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  1. #1
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    Default VFD / Motor Combination

    Looking at build a small VFD controlled motor (about 1 - 1.5HP). I have read through a number of the VFD posts but a lot are from the CNC guys who are more focused on high speed spindles rather than motors so I am hoping someone can help me out with a few questions.

    1. Has anyone purchased a VFD / motor combination that they would recommend? I have looked at eBay and there are plenty of low cost systems (around $150 each for the VFD and motor) but I am unsure of the quality.

    2. I have read that it is generally better to buy a bigger VFD than the motor - is this the general view? How much bigger should I go?

    3. What are people doing to improve cooling at low speeds? I have seen a few examples where people have added computer fans to assist with cooling.

    4. What is the frequency range you can practically get out of a VFD? It seems that you can't go much lower than 20 - 30 Hz but you get go much higher (200Hz plus)?

    Thanks,

    Grant

  2. #2
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    Quote Originally Posted by grunto View Post
    Looking at build a small VFD controlled motor (about 1 - 1.5HP). I have read through a number of the VFD posts but a lot are from the CNC guys who are more focused on high speed spindles rather than motors so I am hoping someone can help me out with a few questions.

    1. Has anyone purchased a VFD / motor combination that they would recommend? I have looked at eBay and there are plenty of low cost systems (around $150 each for the VFD and motor) but I am unsure of the quality.

    2. I have read that it is generally better to buy a bigger VFD than the motor - is this the general view? How much bigger should I go?

    3. What are people doing to improve cooling at low speeds? I have seen a few examples where people have added computer fans to assist with cooling.

    4. What is the frequency range you can practically get out of a VFD? It seems that you can't go much lower than 20 - 30 Hz but you get go much higher (200Hz plus)?

    Thanks,

    Grant
    Grant,

    1. why buy a combination? Unless you get a discount, better buy VFD and motor separately. Choose the number of poles that the motor has, according to your desired base speed. Base speed is the rpm the motor does at 50Hz, just as if it was plugged in to mains without a VFD. Roughly speaking, a 2-pole motor does 3000rpm, a 4-pole motor 1500rpm, a 6-pole motor 1000rpm, an 8-pole motor 750rpm. A 4-pole motor of 1HP is usually in an IEC71 frame, a 4-pole motor of 1HP is in an 80 or 90 frame, and so forth, the more poles the larger the motor for a given power. Motor quality determines price. Very generally, the higher the letter designating the wire insulation class, the higher the motor quality and the more it costs.

    2. yes buy the VFD one or two sizes bigger than the motor unless you are prepared to run the motor at a reduced chopper frequency (= more noise) and can provide a well ventilated enclosure for it. The hotter the VFD runs and the higher chopper frequency you wish to use, the more you must derate its maximum output power. VFD's are often rated for a given power at their lowest chopper setting and at an ambient temperature under 30C. A slightly larger VFD will also have somewhat better dynamic braking capability. In general, you must derate the nominal VFD power by 10% for each 10 degrees hotter room temperature than 30C, derate another 10% if you wish to run it at full chopper frequency.

    3. in my experience, if this is for a home shop application on an ordinary lathe or mill, the duty cycles are so low that no extra cooling is necessary. It really depends what you want to use your VFD/motor for. For example, on a manual lathe one often stops the spindle to chuck, or to take contol measurements, or to clear away chips, or to go to the fridge and study a drawing...... plenty of time to let the motor cool down. In an industrial application thins may be different.

    4. The lowest usable speed depends on the type of VFD, the maximum speed depends mostly on the motor.

    A V/Hz type VFD has only very little torque left below 20 Hz, and when loaded will run erratically below some 10Hz. For machine tools, speed ratios of 4:1 are practical.
    A Sensorless Vector type VFD has usable torque down to below 5Hz and remain smooth running down to under 1HZ. For machine tools, speed ration of 6:1 and up to 8:1 are practical.
    A closed loop sensor VFD is best, but still too expensive for home use. Speed ratios above 10:1 are possible.

    A generic motor in the 1HP range has near nameplate power up to some 100Hz, has reduced but usable power up to some 130Hz, and above that it cannot be used anymore for metal removal. It remains usable to drive smaller drills and endmills up to some 150Hz. But be cautios with a 2-pole motor, it spins at well above 6000rpm at 120Hz and its rotor is only designed for 3000rpm. No problems with 4-poles and higher though.

    A VFD rated motor costs much more to buy, but can be used at higher frequencies. There are also very special VFD motors that can run at up to 400Hz, but too expensive for home use.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Default

    Quote Originally Posted by grunto View Post
    Looking at build a small VFD controlled motor (about 1 - 1.5HP). I have read through a number of the VFD posts but a lot are from the CNC guys who are more focused on high speed spindles rather than motors so I am hoping someone can help me out with a few questions.

