ODP or TEFC?

[Scott Colba]

Hey Guys,

So I am getting ready to replace the single phase motor on my Woodfast M408 with a 3 phase and VFD, and I was wondering about the type of 3 phase motor I should get, regarding to its seal. ODP (Open, drip proof) or TEFC (Totally enclosed, fan cooled)? Normally I would think that TEFC is the way to go, but I was wondering if, since the motor is enclosed in the lathe leg, ODP would be ok since it keeps it out of the mess and dust? It's about a $40 difference in the motor price, so not a big deal, but sometimes every bit helps.

Thoughts?

Thanks!

 

[Rob Wallace]

Scott:

I would go TEFC without question.....for a $40 difference, this is a no-brainer. Spread that $40 out over YEARS of well-performing lathe use, and it really is nothing. The risk of having complications caused by infiltrating dust should be considered seriously, and the peace of mind of using a TEFC should pay for itself in short order. My concerns with a motor enclosed in the lathe leg would be one of heat build-up and heat dissipation when the lathe is used for long periods. I'd check the heat ratings specs of any motor I would choose to use to be sure it will dissipate sufficient heat in an enclosed space.

Bill Boehme, our resident electrical engineer, will likely have additional information for you to consider in selecting your motor.... .....TEFC wins from my perspective.

Rob

 

[Dale Miner]

What Rob said.

 

[Bill Boehme]

What Rob and Dale said. The enclosure really is not dust proof, it just reduces the dust a bit. If I am not mistaken, the Woodfast has ventilation slots.

Over time there will be some dust buildup inside of an ODP motor and that will reduce the cooling efficiency of the motor. ODP motors are really intended for use in a non-dusty environment and also they should be more or less open to free ambient air for adequate cooling.

Since you will be using this for inverter duty, there are some other motor parameters that are much more important than deciding between ODP and TEFC. These include insulation class and temperature rating.

Insulation class refers to the ability of the insulation of the motor windings to withstand high voltages without arcing between adjacent wires. This is a problem with motors used for inverter duty. Very high voltage switching transients are generated that can be well in excess of a thousand volts. The symptom of this kind of failure is a gradual decrease in performance over an extended time. Motor manufacturers have addressed this problem by designing motors that are rated for inverter duty that have heavier duty insulation that is able to withstand the voltage transients without breaking down the insulation. I would recommend trying to find a motor that has an insulation class of E or better (i.e., class F or G or H). Sometimes motors may be rated for inverter duty with an insulation rating lower than E. In those cases the manufacturer may have used high voltage varnish to encapsulate the windings which enables them to withstand higher voltages.

Temperature rating is very important for motors used in inverter duty. The reason is that at slow speeds, the motor is not getting adequate ventilation from its cooling fan. For this reason, having the motor in an enclosure is a bad idea because the motor will run much hotter than it would if sitting in open air. It would be a good idea to consider getting a motor with a built-in thermal overload switch (not a fuse, but a switch). Also closely monitor the motor temperature for a while to make sure that it is not running hot all of the time. Consider replacing the front panel of the motor enclosure with something that will allow more cooling air to the motor. Hardware cloth isn't very pretty, but it may be necessary if you find that the motor is cooking itself.

Unfortunately, you will find that motors designed for inverter duty cost somewhat more than general purpose single speed motors. Some woodturners decide to take a chance with the lower cost motors. How long they last is highly dependent up many factors such as the type of inverter, the speed and load, temperature, and phase of the moon which makes it impossible to quantify.

 

[Scott Colba]

Wow, fantastic information guys, thanks!

I tell ya, looking at it the way you do, Rob, it really is kind of a no brainer.

Bill, wow, that's great info! I am indeed looking at inverter rated motors, to the point of only looking at ones that are labeled inverter duty. This comes on the tail of (probably) killing the original 3phase motor of my '66 unisaw.

So given the information you mentioned about insulation rating, can I look at a motor that isn't explicitly described as Inverter Duty, but has an insulation rating thats or E or higher (though I would probably start with F or higher for one not named inverter)?

As an aside both of the inverter duty motors I am looking at are insulation range F. Also I looked a the bottom side of the lathe and there is a good sized opening, so not as enclosed as I thought. That's probably good for temp, but also so that I don't have to take the bed off to change the motor!

Thanks again for the great info!

 

[Bart Leetch]

Bill is there anything wrong with rigging up a small fan to blow on the motor so that even at lower speeds you would have a constant supply of air flowing over the motor?

 

[Bill Boehme]

... So given the information you mentioned about insulation rating, can I look at a motor that isn't explicitly described as Inverter Duty, but has an insulation rating thats or E or higher (though I would probably start with F or higher for one not named inverter)?

Scott, my guess is that any motor that you find with the desired insulation class rating will also be rated for inverter duty since the main driver behind going to the insulation that is able to withstand higher voltage spikes was to be able to withstand operating with an inverter. I would suggest checking with the manufacturer if you are not sure.

Bill is there anything wrong with rigging up a small fan to blow on the motor so that even at lower speeds you would have a constant supply of air flowing over the motor?

Bart, it depends on how you do it. No you can't just set a little fan next to the motor and let it blow across it because it most likely would only cool one side of the motor if even that much because the flow is mostly turbulent. If you look at the design of a TEBC (totally enclosed blower cooled) motor you can get an idea of what might work -- basically, it is a ducted shroud with a powerful blower at one end. The goal is to have laminar airflow along the axial direction of the motor. If that seems like more air than you would get with a TEFC (totally enclosed fan cooled) motor, you are correct, but a motor gets hotter when used in inverter duty because of higher current and lower efficiency at low speeds.

Here is an example of a blower cooled motor



The main thing is that you need fresh air to cool the motor adequately. If you have the motor in an enclosure that is mainly recirculating the same air to cool the motor then the air will get as hot as the motor and there won't be any cooling benefit. Motors perform best when located in an environment where the ambient temperature is about 75° F.

 

[Scott Colba]

Thanks again, Bill! Super helpful stuff!

I think, in addition to the inverter duty motor of course, I'll cut a hole in the upper part of the housing and add a large case fan or small exhaust fan, just to keep the fresh air moving through. That should draw the air from the opening below, across the motor, and exhaust out the top.