD is not recommended as it results in huge Inrush (starting current) on the VFD and since VFD has IGBTs directly at the O/P , it might result in Overload / Over-current tripping of the VFD and may be permanent damage to VFD.

All VFDs have Forward / Reverse Control terminals from where u can ensure forward / reverse movement of the motor.

To have more torque enabling the Vector Control Mode and Torque Boost mode in VFD programming should help.

rgds

Sanjeev

Not really reccomended to disable the output as the frequency drive does not see the load while you switch. This may blow the unit! Check with the manufacture but the motor load may still be used to sink the drive output. If you open the output the drive voltage climbs to the source voltage and when you reconnect ????

A drive is a advanced solution for starting and protecting the motor.

For starting the motors - It was D-O-L initially, then came the manual star -Delta type, Automatic Star-Delta.

Then we also had Auto transformer type starters, and soft starters.

And then came the Drives, with loads of built in features and protections, along with various staring options. And now we have a trend of having dedicated drives for specific applications.

Mostly the Drive manufacturers recommend not to use any switching device between the Drive and the motor. Along with the points mentioned in above posts, it also affects Auto-tunning of drive (if the feature is being used) and hence wont be as effective as it should be.

There are parameters in the drive which can give you high torque valve in beginning - 1. torque boost parameter or 2. sensorless vector control mode - it gives very high torque even at low rpms.

Hello Johnny,

FUJI inverters are prepared to change from inverter - mains and back again on its I/O, so sequencing is done internally, this is normally to facilitate mains bypass inverter starting.

Is your supply 415V also and what rating motor are you using (kW, A etc) as you do not state??

Maximum torque and minimum acceleration time is normally achieved d-o-l, the inverter could approach the same if oversized, thne you could get near pullout torque during the whole acceleration phase if you are careful with either ramps or the current limit control, theoretically then superior (faster) to using the control delay, stall and pullup currents when dol started.

What is the pullout torque rated motor current?

Starting in delta at rated 220V winding will overflux the motor, saturating it causing poor performance and burnout.

Best method is to connect at rated voltage and have enough peak current in inverter to approach max motor torque (pullout) during accel, the current limit could be set just below this current to negate 'stalling' high currents occurring beyond its optimal performance limit.

Otherwise, if you have strong supply, start d-o-l and then switch to inverter, output switching easily sequenced by the inverter as needed.

However, this is unlikely to be better than selecting the correct size inverter for your application and controlling it optimally.

I have applications starting at well over 200% torque on high inertia applications up to 10x / minute with no probs but ultimately you need to consider all aspects of the design.

What is your application as a higher peak current is also often needeed (by oversizing inverter rating) on crushers, for example, or the rapid stop/start outlined above.

Switching on the output with the relevant flux delay times you are concerned about can upset the vector calcs but doesn't affect the output and is 'ignored' in v/f mode.

Hi Malcom The Motor is a rewound 11kW with Tmax/Tn = 3.5. nominal torque is 72NM. The rewind voltage is 380/220 which will raise the rated speed running from a 415 Volt VSD to about 3000 RPM. At rated torque this raises the motor power to about 22kW. The VFD is a LEnze 9329 30kW rated 150% for 60 seconds. The application is an electric vehicle. I already have the VSD and am not likely to try to purchase a larger one. Since the motor/VSD combination will be current limited by the VSD (89 Amps) the idea is to switch to Star mode to get to about 50km/h then switch to Delta. I can change parameter sets in the VSD during Star/Delta changeover to have correct motor identification in both modes. The question is, with correct sequencing, drive disabled, change star/delta (using a reversing contactor), re-enable drive - will the drive be damaged?

It helps a lot. Thanks Malcom.

What brand of VSD are you looking at? You may be able to get a VSD that will give you that functionality, ie. 6 motor ouput terminals.

Do you specifically need the Star connection to get the torque. Talk to the VSD supplier and look at the torque curve of the motor. I've used VSD's before that would give full rated torque for a motor as low as 12.5 hertz. So the VSD may be able to do the job for you, without having to do the star/delta changeover.

If you do go for the star/dalta change over you may also need to look at is the configuration of the VSD itself. Generally you need to program the VSD for Delta or Star otor configuration, I guess that's so that the VSD knows how to manipulate it's output, and what feedback or response it expects from the motor. This is probably doable. You should be able to us an digital input to switch the VSD to auxilliary configuration.

I've never done this before, so I can't really comment on the rotor magnetism effect on the VSD, but I would have thought that as long as the VSD senses what it thinks it needs, it'll be OK. You may have to inhibit any alarms for the duration of the changeover point, or put delay times on certain alarms, to allow the the changeover period.

Hope this helps.

Cheers,

Tim

The Fuji is rated at over 200% torque at 1Hz demand but peak torque capability is often 250-300% in frequencies up to 40Hz.

The performance is also dependent on the motor, obviously. This data was derived used with standard Fuji motor.

Thanks sparkytip. It sounds like the drive won't suffer. The rest I can handle. The rotor magnetisation decay time was one of my main concerns. When the drive is disabled all alarms etc. are inhibited. This brand also inhibits gate drive during disable. I can change parameter sets when doing the star/delats change to keep motor ID info correct.

The VFD is likely to be damaged during the switching even when idling unless you give many seconds of demagnetizing time.

