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Boucher
07-29-2012, 06:02 PM
How do they come up with 1725 rpm three phase motors? All the ones that I encountered in the water well applications were 3600 until the VFDs came along.

John Stevenson
07-29-2012, 06:14 PM
1725 is a 4 pole motor, 3600 is a 2 pole motor

flylo
07-29-2012, 06:19 PM
I've seen RPCs made from 3500RPM motors but have only used 1725rpms. 1725 RPMs are plentiful here. I just bought a new 15HP for $150. A companies spare stock.

MaxHeadRoom
07-29-2012, 09:08 PM
How do they come up with 1725 rpm three phase motors? All the ones that I encountered in the water well applications were 3600 until the VFD’s came along.

Any induction motor can never run at synchronism, on 60hz that would be 1800, but factor in the slip frequency and you arrive at 1725 for a 4 pole motor, a 2 pole motor the sync freq would be 3600, but is usually around 3420rpm.
Max.

J Tiers
07-29-2012, 09:35 PM
Expanding on that.....

The rotor needs a magnetic field in order to develop torque, but it has no way of connecting wires to it to provide a field.

What happens is that the difference between the synchronous speed (1800 or 3600 rpm) and the actual speed is used to generate a current in the large coarse 'windings" of the rotor (the aluminum or copper bars you see cast into the rotor). That current provides the magnetic field, and thus the torque.

So if the motor ever went at 1800 or 3600 rpm, the rotor magnetic field would disappear, and the motor would slow down until there WAS current in the windings sufficient to produce a field that will run the motor at the load applied.

As the load becomes heavier, the required magnetic field increases, and the rotor slows down a bit more until a balance is reached between the load and the added field produced by slowing the rotor. At some point, there is no added field from additional slowing (due to slip frequency vs inductance) and the torque "breaks down, stalling the motor.

The rated speed (1725 rpm, 3450 rpm, etc) is at the rated torque /power of the motor. At less load it will be faster, at overload it will be slower.

Lew Hartswick
07-29-2012, 09:43 PM
Along these lines: Since for single phase 60 Hz 3600 synchronous speed the
motors are referred to as "2 pole" and the 1800 as "4 pole" WHY aren't the
3 phase synchronous ones referred to as "3 pole" and the 1800 as "6 pole"
Or should that be 6 pole and 12 pole? :-)
I'm serious, it just bothers me to see this sort of "inaccuracy" in a technical
subject.
...Lew...

MaxHeadRoom
07-29-2012, 10:31 PM
What 3ph synchronous are you referring to? the formula is 60xhz/pole pairs.
This also applies to synchronous motors, IOW if you (could) run a 8 pole, P.M. AC servo on 50hz, it would result in 60x50/4 = 750 rpm.
Max.

flylo
07-29-2012, 11:48 PM
Is that why 3 phase inverter duty 3HP Daytons are 1110RPM & 12 wire? I have 8 new ones weighing 115# ea.

lakeside53
07-30-2012, 12:22 AM
Yes, that's a 6 pole. Accessible wire count is a different issue. The 12 wire configuration just allows you a lot of options for voltage, starting and run.

Blackadder
07-30-2012, 03:47 AM
if you excite the rotor with an external power source you can run at sync speed but these are very special motors , we used to use them at Stanton ( for JS ) to power the large compressors and as a side effect if you over excite them the can be used for power factor correction, but by ek did they take some starting up these like a few more were the domain of the duty sparks to start up

other were the ac/dc converters these had open knife switches ( the live part were not covered up ) and most of the 3kv up pumps

Stuart

J Tiers
07-30-2012, 08:29 AM
Blackadder... that would be a synchronous motor...... same as a generator (alternator), just run with power going IN instead of OUT.

They do not self start unless started as an induction motor using damping bars in the end of the poles......

Blackadder
07-30-2012, 08:38 AM
sorry for not using the correct term I do know but after 20 attempts to spell it ( I have dyslecsia ) and its bad today

we used to get them up to speed with a donkey motor

Stuart

MaxHeadRoom
07-30-2012, 09:44 AM
if you excite the rotor with an external power source you can run at sync speed but these are very special motors , we used to use them at Stanton ( for JS ) to power the large compressors and as a side effect if you over excite them the can be used for power factor correction, but by ek did they take some starting up these like a few more were the domain of the duty sparks to start up

other were the ac/dc converters these had open knife switches ( the live part were not covered up ) and most of the 3kv up pumps

Stuart

I converted a few of those old compressors from the original rotary DC excitation generation to a solid state P.S.
These were about 100hp, single lung compressors and they were run up as a induction motor and the rotor winding was used to detect the slip frequency, when the slip came down to below ~5cycles, the DC was injected and they then came up to synchronism.
Max.

