Question(s) for 3-phase Motor/VFD Experts

[Bill Adamsen]

The 3-phase motor on my dust collector (3hp) is controlled via a Teco FM-50-203, setup shown on the page Remote Powered VFD. I use a linear taper potentiometer (shown on attached photo below) to control the speed, which I reduce dramatically to use the system for clearing the air following tool operations which create dust not picked up by the dust collection system itself. I don't have a tachometer, but I'm guessing the rpm are 1/10th or even less of max when just used as an ambient air filter. I could probably "manually" adjust the frequency to approximate the speed, but I'm not sure it's relevant to the question. I used the Teco programming guide to set F_11 to 1, which gives 0-max frequency adjustment via the potentiometer.

Operationally, this system is not the best. The potentiometer doesn't reset when the machine is turned off. So, if I want to use a tool, I switch the remote control (see photo below) hanging on my belt, and realize I need to walk over and turn the potentiometer up. Once done with the machine operation, if I wish to keep the collector running to filter air, I need to walk over and turn the potentiometer down to the appropriate level. And then of course walk around and open up the gates I want to use to filter the air entrained dust. And then reverse when done. Too much walking and remembering.

Questions

1. Is there a function on the Teco FM50 (JOG or SP1 or Set Speed?) that would allow me to use a switch (thinking the second channel of my remote) to activate a pre-set frequency reduction? This would solve all the electrical control issues mentioned above, and improve the user experience considerably.
2. Is there any equipment-related risk at running the 3-phase Baldor motor at such a slow speed? The vfd has never (to date) logged a fault.

[Rod Sheridan]

Hi, the motor will have reduced cooling at lower speeds, however the motor load will also vary with the cube of the fan speed so running the fan at half speed results in 1/8 motor load. You won't have a cooling issue with the motor.............Rod.

I'm sure there's an easy method of providing a 2 step speed control for your VFD, I'm just not familiar with yor VFD...

[Bob Winkler]

I'm sure something could be rigged with a relay switching in a second potentiometer. This would give you a low speed and high speed pot. As an alternative, since the pot only controls an analog voltage to the VFD, discrete resistor dividers and a 2 pole toggle switch could be setup for two speed operation (kind of like your car fan).

In the end, have you considered a separate ambient air filter that is designed for this task and can be set to run for a certain time interval? it may be cheaper and more reliable.

Bob

[Bill Adamsen]

Folks:

I posted a similar question in the "general" forum of Practical Machinist and received suggestions which gave me the confidence to explored some alternative approaches. I thought I'd share back what I learned.

My remote has two channels ... essentially two independent latching switches. I connected those to the VFD (Teco FM50) in the following fashion. Connect the first switch to FWD control and common, second switch to SP1 and common. I then programmed an SP1 frequency (F_8) and set the multi function input (F_19) to #2 ... SP1. That was it! The first button on the switch now turns the DC on and off, and the second switch alternates between normal frequency (speed) and SP1 frequency (speed). There are two user downsides I've discovered so far. First is that I now might need to push two buttons rather than one - but that can all be done remotely. For instance, had I turned off the machine when the velocity was on low, simply clearing ambient air, then when I turned it on I would need to switch the frequency from SP1 to normal. Similarly, if I had turned it off on SP1, that is the mode that would function if activated with the manual switch. I need to explore whether turning it off can reset to the normal phase. But even with that anomaly, the solution works for me.

The whole modification took about 30 minutes, about 90% of which was reading the manual to determine the logic.

[Bill Adamsen]

...have you considered a separate ambient air filter that is designed for this task and can be set to run for a certain time interval? it may be cheaper and more reliable.

Bob

How would it would be cheaper or more reliable? It certainly would be a separate tool requiring storage, spares, maintenance, switching on/off, an extension cord. I really like the fact that my dust collection system is able to put the expensive installation, infrastructure and filters to additional use. But if my thinking is wrong on this I'm open to change. I have virtually zero dollars (just time) invested in this approach. Part of the decision to use this approach was the many posts that question whether separate air filtration units really work. The idea of a timer might have merit, but probably better would be a sensor that shuts down once a certain air quality was attained.

[M Toupin]

I'm guessing the rpm are 1/10th or even less of max when just used as an ambient air filter.

I get what you're trying to accomplish, but a DC isn't a very effective ambient filter system especially at that low of a rpm. My main concern though is driving a motor at 6hrz and 180ish rpms for any length of time isn't going to be kind to the motor. I would also guess since it's a Torit it's a good quality motor (Baldor etc) so when you fry your motor it'll cost more to replace than a ambient filter system to begin with.
Just my 2cent
Mike

[Mike Henderson]

Running the motor that slow should not be a problem for the motor. It should not get very hot because it's not loaded very heavily. But I question how well it will work to "clear the air". It's not moving much air at that speed.

Mike

[Bill Adamsen]

The rpm are completely programmable via the VFD. I chose that speed simply because by feel - it seemed to be moving a "fair amount of air" through several larger (six inch) simultaneously open gates - and was quiet enough to listen to the radio. Was it as fast as the chip removal required by the larger machines when operating? Absolutely not. But then the dust particles I'm trying to clear are of a much different nature ... light enough to be "in suspension" in the static air column. The filters I have on the machine are of the same type that B. Pentz had recommended for his AAF. The comment about the cost of the motor is well founded. No question that using a small vortex blower would remove some work from the larger motor.