certain kind of motor to hook up to a vfd

so I will be buying a 5hp motor to hook up to a vfd that will be install on my wooden lathe
(I know its over kill)
so my question is since theres so many different typeS of motors out there what should I look for on the nameplate that will be fine to install with a VFD?
I have 3 phase power that is run from a RPC (15HP)
also what type of vfd is good to buy ?

thank you

It depends how much you want to vary the motor speed. A VFD (inverter) rated motor has better insulation on the windings that allow it to run at a very low speed without damaging the windings. A standard 3 phase motor will work fine if you are not going to run below around 50% of rated motor speed. Automationdirect.com is a good source for both motors and VFDs. You can compare prices for standard and inverter rated motors there.

thank you for the quick reply
so lets say the top rpm for the motor is 1800 rpm
but sometime I will be running it at about 300 rpm. so it would be better to get an inverter motor. (right)

since I will try to look one on craigslist how can I tell if it is an inverter or standarn motor?

also what do I look out for on the nameplate?

thank you

Most of the "inverter duty" motors I've seen have it printed right there on the name tag, not in small print, usually large font, sometimes just a sticker on the motor.

You'll want a service factor of 1.15 or better to get the full 5hp out of it. As mentioned inverter rated motors do have better insulation to deal with inductive voltage spikes from the harsh switching of the VFD, but I don't think that is as big of a deal.

What you do want to find is a motor with a good turn down ratio. This is the ratio of running speed to rated speed that full torque can be used without the motor overheating from the cooling fan spinning too slowly. A 10:1 turndown ratio will allow you to use a 3600 RPM motor at 360 RPM. 20:1 would go down to 180 RPM. It is wise to leave some headroom here if you can for longevity, since this directly relates to the motor's running temperature.

ODP (Open drip proof motors) may have better turndown ratios since they blow air directly across the windings, but those motors cannot be used in a dusty environment. You will need a TEFC (totally enclosed, fan cooled) motor for this application.

I am not affiliated with them in any way, but I get most of my motors from surpluscenter.com , simply because they have new surplus motors for fairly cheap. Their listing generally will not state the turndown ratio, so you will have to search the model number and find more information once you have some motors narrowed down. I am not sure of your budget.

I would oversize the VFD a little bit and run it off of single phase, not the RPC. The poor voltage balance of RPCs combined with the rectification stage in most VFDs mean current balance may be so terrible that it's practically single phase anyway. There are many rules of thumb for oversizing VFDs for this case, but the actual effect it has on the VFD depends on a lot of internal factors, so you need to consult the manual or manufacturer of actual VFDs you are considering to know what size you need to buy.

Strostkovy said:

I would oversize the VFD a little bit and run it off of single phase, not the RPC.

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I agree with this.

A few points:
- a good cast iron industrial motor will generally have a 10:1 variable torque and a 4:1 constant torque spreed turn down rating - a lathe is considered a constant torque application. However this assumes a 104 degree F ambient temperature and at 100% amps. Reduce the ambient temp and you can run the motor harder. Heating in the motor from the amps is a square function so reduce the amps some and you can run the motor that much harder. Run it at 50% amps and you can run it quite slow with no worries.
- a true VFD motor will have an external blower so that the motor has the same cooling at all speeds. The 3 hp spindle motor on my cnc has a muffin fan on it instead of a shaft mounted fan.
- I would not worry about voltage spikes hurting the insulation in a modern industrial quality motor. Keep the leads as short as possible.
- the service factor is to compensate for power quality variations when the motor is run across the line. A small voltage variation causes a larger current variation causing the motor to have to be derated - hence the service factor. You don't have a power quality issue when run on a VFD as volts and amps are balanced but additional harmonics cause heating in the motor so a 1.15 service factor motor becomes a 1.0 service factor motor when run on a VFD.
- you want a motor with class F or H insulation

Sounds like the 5 hp motor is oversized for the application. If so, buy a VFD rated for 5 hp single phase input and call it good!

I recently shopped for a 5 hp 1800 hp 3 phase cast iron industrial duty motor. It was around $350 from a local dealer.

This issue has recently come up for me as well. Would it be a workable situation to add an auxiliary fan to a motor? I have a 2hp 3ph motor that I wish to put on a drill press using a vector drive. It has cooling vents on both ends of the motor case. The vents are small for a motor this size, and I don't see a fan on the shaft. I will be using this primarily for metal, turning hole saws and larger drill bits. I need to have a low rpm capability. Could I put a muffin fan on one end to push air through the motor. I looked for an answer to this online, but didn't find very much.

chancey47 said:

This issue has recently come up for me as well. Would it be a workable situation to add an auxiliary fan to a motor? I have a 2hp 3ph motor that I wish to put on a drill press using a vector drive. It has cooling vents on both ends of the motor case. The vents are small for a motor this size, and I don't see a fan on the shaft. I will be using this primarily for metal, turning hole saws and larger drill bits. I need to have a low rpm capability. Could I put a muffin fan on one end to push air through the motor. I looked for an answer to this online, but didn't find very much.

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Yes you can, but is the motor really getting hot? Remember that surface temperature is not a good indicator of internal temperature. The motor can run safely a lot hotter than you think. I have motors running at 120 degrees C surface temperature that are still running with spec - class F insulation on a VFD.

If you do add the fan, you need an fan housing to direct the air across the motor. You can also make a metal round shield that goes around the motor and the air is pushed between the motor and this shield. This is the most effective way to cool a motor like this as the motor sees all the air.

