VFD vs Freq Converter

[vanskat]

Trying to weigh speed control options for a 3ph 220V AC induction motor. The easiest (and more expensive option) is to use a VFD.

However, I want to take up the challenge to implement the most economic solution while not sacrificing too much functionality. Was looking at the SMC-2 controller from AB but need to know what else is out there as I have never had the chance of working with freq converters..

Any suggestions?

 

[Skogsgurra]

I am not so sure why you diffentiate between VFD and frequency converter (or inverter). They are (to me, at least) identical.

If you are looking for variable speed in general, then there are quite a few options. Ranging from mechanical variators, DC drives, hydraulic drives, eddy current couplings and so on. There are also wound rotor asynch motors with controlled feed back of slip energy, cyclo-conveters and matrix converters - they all do their job in their special applications, but they are not mainstream.

There is one class of variable speed drives that can be used with some advantage in fan and centrifugal pump drives and that is the stator voltage controlled asynchronous motor. It has a high rotor resistance and its slip can be controlled by adjusting the stator voltage. Simple triac controllers are used and the total efficiency is quite good due to the fact that the speed range is limited and that the torque goes down as speed squared. So you will still have just below 75 percent efficiency at half speed - and that is not at all bad considering the simple and cheap technology.

But - as I said - I do not know if I have understood your question.

 

[jraef]

Caution!
The SMC-2 is NOT a speed control device of any kind. It is a soft start controller. You CANNOT use it to change the motor speed, only the rate at which it gets to full speed, and even then only within the limits of the motor! If someone is trying to sell you the SMC as a speed control device, they are not worthy of your business.

"Venditori de oleum-vipera non vigere excordis populi"

 

[vanskat]

Ok let me clarify..Yes the SMC is a soft start unit that allows adjustment of ramp time and offers several preset starting modes..similar to other makes like the Saftronics EZ6 etc...

Also I understand that a VFD is basically a freq converter. I have installed several from AB to Control Techniques.....

Just trying to do something a bit different/creative from a packaged product point of view. Instead of spending on a full blown VFD (which maybe overkill for the application), I am looking at the alternative -- a variable freq converter,,something simple and functional.

System requirements are simple:
1. Ramp or soft start
2. Simple speed control (really a nice to have. no auto control or analog speed reference required.)

Want to try something I have never done before!

 

[jraef]

To offer something on a more positive note then, you really only have 3 choices (assuming it is an AC motor within the range of what the SMC-2 was capable of). Do a keyword search on some of these terms to find more posts regarding them.

1) Electronic speed control, which means a PWM VFD (which by the way IS a frequency converter). There really isn't another practical way to change frequency and voltage, both of which are necessay if you want to change the speed of the motor itself. Although several older technologies were at one time available, they are no longer widely supported and you realistically only have one technology choice left available to you now. Within that realm thee are a myriad of choices and features, but from a basic technology they are all PWM inverters. I understand that you think they might be too complicated, but believe me, if you look for a simple stipped down model with a built-in speed pot, you will not find anything else less expensive. And remember, no matter how else you do it you will still need to start and protect the AC motor.

2) Electro-Mechanical speed control, consisting of magnetic drives and eddy-current drives for the most part. These offer some possible advantages and other disadvantages over Electronic, many of which can and have been debated in this forum ad nauseum. The AC motor is run at full speed, but the clutch mechanism slips to allow the output shaft speed to vary. The main disadvantage is in additional losses in the power system that translate to increase operating cost. still, if you do not use it often enough to make that an issue, they might be a good choice.

3) Strictly mechanical VSDs such as a VariDrive, which use variable pulleys to change the speed mechanically. Again the motor runs at full speed and the mechanical system changes speed by changing the sheave ratio. Although this system is highly inefficient compared to the others, it does have the benefit of being able to provide mechanical advantage by virtue of the sheave (gear) ratio multiplication of torque.



"Venditori de oleum-vipera non vigere excordis populi"

 

[vanskat]

Yep, V/Hz ratio prevails as it relates to speed control. Oh! Another complication is that the motor is not inverter duty. So I've got to compensate for that.

The 30hp motor is already coupled to a reduction gearbox etc.. Bottom line is that soft start maybe the way to go. I should be able to apply mechanical retrofit for simple speed control..Just thought that there was something esle out there!!

Thanks