In an earlier thread I asked about magnetic starters, since I am looking at a cab saw and thought I had a single phase motor to replace the 3 phase motor in the saw with, but unfortunately my single phase is not TEFC, which the saw requires. A search here suggested using a VFD to drive the 3 phase motor (and yes, I know a rotary is superior, and no, I am not considering a static converter). Does anyone have experience with this configuration? I would like to hear opinions! TIA.
Anyone have experience with VFD and 3-phase motor?
- Thread starter Thruxton
- Start date
Your mixing your termanology. A VFD is just that a variable frequency drive. Rotary and static are phase converters to allow running a 3 phase motor on a single phase input. I run multiple pieces of equipment from a rotary in my shop and am happy with it. With a saw I assume that you do not need variable speed so either would work for you. Horsepower will determine which will be most cost effective.
lg
no neat sig line
lg
no neat sig line
rsanter
Well-known member
Use the VFD, it will convert single to 3ph and anymore is cheaper than the rotary converter.
Bob
Bob
Don't think I mixed my terminology at all. As rsanter (above) and others have pointed out in other threads, a VFD (and you are correct, it stands for variable frequency drive) can be used to run a 3 phase motor from a single phase power source, and therefore is functionally comparable, in that usage, to rotary and static phase converters. And it can, as you noted, function as a speed controller, which you were correct in supposing that that is a use I am not interested in. My question was about how effective it is as a phase converter "substitute"- I have read pros and cons, and wanted feedback from those who use a VFD for the purpose I gave. But thank you for your contribution.
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Thedroid
Well-known member
I can't think of any cons compared to a phase converter. Ive used vfd's numerous times, and its always been a plug and play ordeal.
I've got one on my mill,one on lathe, one on drill press. all 3 phase motors. run from single phase. I think the only thing about using it on the saw you don't want the vfd to stop the motor suddenly (blade nut might get loose) let it coast down.
pete
pete
There are a lot of folks who mount a NEMA type 1 enclosure VFD on a saw or other equipment, it is IMO, hackwork but for a 3 HP or smaller a drive can be purchased quite reasonable, I bought a GE/Fuji 3 HP drive for less then $200 shipped for a shaper (2 HP 200V 3Ø). My drive was mounted in a can on the side of the machine.
Here is a unwired photo on the Rockwell HD shaper.
Here is a unwired photo on the Rockwell HD shaper.
tdkkart
Well-known member
Braking/coast down speed is a setable parameter every VFD that I've dealt with. You can usually turn it off completely so that the motor will freewheel down normally.
I too have VFDs on a lathe and a mill at home. I use the variable speed ability on the mill but not on this lathe as it has it's own. On my last lathe I used the variable speed because it was a step pulley machine. It's up to you.
There's not very many reasons to use a rotary converter any more except for some special applications. I know that the control circuitry on CNC machines doesn't like running on VFD generated power.
hidollartoys
Well-known member
As a rule, when using a VFD to convert single phase to 3 phase, the VFD will need to be "upsized" to 2X the required horsepower. This becomes costly for motors of 5hp and larger. 1hp and 2hp motors are easly driven with VFD's.
This does not apply to VFD's w/ single phase input 3 HP & less, they may be used at their rating, above that a 3Ø VFD that can have the phase loss protection disabled is needed & 50% derating applies in most cases.(5 HP motor requires a 10 HP 3Ø VFD).
For smaller motors, a VFD is a cheap way to run 3Ø equipment, 1 HP & smaller motors can be run w/ a 120V input VFD & 200-240V 3Ø output. I would avoid "no-name" ChiCom drives off eBay though, "cusstomer" service is lacking w/ them.
tdkkart
Well-known member
I've gotten my last couple drives from here:
http://www.dealerselectric.com/variable_frequency_drives.asp
Had good luck with the TECO brand.
http://www.dealerselectric.com/variable_frequency_drives.asp
Had good luck with the TECO brand.
rsanter
Well-known member
A VFD is as good or almost as good as a RPC, the VFD will retain 92% to 98% of the original rated HP of the motor
Bob
Bob
The sentence above in your original post leaves me wondering if your talking about a rotary VFD. If you know the difference then good.
I also said that "Horsepower will determine which will be most cost effective." and that is the point. Without knowing the hp of the motor you're trying to drive then blindly saying use a VFD is just a swag. As Norcal points out there is a point that you have to derate the VFD and it starts to get very expensive to use a vfd as a phase converter. There is also a point at which a static converter may be the right choice, that is if you have a big tool that will not be run anywhere near capacity.
lg
no neat sig line
lg- you're right, I need to be sure to post ALL the info- I know I get annoyed when others leave crucial stuff out. It is supposed to be a 2 HP 3Ø motor but I would not be surprised to find that is 3 HP, this machine I think usually had a 3 HP. I'll find out this week if it's still for sale.
hidollartoys
Well-known member
I usually highly recommend the VFD options but you asked for reasons for NOT going with the VFD.
A VFD can cause some AC line notching and high frequency harmonics reflected onto your power system. IF this is a problem you can add line reactors (inductor) or harmonlic filters that are designed for VFD applications.
A VFD can potentially cause premature motor failure in very old motors. Real old motors built before VFD's were invented, when operated on a VFD may run hotter than on a pure sine wave voltage. This is due in part to the motor laminations and causes of the PWM voltage. Note though that a static or rotary phase converter will not have a pure sine wave like 3 phase from the power company either, and the VFD might be the best choice aside from the grid power. To further soften the pwm voltage to the old motor you can install a "load reactor" (basically a 3 phase inductor) between the motor and VFD. I usually recommend a load reactor on older motors.
