Variable Speed Drive

Hello Sisra

The costs of the additional features are no really and issue, as most of these features are done in software and do not require additional hardware to be added. There are some costly itmes that are included in the better drives, such as DC chokes, Dynamic braking etc, but these all do actuallly improve the performance, and there are drives on the market without them already.

I do not believe that there is much on the cheaper drives that could be left out without affecting the performance/reliability of the drives.

Best regards,
Mark Empson
http://www.LMPhotonics.com | http://www.LMPForum.com

Years ago I did away with VFDs for downhole oil recover pumps that were variable speed drives with above ground VFDs. I used volts per hertz generators on diesel engines and throttled the engine up and down and the pump motor followed. No VFD.

I'm mechanical as well but have worked hands on with VFD and DC variable speed drives. I have found VFD's remarkable in the way the costs have plummetted over the past 20 years. Even good major brands can be had very inexpensively. I have however found them infuriatingly limited - especially at the low end. One additional exceptional problem I've encountered is even if I use a very expensive, very linear rheostat for frequency(speed) control the response if anything but linear. 95% turn = 100% down to 90% speed adjustment, the remaning 5% turn is a precipitous 90% to 0% speed reduction. The best bad solution I've found for my applications was DC variable speed drive - 120 or 230 VAC in and DC out. DC is a convoluted system though - the drive needs to match the motor or else...

Good luck

hi Sisira,

One cost effective solution of VFD is Eddy current clutch, Though bulky and less efficient, it is simple, sturdy , reliable and ofcourse cheap. Check availiability and suitability to your specic need. It was used in olden conveyer drives etc.

K.A. Suresh

Hi Sisira,

I have worked in the printing industry for a long time and before vfd were an option, we always used dc drives. These are cheap, very easy to install and every motor manufacturer knows everything about them so if it breaks it will be easy and cheap to get it repaired. Why don't you look at some of these very long established systems and well proven in the field.

If you could elaborate on the nature of the design, in particular starting torque i may have a few ideas for you.

Dave

What is the application? As Conscia pointed out VFDs can be a real pain at the lower end especially when other machines depend on the VFD to deliver at a precise rate at the right time.The old DC variable speed motors still work better under those conditions, but in my experience some of the more upscale digital controllers (PLCs) are getting good at controlling output at higher frequencies, (45 to 60 Hz); of course that means more cost but it may be worth it.

If your machine does not require the ability to frequently vary the drive speed then a suitable mechanically adjustable eccentric friction drive would fill the bill with about 95% efficiency. They don't take up a lot of space and are real work horses. I've used them in assembly machine applications that ran 24/7 and averaged 8 years of service before needing any repairs or replacement parts.

http://www2.nord.com/cms/us/product_catalogue/mechanical_variable_speed/mechanical-variable-speed.jsp

The above is just one of many companies who manufacture these drives.

I hope this helps.

WOW it seems like VFD's are getting a bad grade at the lower end speed and torque requirements. I must input as to my experience, I have been using VFD's in machines that I design for about 13 years now. In my particular application low speed torque and speed control is absolutely critical. What I have learned over the years it that set up of a particular manufacturers drive may be quite different than another. However one thing remains the same with all drives, you get what you pay for. Bells and whistles can be attractive but firmware is even more important. I have worked with several different drive manufacturers to improve low speed performance. we need to keep one very important thing in mind, Drives are quite generic. All manufacturers try to accommodate as many applications as possible. When speed control is critical the only way to go is a drive that is listed as constant torque and it must have dynamic braking.

I have done extensive testing on several different drives from the low end to the highest. the low end drive had very Poor performance at the lower speeds. The highest cost unit had the ability to perform but the manufacture would NOT modify their firm ware to suite my application.

I am not in the business of selling drives how ever I would mention the manufacturers that I buy from with my opinion of each of those.

I thought that was the whole issue here, you have to pay a lot more to get all the bells and whistles.

