Teco N3 tuning: VFD running 2 hp SB13 lathe - running at > 60 Hz.

Hello!
I installed a Teco N3 VFD on a South Bend 13" lathe with a 2 HP 3 phase motor. This couldn't have been much easier. Bring in 220v single phase, wire the motor to the VFD, run Auto-Tune and it runs very nicely. I checked the settings it came up with and it's pretty much what the motor label shows.

Now I'm wondering: Just for kicks I set the max frequency to 100 Hz and gave the machine a spin with no load. The motor is spec'd for 60 Hz and it seems to run fine at 100 Hz, just faster.

Are there limits for "overclocking" a motor and running faster than the rated frequency? If 100 Hz is OK, what about 120 (double-speed). What happens at higher frequencies? I would expect that at higher frequencies, it's like putting a heavier load on the motor and it will generate more heat.

Anybody have experience doing this?

Funny, I did a Google search on VFD overclocking and found a thread on this forum that deals with just that topic:

http://www.practicalmachinist.com/v...over-speed-can-3-phase-motor-tolerate-214146/

Your torque is dropping, fast, at 100Hz and at 120Hz you can probably stop the shaft with your (gloved) hand. Also, you have no way of knowing if the bearings and cooling of the motor can handle that higher speed. Only the motor mfr can tell you for sure, everything else you hear is just an opinion or a guess and you are potentially trusting your LIFE to that answer should the bearings turn into shrapnel.

Jraef said:

Your torque is dropping, fast, at 100Hz and at 120Hz you can probably stop the shaft with your (gloved) hand. Also, you have no way of knowing if the bearings and cooling of the motor can handle that higher speed. Only the motor mfr can tell you for sure, everything else you hear is just an opinion or a guess and you are potentially trusting your LIFE to that answer should the bearings turn into shrapnel.

Click to expand...

Motors run on drives like that are constant hp above nameplate speed, so torque is not dropping 'fast' at all. It will go down
linearly as the speed increases above 60 cycles. Given that the motor is inside that cast iron base housing in that lathe, I
doubt there will be much shrapnel released even the entire motor housing grenades. Running at 100 cycles is pretty
mild overspeed. Cooling however is more of an issue if the motor is run well underspeed, under substantial load, for
a long time. Like, 10 hz or so.

If that's likely a muffin fan is a good idea on the motor housing.

OK, so if the motor is constant hp and torque drops off about linearly with increased freq, then this implies that at lower freq there may be more torque available but with a risk of overheating the motor, especially at very low speeds under heavy load. At higher speeds, the concerns are heating of bearings, perhaps some vibration if the balance is marginal and loss of torque.

I read somewhere of a guy who has run his vertical mill motor at 3x speed for years with no issues. Obviously, he hasn't suffered from any loss in torque, so he's not working the machine hard.

I'm loving this VFD!!

Torque remains mostly flat below the base speed based on the CT ratio, so for many conventional 3 phase motors this is 4:1 or about 15Hz, but Hp drops off in a linear fashion. Above the base speed torque falls off in a non-linear fashion, this is often described in the motor spec sheets and typical motor manufacture poop sheets. Typical smaller 3 phase motors may maintain constant Hp to about 1.5X their base speed, beyond that Hp drops off depending on the motor design. The base speed RPM of the motor also plays a factor in over speeding a motor, as well as a number of other limiting factors. A 1750 RPM motor often uses the same bearings as a 3500 RPM motor, but the latter cannot run at 2X its base speed. Inverter/vector motors, TENV, TEBC are designed to run over a wider speed range, a standard motor is not.

My usual recommendation using a standard 1750 RPM motor up to 5Hp is in using a VFD speed range of 20-90 Hz, inverter/vector motors usually are good for 20-180 Hz depending on the design as there are many variables. Sure you can try to run a motor to any speed you want, but running one at 3X its designed specifications for years would not be wise. On a lathe you need to change the belt drive ratio to optimize the motor speed window to spindle speed, you also need to account for the mechanical ratios and how they affect calculated Hp/Torque at the spindle. You loose too much Hp if you plan to run your lathe at 15 or 20Hz and expect to turn a heavy load, one reason why most factory VFD lathes up size their motors by 1.5-2x. You still need to adhere to the machines designed operating speeds no matter how fast you want to spin the motor. On a mill, you optimize top speed in accordance with the bearings and cooling, higher speeds are usually associated with smaller bits, so torques loss becomes less relevant. I use an inverter/vector motor on my lathe which I run from 20-125 Hz, the drive pulley ratio was changed to be close to it previous maximum speed (2000 RPM). This gives me a much wider dynamic operating speed range, typical of factory VFD systems for a smaller manual lathe would be 40-400 RPM and 200-2000 RPM. Going beyond this speed would lead to spindle failure due to lubrication limitations. My mill came with a factory VFD designed to run at 20-200Hz (40-5000 RPM with a TEBC motor) and I use a custom control system.

VFDs can provide a lot of functions/features, but you need to optimize the complete system and know the limitations. Your major limitation is the the lathe design, drive system and spindle bearings, if I recall the maximum speed is 1400 RPM. So the major benefit of the VFD in your case would the ability to adjust the speed dynamically, and to a lesser degree controlled acceleration and deceleration because of the drive/belt limitations.

Mksj:
Excellent commentary on motors and VFDs! Thanks so much.
Dan