There is a BIG price jump in the VFD cost to go from 3HP to 5HP 230V if you want a single phase input. Most VFD manufacturers offer 3HP max without needing to de-rate the VFD for single phase input. A few people MARKET a 5HP version, but what they are really doing is selling you a 10HP drive with a label that says it is a 5HP single phase input (which is what you would do anyway). You can tell by comparing the prices for both of versions and see that they are identical. So if 5HP is overkill, I would consider sticking to 3HP. On a wood lathe, you can hurt yourself with anything over 1HP...

And beware of the cheap crap Chinese drives (that must remain nameless here) on Fleabay and the like that CLAIM to be rated for 5HP single phase input. They are LYING about that and don't care at all that you find out only AFTER you buy it... The fly-by-night resellers don't understand English, let alone the technology and they have mistranslated something, but they don't care (or are doing it on purpose to dupe people), then they disappear when they sell out their stock.

A would opt for a totally enclosed motor, a TEFC the cooling probably becomes an issue below around 15 Hz, a TENV would be more optimal if going below 15 Hz. A TEBC may be more ideal over a wide RPM operating range, but much more expensive and larger. Most newer inverter motors have a something like 10:1 constant torque so that would not be an issue (vector motors are 1000:1 or higher), but at 300 RPM you are only getting around 0.8 Hp from a 5 Hp motor. Most 4P inverter motors in the 3-5Hp range will spin up to 2X their base speed and vector motors to 3X their base speed and maintain their Hp. So you would be better off changing the pulley ratio of the wood lathe to over speed the motor to something like 2 X its base speed (or 3X with a vector motor). If you are using a 3600 RPM motor currently, then go with an 1800 RPM motor and over speed it. Typically at speed you will be turning smaller diameter work and torque is not an issue, low speed larger diameter and torque/Hp is an issue. I would look at 2 options based on the pulley size ratios you can fit.

First would be to use a 3 Hp vector motor and use it from something like 30-180 Hz. You will have 1.5 Hp and full torque at 30 Hz, full Hp at 60-180 Hz. The cost of a 3 Hp VFD is about 1/3-1/2 of a 5/7.5 Hp VFD and the motors can be purchased for around 600-700 or less. I purchased the motor below for a lathe (accepted a lower price), also look at the BlackMax Y541 and the Baldor IDNM series. The physical size of the motor will be smaller and shipping costs less.
LINCOLN LM06119 INVERTER DUTY ELECTRIC MOTOR 3HP 1800RPM 3PH 60HZ TENV 184TC | eBay

Second would be a 5Hp inverter or vector motor, so consider something like TENV or TEFC, run the motor from 20-120 Hz. At 20 Hz you will have ~1.7 Hp, but much more torque. There are lots of 5 Hp motors in this category at a price close to the 3 Hp above, but they are larger and heavier. There is the option of going with a rolled steel case TEFC like the Baldor below which would work fine in this application and are less expensive and weight/motor frame is less then say a cast iron TENV motor.
LINCOLN LM06125 INVERTER DUTY MOTOR 5HP , 213TC , 3 PHASE , 1800 RPM , TENV | eBay
EM3615T BALDOR 5HP MOTOR 36G271S268G1

VFD options, as others have mentioned you would be looking at a 3 Hp then something like the WJ200-022SF, Teco A510 or E510 (E510-203-H1FN4S-U is their NEMA 4/4X/12 Series), Fuji Electric, etc. These range between $300-500 based on the enclosure rating for a 3 Hp single phase input. If using a 5 Hp motor, I would go with the 7.5 Hp 3 phase input VFD and derate for single phase. Yaskawa does make a single phase input VFD, CIMR-VUBA0018FAA that I have used, so also an option but you are looking at the $600-700 range either way you go and you need a larger service and associated costs. You will probably want an external braking resistor for the VFD, in particular if you are turning heavy materials.
Yaskawa CIMR-VUBA0018FAA, 5 HP, 200-240V, VFD
Hitachi, WJ200-055LF, 7.5 HP, Variable Frequency Drive 230 Volt, 3 Phase Input, IP20, at Dealers Industrial

With 3 phase input VFDs running on single phase or off of a RPC, using a DC buss choke is recommended to diminish the increased harmonic distortion and also decreases the input fusing requirements due to the current peaks. You are not using the VFD at it's maximum ratings for any length of time, so there is a wider margin of reserve for the VFD in this application so a VFD derating of 1.5 for single phase input is acceptable.

IMO, there's way too much complexity here for what this machine needs. It sounds like he's running a 5hp motor to a wood-turning lathe spindle... that's a lotta flying wood.

My experience has been that darned FEW motors are unworthy of VFD operation. It also has been, that for most applications, under-driving the load, and programming the VFD for overspeed operation yields incredible performance all around. I typically run a 3:1 or 2:1 drive ratio, and set my VFDs to run up to 120 or even 210hz to get the tool speed back up into, and in some cases ABOVE what the original setup offered.

I yank ALL my VFD-driven motors' cooling fans, and attach constant-speed fans on the housings. This solves the low-speed cooling flow problem, and eliminates the air-raid-siren wail of a 1800rpm motor spinning at 6500rpm. Instead, it's all calm, cool, and collected at every speed.

For powering some of my machines, I have VFDs that accept 240 single. I have some that weren't 'advertised' as accepting single, but they do, and in both cases, I 'de-rate' them a bit to prevent overload of the input bridge.

For those machines which require more spindle power, I use a dry transformer wired backwards to boost my 240v mains to 480, then I feed 480 single phase into a 480v VFD, and wire the motor for 480, program it, and call it good.