    1. Has anyone purchased a VFD / motor combination that they would recommend? I have looked at eBay and there are plenty of low cost systems (around $150 each for the VFD and motor) but I am unsure of the quality.
    I would rather get a used brand name motor like a Leeson or ABB, than a new no-name motor.

    2. I have read that it is generally better to buy a bigger VFD than the motor - is this the general view? How much bigger should I go?
    That's what I heard as well but more important is to use a bigger motor than usual so that you still get some power at low speeds.
    Remember at 25Hz is will have half the power that it has @ 50Hz

    3. What are people doing to improve cooling at low speeds? I have seen a few examples where people have added computer fans to assist with cooling.
    I have done this for one motor and it seems to work OK.
    The other idea I've have for cooling is for a wood working application and that involve ducting air from a dust extractor to run over the motor.

    4. What is the frequency range you can practically get out of a VFD? It seems that you can't go much lower than 20 - 30 Hz but you get go much higher (200Hz plus)?
    The range to some extent depends on things like the quality of the motor bearings, application patterns and cooling.

    A 2 pole, 3000 rpm @50Hz, motor can usually run continuously @ 60 Hz (3600 rpm) with out a problem but above 4200 RPM (70 Hz) in the long run the bearings may start to have problems. I doubt wether a 3000 rpm @50 Hz motor will even get to 150Hz let alone 200rpm. The 2 pole motors I have tried all start doing funny things and losing power above 120Hz

    A 4 pole (1450 rpm at 50Hz) can usually go to 120 Hz (3600 rpm) without too much trouble but above 140Hz you run into the same problem with bearings as the 2 pole motor.

    On my DP which has a 1440 rpm motor I use anywhere from 10 to 120 Hz fair regularly. Drilling is usually a short term operation so the load time V free wheel time is usually small.
    On my Hercus lathe I typically use 20 to 70 Hz, same on my WW lathe.
    On my WW linisher I uses 20 to 60Hz - I could go to 70 Hz and still be in an acceptable linear speed for wood removal but it's fair ripping the wood off at this stage as it is.

  4. #4
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    Default brands and quality

    Re small 3 phase motors. There are many brands out there, of varying quality .

    Ive seen a few Chinese made motors fail and I avoid them if I can . Id like to know what are the good brands to look for . Toshiba, Teco , ASEA and others seem to be Ok. I have a nice Hyundai 3hp made in Korea of all places .

    Mike

  5. #5
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    I was taking to a motor repair guy the other day and he was saying that pretty much all the motors come out of china these days. You have to go for the brand names if you want the quality - although the no names can be pretty good still.

    Michael

  6. #6
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    Quote Originally Posted by morrisman View Post
    ........ Toshiba, Teco , ASEA and others seem to be Ok......
    You will find that all the big, well known, established motor manufacturers do manufacture in many countries all around the world, including China. You will also find that most if not all new small HP motors sold in Australia nowadays come from China. They are just as good and well made as if they came from any other country.

    The problem is with lowest cost "noname" and "fantasy name" motors out of China, that sometimes can be unbelievably poorly made. The absolutely worst example for me was, when the motor in my Aldi concrete mixer failed. Never seen a worse made motor. Ok, I only paid $279 for the whole mixer - a good quality ABB or Toshiba or Baldor 8-pole motor would cost alone at least $150.

  7. #7
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    Default

    I think to get any meaningful information for tour specific application, you need to tell us what you want to drive with it.....
    If it was foe a variable speed fan to keep your shed cool in summer, then the info you are looking for I add very different to driving a compressor for sand blasting.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  8. #8
    BobL is offline Member: Blue and white apron brigade
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    One thing you need to be aware of when going to a bigger/higher rated VFD is that it will be capable of delivering much more current than the motor is capable of using safely before the VFD trips out. On some VFDs the max current can be reprogrammed to protect a smaller motor but if this is not so I can see possible problems looming. This may also call into question as to why bother going to a bigger unit in the first place especially if your usage pattern is expected to be light or intermittent.

  9. #9
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    Quote Originally Posted by BobL View Post
    One thing you need to be aware of when going to a bigger/higher rated VFD is that it will be capable of delivering much more current than the motor is capable of using safely before the VFD trips out. On some VFDs the max current can be reprogrammed to protect a smaller motor but if this is not so I can see possible problems looming. This may also call into question as to why bother going to a bigger unit in the first place especially if your usage pattern is expected to be light or intermittent.
    Bob, I am not aware of any standalone VFD made in the past 15 years, that did not have programmable motor current and motor protection. But maybe you meant VFD's that are directly built into motors, and yes these would usually lack any user accessible adjustment.

