Bridgeport J head Spindle RPMs, 50hz or 60hz values?

The title is the question; on J-head the step pulley RPM high-speeds are 660/1115/1750/2720 RPM values, but the motor plate gives ratings for values both at 50hz and 60hz. While I've heard it talked about a lot around here with the presumption being these step pulley speeds are for 60hz, I've never actually seen anything which confirms they are for 60hz. So I would just like to pose the question to alleviate my concerns that I'm calculating VFD-produced RPM values correctly. Are the J-head pulley speeds on the front plate indeed for 60hz operation? Thanks!

Doh, and just like that I answered my own question as soon as I posted; I said wait a minute the pulley on highest speed is 1:1.5 ratio, the motor plate says 1800RPM @ 60hz. 1800*1.5=2700, and the front plate says 2720RPM, that must be 60hz! Sorry for the litter.

Chinese laser tachometers are dirt cheap and mine works fine. Snip off a piece of the included reflective tape and stick it to the machine spindle and you are ready to measure.

Digital Tachometer Non Contact Laser Photo RPM Tach Meter Motor Speed Gauge New | eBay

You probably know that induction motors do not actually run at 1800 RPM. Motor nameplates usually rate the HP at 1725 or 1750 RPM.

Larry

L Vanice said:

Chinese laser tachometers are dirt cheap and mine works fine. Snip off a piece of the included reflective tape and stick it to the machine spindle and you are ready to measure.

Digital Tachometer Non Contact Laser Photo RPM Tach Meter Motor Speed Gauge New | eBay

You probably know that induction motors do not actually run at 1800 RPM. Motor nameplates usually rate the HP at 1725 or 1750 RPM.

Larry

Click to expand...

Larry, no I did not know that--I had taken the value to likely be accurate--you're saying the thousands of users without tachs and who are using VFDs on BPs have been incorrectly calculating their RPM values using those name plates, uh oh! Oh wait, if we use '1740' as an arbitrary value within the range of what you specified, it turns out to be under 3.4% different from 1800RPM, so it probably wouldn't have caused too many people problems in their SFM range calculations. Why wouldn't the motor plates say what you say, from Bridgeport? Mine states 1HP / 3.5A @220V(3ph) / 60Hz / 1800RPM, on the plate.

Hey, I've got a quick question I'll squeeze into this thread before abandoning it. On the subject of VFDs; I have heard 'torque plot curves' referred to around here before when talking about the stock motor being run with a VFD. I think what most do, like myself, is run the tallest pulley config--the 1:1.5 (or 2:3) where the 2720RPM spindle speed occurs at, and if they need less, say 2000RPM, take 2000/2720=0.735 and 0.735*60hz=~44hz so we would set the VFD to 44hz and presumably run close to 2000RPM in the tallest drive pulley. OK, now about these torque curves for the motors--in the previous example our motor is at 44hz but at a torque limited pulley arrangement. It looks like the 3rd pulley step is close to a 1:1 drive, and it's rated at 1750RPM there. So what if we take 2000/1750=~1.14 and 1.14*60hz=68.6hz; how would we choose which arrangement would have more tool driving power? Torque is less at higher frequency, but there is a higher mechanical advantage at the point where the higher frequency would get used, so how could we calculate which setup has more power to the spindle?

I got my first machine tool with an induction motor about 70 years ago and have owned dozens more since then. I cannot recall ever seeing one with the RPM blank filled in with 1800 (the synchronous speed) instead of the rated speed.

Induction motors have a "synchronous speed" that is based upon the frequency (60 Hz) and the pole construction of the motor. The possible synchronous speeds are 3600, 1800, 1200, 900 and rarely, 600. In practice, the load and friction will make the motor run a little slower than synchronous speed. The speed difference is called "slip." When the motor designer/maker makes a motor of a specific power rating, like 1 HP, they assure that it will perform that value at a specific speed and then put that rated speed on the motor nameplate.

The common rule for VFD controlled induction motors is constant torque from zero to 60 Hz and constant power above 60 Hz. That said, there may be electronic tricks that can alter that relationship on the latest design VFD's.

If you want the greatest possible power at the spindle when using a VFD, use the pulleys to get the speed you want without setting the VFD below 60 Hz. A 2 HP motor at 60 Hz will be a 1 HP motor at 30 Hz. The motor will still have 2 HP at 120 Hz.

Here are a couple of websites that might help you understand what I wrote.

Variable Frequency Drive: Torque vs. HP - Kurz Industrial SolutionsKurz
Difference between base speed and rated speed of induction motor | GoHz.com

Larry

Just looked at my 1983 J head, motor by U.S. Electric Motors. Motor nameplate shows both 50 and 60 cycles with corresponding rpm's of 1420 and 1730.

Spindle speed placard shows rpm for 50 cy/60 cy

In direct they are: 550/650, 920/1115, 1500/1750 and 2300/2720

And in back gear: 67/80, 115/135, 180/210 and 275/325.

David

Last summer I did some calculations for feed and speed. Those speeds on the front plate were pretty close for common materials.
I use the old gear drive box that most people remove. The feeds on the drive box plate are close to what I calculated.
Up to then I was watching all these discussions with VFD's on BP's to get all the different speeds in between the original speed steps.
Something to appreciate but I'm going to live with what I have for a while longer.