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Question on Motor Cooling While Running on a VFD

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Plastic
Joined
Feb 28, 2018
Location
Michigan, USA
I have searched and read a few threads on this topic and seen a few things around the internet but would like to see if someone with some experience can clarify when I should add a secondary or separately driven fan to a motor run on a VFD.

To start with a little information, I am going to put a VFD in a manual lathe. The motor is TEFC, 3 HP, inverter rated, and has speed ranges of 20:1 VT and 10:1 CT. As it is in a lathe, it appears to me that the continuous torque number would apply. Does this mean that an externally powered fan would not have to be installed if I do not exceed this 10:1 speed rating? I am not against adding a fan, but don't want to put one in if it is not necessary either. This lathe is at another location and will not be moved for a few weeks so I don't have everything figured out as for as speed ranges but I am trying to get as much lined up as I can before its arrival. Off the top of my head I am thinking it will be run within the 30 Hz to 120hz range but I need to check the pulley ratios when I get access to the lathe again.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
Inverter rated motors require fans less often, if at all.

The point of added fan is that the internal fan becomes less effective with speed reductions. Under maybe 50% speed, the usual regular motor will not cool as well. It is hard to give a definite number, as the fans vary, from actual fans to cast in extensions on the rotor *, etc.

It also depends on the load. If you are running full torque (full current) at low speeds, you will probably need an external fan at higher speeds than if the slow speeds are also at reduced torque (current).

Part of "inverter rated" is improved cooling. You should be able to refer to the motor specifications to find the cooling requirements vs load and rpm on any "inverter rated" motor.


* the rotor extensions are actually pretty good, as they not only move air, they also act as cooling fins. The shaft-mounted fans have essentially zero cooling fin effect.
 

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Plastic
Joined
Feb 28, 2018
Location
Michigan, USA
Thanks for the reply, I checked over the lathe and as the motor is still running though a gearbox it looks like the frequency range required will be in the neighborhood of 29 Hz to 118Hz. I think I will proceed without a fan and monitor the motor temperature.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
I think you misunderstand the what the 10:1 ratio is, it is the range below the motor base speed at which the motor maintains constant torque, but has nothing to do with the cooling. Cooling depends on a multitude of factors, but in the application you are considering you should have adequate cooling down to 15-20Hz with the typical duty cycle of a manual lathe. This is also indicated by the specific motor manufacture. There are also limits on the top end, so although the mechanical speed of smaller motors is higher, there performance and cooling fan limitations, so inverter duty motors I usually set the top end in the 100-120Hz. Vector type motors have a much wider operating window, usually a constant torque ratio of 1000:1 or higher and the top end is around 200 Hz for a 4P motor with a base speed of 60Hz. They are usually TENV or TEBC, so cooling is not an issue, they also may have thermal overload sensors installed in the windings. These work well on mills where you need a wider speed range. In a gear or belt drive lathe there are other limitations as to the rated speed of the the drive train. Many factory installed VFD lathes are 2 speed design, the motor is up-sized to enhance the low speed operation.

Although your motor may have constant torque of 10:1 (down to 6Hz) the Hp declines in a linear fashion below the motor base speed, and you loose the mechanical advantage to the gearbox ratio, so you will have be limited ability in turning ability on larger diameter work at lower motor Hz (speeds) with the stock size motor.
 

markz528

Hot Rolled
Joined
Sep 25, 2012
Location
Cincinnati
Don't overthink it. Are you going to run the lathe hard at low speeds for an extended period of time? Most don't.

My lathe is a 3 hp 1800 rpm TEFC motor rated 4:1 constant torque. No matter what I do to the lathe, I don't blink. I never change the belt ratio (leave it on 3rd ratio out of 4 - higher number is faster) and I overspeed it some. The WEG drive runs it just fine with high torque even at minimum rpm. Torque does the cutting - not horsepower.

A good modern insulation motor will be just fine in your application. My good buddy is product manager for a large motor manufacturer, and he said I would not believe how much they had to abuse a motor to get it to fail during UL testing.

I require TEFC motors to get tested on a VFD at 4:1 speed. Granted these are large motors, but it is not uncommon for the motors to have to run 12 hours at rated torque and 25% speed before the temperature stabilizes.
 

JST

Diamond
Joined
Jun 16, 2001
Location
St Louis
........................... Torque does the cutting - not horsepower.

...........................

Sort-of.

Torque forces the cutting tool through the metal.

Power (HP) controls how fast that gets done. In other words how much metal you can cut in a given time.

A 1/100 HP motor could happily operate a big K&T mill (assuming you slow it down with gears etc, and overcome all the friction). And it could cut metal. It could have the exact same torque at the cutter as the standard motor. But, the speed at which the cutter moved would be so slow as to be un-usable for any normal purpose.

The difference in HP determines how fast you can force the cutter through metal, and so, how much metal you can cut.

Not only that, there is a difference in torque.

When you set for a high speed, and slow the motor, you lose two different (but related) ways.

1) You operate the motor at a lower maximum HP.

2) You lose the torque multiplication that would otherwise be available due to the mechanical speed change.

The result is that you must operate the cutter at a lower depth of cut, because your torque is closer to the motor torque itself, not multiplied by as large a speed ratio. AND, the lower HP provided to the cutter means that you end up removing metal significantly slower.

If you just had the lower torque, with the original HP, you would still need the lower DOC, but could run the cutter faster; net result about the same metal removal rate.

If that is unimportant to you, then knock your socks off, go for it.

Otherwise, you may want to think again.
 

ADV

Plastic
Joined
Feb 28, 2018
Location
Michigan, USA
Thanks for all the information. I should have mentioned that this lathe isn't going into commercial use. This lathe isn't something we would use at our shop. One of the shops I work with had this lathe with a known bad drive and were going to scrap it so I picked it up for my home hobby shop at a price that made it practical to put money into a drive.
 








 
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