The VFD is not likely to give you more starting torque in delta as it is current limited. If you try to accelerate quickly, the current will be limiting factor. At starting, the VFD cannot give you 5-10 times the operating current like the main feeder (unless you buy an oversized VFD)

Fast acceleration and VFD are not compatible when large torque are needed even with a vector controlled unit. Expect many times the starting time obtained using an accross the line starter.

Only a servo motor system (or a DC motor + large drive) will match an across the line starter but $$$.

The flux delay time can upset the vector calcs but does not damage the inverter from my experience. Since they are designed for mains - inverter changeover I can't see why you think this would happen.

When you start with the inverter you develop torque as slip increases until you hit pullout torque when the motor would collapse to stall. The 5-10x current you describe is stall current which doesn't happen with correct VSD use so you could estimate that 250% torque may only be about 200% rated current when used with VSD.

Your statement re acceleration is completely wrong as I have applications replacing clutch brakes and servos with very high performance, maybe your experience of VSD is with poor equipment and low performance applications only. For example, 2.2kW accelerating inertia of many times motor i in less than 40mS open loop. Or, 30kW hoist with 180% torque in 2s. Often, people associate servo with high performance and VSD as general use but if you consider arduous applications such as lifts and hoists it is clear high performance with high torque is available from AC inverters.

The standard AC squirrel cage motor has limitations in some respects (e.g. slip / inertia / open loop) compared to dedicated designs (DC/Servo motors but all closed loop) but you can also use PM motors closed loop with excellent characteristics with VSD'S.

I am glad you have success with your drives.

My Hitachi 30HP didn't cut it in a wire drawing line back in the late 90's. It was vector controlled and perfectly tuned for the motor. The starting time was about twice as long as the across the line method. We evaluated it from the amount of wire that accumulated on the ground from the feeding section during startup.

If the feeding section has been a slower, it would have worked.

It just depends on your definition of "fast". I am yet to see a real life result proving that a vector AC drive is faster than across the line starting.

Please prove me wrong with real life measurements of acceleration or speed curves.

Regards

'Fast acceleration and VFD are not compatible when large torque are needed even with a vector controlled unit. Expect many times the starting time obtained using an across the line starter.'

I was replying to the above post where you say fast acceleration and VFD are not compatible.

I cited examples (as I posted) where the contactor has hardly even pulled in (for dol)before an inverter controlled motor is up to speed given the coil and flux delay constants. You can remove the flux delay by pre-magnetisation in AC drives (usually closed loop which you cannot dol).

In my previous post I did say inverters can approach dol performance, NOT surpass it although, theoretically, this may be possible. You're correct to say that to achieve high torque you need a drive sized to allow the current for that peak. I also advised this and asked re the motor pullout torque current.

It seems you are challenging me to prove something I didn't say but my main point was that high performance and fast response is possible but me sending data to prove it I think is a little futile since your experience seems to show you something quite different to mine.

Naturally, inverters have improved greatly over time but your failure on your applicatioon is quite hard to comment on unless I have a much greater understanding of your application, difficult without familiarity. Maybe you were sold a dud by someone who hadn't done it before and obviously the outcome was disappointing!

If inverter control at most frequencies achieves well over 200% (up to 300% depending on motor) and dol is 250-320% whiklst accelerating, why do you think VSD is 'many times' longer than across the line?

I believe high performance is possible from my experience replacing servos and clutch-brake and even mechanical clutches in applications, also from lift and hoisting data, your experience tells you something different with your Hitachi application and I would generally concur that expecting better than dol performance would not be something I expect - but the level of performance is much better than I thought your first post implied.

The new Hitachi drives are much improved.

http://www.ctiautomation.net/Hitachi-VFD-Drives.htm

starting torque in star mode will be 1/3 the torque in Delta.So question of getting max torque does not arise.The star / delta starter is used to reduce the starting current duration .A vfd also achieves the same smoothly .You need to coordinate the driven eqipment torque requirements with that of the motor and ensure sufficient accelerating toruque to run up from zero to full speed is availble so that motor can accelerate the load to required rated rpm.The purpose of ur trying to use star / delta is not clear to me.

I think Johnny is using 380V 50 Hz star winding at maximum torque to gain best acceleration possible to this speed given that this will be 200% + rated current at this point and he has a substantially oversized inverter.

At half speed (approx), changing to delta, he changes the inverter settings to the new data and the motor rated power / current is now root3 times more but he can achieve min 100% torque and more as he accelerates to a higher speed without underfluxing until he hits 87Hz (then on again, I believe. Now, of course, he has less current in hand comared to the 'new' motor rated current.

This is what he's trying to achieve, I think.

Right on the mark Malcom. Spreadsheets using CD, battery droop, VFD current, etc. show that switching takes 0-60k/h time from 11 seconds in Delta to 8.2 seconds in Star. More importantly the climbing gradient goes from about 13 degrees to 20 degrees. None of this would be needed if the controller could provide 180 Amps but that is just too large a controller. I can put it on the road in just Delta mode and performance would be adequate but for the sake of a reversing contactor (helpful because of the interlocks) I get an electric "gear change".

VFD output current and acceleration ramp can be programmed into VFD controls. Voltage source VFDs can be set for different V/Hz ratios controlling voltage according to the required speed. There are short time or starting time overloads available also. You do need any star/delta type configuration.