    Going to a bigger VFD may be necessary, because of many situations requiring to derate the VFD nameplate power, see my post above. For example, a 1HP VFD operated at its highest chopper frequency (to reduce the audible noise) and enclosed in a box where in summer temperature can rise to 50C, has to be derated by 30% (10 for the chopper frequency and 20% for the higher temperature) and will only be able to deliver 0.7HP on a hot day and 0.9HP on a cold day. I am sorry, but that is the way how manufacturers rate their VFD's.

    Going to bigger drive (bigger motor and bigger VFD) IS necessary when people want to do away with existing gearboxes and the need for gear changes. For example, if a lathe had a 1HP motor and a 3 speed gearbox, and the user wants to have variable speed without the gerabox, a 3HP drive will be necessary to retain the same overall metal removal capability. Think about it, it is like driving a car without gearbox, to climb a hill in 4th gear you would need a MUCH larger engine. For home use, this is usually too expensive, plus extensive modifications would be necessary to remove gear or belt drive, accommodate a much larger motor etc. For this reason, the vast majority of home shop VFD upgrades retain the "mechanical advantage" of a gearbox or step pulley drive, and choose a motor the same HP rating and size/frame of the original motor.

  10. #10
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    Im looking at adding some form of speed controller to my mill/drill so I don't have to keep changing the belts around all the time, is there a single phase to single phase VFD out there available? or do I have to use one of those jay car kits or do a 3 phase motor conversion so can run a VFD.

    Cheers

  11. #11
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    In theory you could run a single phase motor from one feed of a VFD but it would have to be one made for it (that includes a neutral) and the motor would have to be one that does not have a start switch that switches on speed.

    3 phase motors are not prohibitively expensive - much simpler to do a swap as several people here have done.

    Michael

  12. #12
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    Quote Originally Posted by RVK 355 View Post
    Im looking at adding some form of speed controller to my mill/drill so I don't have to keep changing the belts around all the time, is there a single phase to single phase VFD out there available? or do I have to use one of those jay car kits or do a 3 phase motor conversion so can run a VFD.

    Cheers
    Yes there are VFD variable speed controllers for single phase induction motors. But these are designed for things like fans, which do not require any torque at all at lower speeds. I have also seen them used on a scroll saw (Hegner), same thing no need for torque at low speeds and a very narrow speed range serves the purpose.

    For a drill press, you definitely want a real 3 phase motor with a single phase input / 3 phase output VFD.

  13. #13
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    Quote Originally Posted by grunto View Post
    Looking at build a small VFD controlled motor (about 1 - 1.5HP). I have read through a number of the VFD posts but a lot are from the CNC guys who are more focused on high speed spindles rather than motors so I am hoping someone can help me out with a few questions. 1. Has anyone purchased a VFD / motor combination that they would recommend? I have looked at eBay and there are plenty of low cost systems (around $150 each for the VFD and motor) but I am unsure of the quality. 2. I have read that it is generally better to buy a bigger VFD than the motor - is this the general view? How much bigger should I go? 3. What are people doing to improve cooling at low speeds? I have seen a few examples where people have added computer fans to assist with cooling. 4. What is the frequency range you can practically get out of a VFD? It seems that you can't go much lower than 20 - 30 Hz but you get go much higher (200Hz plus)? Thanks, Grant
    Hi Grant, Here is a link to my description of a bandsaw upgrade I did earlier this year. http://www.woodworkforums.com/showth...t=rob+streeper Cheers, Rob

  14. #14
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    If you want variable speed then don't go with a single phase motor or a Jaycar kit. Go either a DC motor and speed controller of a 3 phase motor and VFD. The 3 phase motor I bought brand new for my mill was $40 and is made in Brazil. Not a bad quality motor either. I have a similar size 3 phase motor waiting to go on my lathe, it's S/H but a reputable brand motor none the less and in good nick, it cost me $30. Just need to keep looking. Fortune favours the brave AND the patient buyer.

    Cheers,

    Simon
    Girl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.

  15. #15
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    Quote Originally Posted by rob streeper View Post
    Hi Grant, Here is a link to my description of a bandsaw upgrade I did earlier this year. http://www.woodworkforums.com/showth...t=rob+streeper Cheers, Rob
    Rob,

    Good read - pretty much covers off everything I am looking at. One question - what is the 3 phase motor normally wired as? I am assuming delta?

    Grant

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