This guy wants something that is cheap and chearfull that will still do the job so DC wins it hands down. I have also worked with a lot of VFD and it is absolutely fantastic especially when you want to troubleshoot both electrically or mechanically but the cost...! We are not trying to make out that VFD is bad, it is just a matter of horses for courses.

Yes Case491 hiding behind the mask (i am referring to your logo!)- By the way I am curious to know what case491 stands for!

Anyway you hit the nail on the head.. at least on the part of the head.

Now will go back to my original question..

Can any smart electrical engineer design a very simple circuit where frequecy can be varied in 2Hz intervals (say over the range 40-60Hz based on 50Hz standard in this part of the world)?

For example this will allow us to run a 1400RPM motor in the range 1260RPM to 1540RPM in steps of 28RPM.....

May be I really do not understand the basic principles of a VFD and giving a stupid idea!!

Over so narrow a range you should have no difficulty using even the cheapest of VFD's. You can get something decent in the way of an OEM board mounted inverter VFD for a little over $100. Takes 110 VAC in puts out 220 VAC 3 pase, and you can have a 4-20 mA input to control the speed, or a potentiometer, or some arrangement of switch contacts with various resistances for stepped speed. You may find you need to test and tweak the resistances to get exactly the speed you want - from my experience these things are not linear. These generally come with full documentation, and are very straightforward to setup and configure.

I wish the guy who said there are some great on the low end would say which.

The best units I have tested from the most precise to the least

1, toshiba Tosvert s-11

2, Telemechanique AVT-31 and 71

3, Mitsubishi E series

4, Allen Bradley ( hard to tune in )

5, Funac

Toshiba and Telemechanique are the same hardware but the firmware is completely different.

Mitsubishi drives are tricky to tune in. they have a strange way of doing things like motor tune.

AB's are very difficult to deal with. for precise control.

Funac is good but expensive

Dave T

Dear Sir,

Try Yaskawa's VFDs; www.yaskawa.com for info and local sales support.

Best regards,

Andy U.

We have designed and installed several VFD pump systems that work perfectly, especially in fire fighting, and community water and sewerage systems, but that is mostly higher frequency applications. I find controlling these pumps between 40-60 Hz works well using a 4-20Ma pressure transmitter to monitor pressure and/or reservoir levels. The PLC uses the signal from the transmitter to control the output with incredible accuracy.

In my experience ramp-up and ramp-down times can be critical depending on the application. If ramp-down is very important then a dynamic brake, or some other braking system my be necessary. If ramp-up time is critical then you may need more Hp or a better control system. Either way the best results cost more money.

Don't expect digital results with analog thinking.

You use PLC's with onboard PID controllers? That's excellent! What brand and model?

Dear Conscia,

Can you help me in giving some contacts who do OEM?

By the way I am from Sri Lanka (for the benefit of those who have never heard of this place it is a small island in Asia near India) and we use 230VAC single phase /400V 3phase- same as the British.

Regards

Sisira

Danaher Motion (HK) Ltd
Unit A, 16 Floor, 169 Electric Road
Manulife Tower, North Point
Hong Kong
Tel : (852) 2503 6565
Fax : (852) 2571 8585
E-mail: victor.lim@danahermotion.com

Their SECO "Bronco Series" is available OEM - Open Frame - I've used these. Let me know if they are gouging you on price in your area, I know that sort of thing goes on, though sometimes it's a function of crazy tariffs and duty.

Danaher Motion owns a broad set of formerly independent companies - Pacific Scientific - excellent very cost effective DC drives, Kollmorgen, SECO and others. If they have a stocking distributor in Sri Lanka it would make many things easier for you.

You might also want to dig around this site.

http://www.directindustry.com/industrial-manufacturer/inverter-71215-_61.html

Good Luck.

Thanks a lot for your help.

I will contact them.

Thanks once again

Sisira

I believe that not all here know about drives with pure sinusodal output.

http://www.nfo.se/

Hope to attract your attention , to know that there is PWM drives and there is another type based on switching circuit principle !!

Good luck to choose the right drive

Takhinen