Applications on motors with extremely long wiring between the motor and VFD, like 500-1000' may have issues with "reflected wave". This should not be an issue in a home shop but is experienced in industrial installations and causes new motors to fail. There are other problems with long leads but the topic is most likely too in depth and irrelevant to this forum.
A VFD uses PWM in the audio spectrum. Some motors depending on mechanical construction can "sing" or act like a speaker at that frequency and the noise, although not louder than the power tool, can be annoying. This occurs mostly on older motors where the windings are not secured as well in the motor. To minimize this a load reactor will reduce the volume, and or you can increase the PWM frequency to reduce it as well.
A VFD can cause bearing currents that can eventually destroy the motor bearings. The PWM frequency can induce a high frequency component into the rotor and this voltage needs a conduction path to ground. The current is low and akin to a small static electricity charge. The problem is that this discharge occurs at the bearing race and can damage the race. In 24/7 motor operation, the motor bearings can fail before they would otherwise. In a shop application, this is almost not a concern. In industrial applications where this is a concern, a carbon brush is placed on the motor shaft and connected to ground.
I still highly recommend the VFD option though but usually I add a load reactor to resolve the possible PWM noise issues and it also fixes other possible problems as well.
The VFD may not be sized accordingly, most manufacturers rate them nowadays with global marketing, marginally. What I mean is that they might be able to supply 150% rated current for 3 seconds and 110% current for 60 seconds. A few years back the US made VFD's were rated at 150% for 60 seconds. The US has changed to Asian ratings to have comparable pricing. More current is more HP, so if your table saw hits a knot in the wood, your 3hp motor is a 4.5hp motor for that amount of time. The motor can do this without damage as 150% for 1 minute is usually within the motors overload curve. A RPC will be able to supply this power but a VFD may not if it's not sized properly. IF you need this capability sometimes you may need to buy a 1.5hp VFD for a 1hp motor.
I didn't include any disadvantages of the RPC or SPC because the list would be much longer.
I've used VFD's for variable speed on my wood lathe.
I've installed VFD's on a 3HP and 2HP knee mills with excellent results.
A VFD can also give you increased starting torque while keeping the motor current below rated current compared to starting across the line which yields less torque and 6-10 times rated current.
Brian
A VFD can cause some AC line notching and high frequency harmonics reflected onto your power system. IF this is a problem you can add line reactors (inductor) or harmonlic filters that are designed for VFD applications.
A VFD can potentially cause premature motor failure in very old motors. Real old motors built before VFD's were invented, when operated on a VFD may run hotter than on a pure sine wave voltage. This is due in part to the motor laminations and causes of the PWM voltage. Note though that a static or rotary phase converter will not have a pure sine wave like 3 phase from the power company either, and the VFD might be the best choice aside from the grid power. To further soften the pwm voltage to the old motor you can install a "load reactor" (basically a 3 phase inductor) between the motor and VFD. I usually recommend a load reactor on older motors.
Applications on motors with extremely long wiring between the motor and VFD, like 500-1000' may have issues with "reflected wave". This should not be an issue in a home shop but is experienced in industrial installations and causes new motors to fail. There are other problems with long leads but the topic is most likely too in depth and irrelevant to this forum.
A VFD uses PWM in the audio spectrum. Some motors depending on mechanical construction can "sing" or act like a speaker at that frequency and the noise, although not louder than the power tool, can be annoying. This occurs mostly on older motors where the windings are not secured as well in the motor. To minimize this a load reactor will reduce the volume, and or you can increase the PWM frequency to reduce it as well.
A VFD can cause bearing currents that can eventually destroy the motor bearings. The PWM frequency can induce a high frequency component into the rotor and this voltage needs a conduction path to ground. The current is low and akin to a small static electricity charge. The problem is that this discharge occurs at the bearing race and can damage the race. In 24/7 motor operation, the motor bearings can fail before they would otherwise. In a shop application, this is almost not a concern. In industrial applications where this is a concern, a carbon brush is placed on the motor shaft and connected to ground.
I still highly recommend the VFD option though but usually I add a load reactor to resolve the possible PWM noise issues and it also fixes other possible problems as well.
The VFD may not be sized accordingly, most manufacturers rate them nowadays with global marketing, marginally. What I mean is that they might be able to supply 150% rated current for 3 seconds and 110% current for 60 seconds. A few years back the US made VFD's were rated at 150% for 60 seconds. The US has changed to Asian ratings to have comparable pricing. More current is more HP, so if your table saw hits a knot in the wood, your 3hp motor is a 4.5hp motor for that amount of time. The motor can do this without damage as 150% for 1 minute is usually within the motors overload curve. A RPC will be able to supply this power but a VFD may not if it's not sized properly. IF you need this capability sometimes you may need to buy a 1.5hp VFD for a 1hp motor.
I didn't include any disadvantages of the RPC or SPC because the list would be much longer.
I've used VFD's for variable speed on my wood lathe.
I've installed VFD's on a 3HP and 2HP knee mills with excellent results.
A VFD can also give you increased starting torque while keeping the motor current below rated current compared to starting across the line which yields less torque and 6-10 times rated current.
Brian
Brian, thanks! I brought up this question in the first place because of some anti-VFD ad claims I had read. Your information clarifies the basis for those claims, and gives me enough information to make the judgement call on how to go (if I get the saw, VFD). I really hope my thread title helps others who are interested in this question find the information you have posted here.