Rosco-P
07-30-2012, 09:50 AM
1725 is a 4 pole motor, 3600 is a 2 pole motor

6 pole motor, found on some older machinery 1200rpm (1140rpm approx.)

cameron
07-30-2012, 10:30 AM
Are, or were, more than six poles used on standard commercial motors?

MaxHeadRoom
07-30-2012, 11:05 AM
The lowest I have seen is 8 pole 855 rpm on 60hz.
You can get synchronous motors which resemble its DC stepper cousin, these run at 72rpm on 60hz, these start and stop within 1.5 cycles, they can also be stalled with no damage.

Max.

lakeside53
07-30-2012, 11:17 AM
If you want lots of poles, look at overhead fan motors.

Hopefuldave
07-31-2012, 04:50 PM
There are, of course, multi-speed induction motors too - they tend to be pretty bulky though, the 3HP motor in my lathe is nearly the size of an oil drum and a definite 2-man lift! It has a 4/8 pole winding for high and low speeds (and a proper Frankenstein switch for arranging the connections) and a 6-pole middle speed winding - the switching gives (approx') 1450, 950 and 700 RPM switchable on the fly without messing about with gears or pulleys, constant HP (and nearly constant full-load current) in the three speeds - add a VFD and I can run spindle speeds from about 5 RPM to 2300 RPM :D
The Intensely Curious can google "dahlander pole-switching motor" for far too much info!

Dave H. (the other one)

Lew Hartswick
07-31-2012, 06:01 PM
The Intensely Curious can google "dahlander pole-switching motor" for far too much info!

Dave H. (the other one)
NOW you tell me. :-) I spent quite a bit of time figuring out the connections
and how one of those worked on a 24inch wood planer for a fellow at school
about three months ago. That delta star switching is really slick.
...Lew...

MaxHeadRoom
07-31-2012, 07:40 PM
Star-Delta starters have been around for a long time, decades ago this consisted of a large manual operated starter, now done with contactors etc.
http://myelectrical.com/notes/entryid/83/motor-starting-star-delta
Max.

John Stevenson
07-31-2012, 07:59 PM
Are, or were, more than six poles used on standard commercial motors?

Quite a few machine tools have made use of this feature for speed change instead of a gearbox.

Holbrook was one and they had a special motor that was 8, 6, 4, and 2 pole that gave 4 speeds.
With a geared back gear arrangement that made a useful machine with 8 speed ranges.

There was a range of pillar drills that did the same bt the name escapes me, might even have been Herbert or one of their spin offs.

J Tiers
07-31-2012, 11:10 PM
Multi-speed motors are usually also multi-power motors.... each speed has a different power, and the slower speed isn't necessarily the most powerful.........

has some of the disadvantages of a VFD turned down..... without the variable aspect to counterbalance it.

John Stevenson
08-01-2012, 04:15 AM
Multi-speed motors are usually also multi-power motors.... each speed has a different power, and the slower speed isn't necessarily the most powerful.........

has some of the disadvantages of a VFD turned down..... without the variable aspect to counterbalance it.

What Jerry says is very true but the designers of the day were running with what they had, especially on a performace / cost basis.

The first viable speed units I bought and fitted were DC units as the AC inverters of the day were expensive, unreliable and didn't give the same results as DC units.

Time and development has moved on though and if we had the inverter technology before machines like the 2J varispeed Bridgy would not have need to exist.

My Beaver CNC which was built towards the end of that era, late 1980's, and was not shacked by cost as much had a large oil cooled Danfloss inverter that looked like a stick welder in size, weight etc and this drove a 7 HP motor thru a two speed timing belt arrangement via air clutches. This was all that was needed to go from 200 to 4,500 rpm.

Fast forward to today and if Ii was buying a Bridgy type machine [ Hawk - spit - ding ] I'd go for the step speed in a heartbeat and fit an inverter. Far more quieter and reliable

J Tiers
08-01-2012, 08:29 AM
What Jerry says is very true but the designers of the day were running with what they had, especially on a performace / cost basis.



Absolutely..... and I am NOT "knocking" it..... it is a very good solution, and you can usually trust the designers to have been aware of the issues and to have countered them to the best of their ability.

I was actually thinking more of a person looking at a machine and thinking "I'll just put a two-speed motor on this and everything will be great"..... Maybe it will, but one needs to consider the various factors before one KNOWS that it will be better. No sense putting on a fancy (and expensive) motor only to find that there are "issues" that were not anticipated.

In engineering (and machine powering falls into that for sure) a little calculation and planning is generally a whole lot better than a 'cut and try" approach... despite the fact that engineers may use the latter as well.

Hopefuldave
08-01-2012, 05:52 PM
Multi-speed motors are usually also multi-power motors.... each speed has a different power, and the slower speed isn't necessarily the most powerful.........

has some of the disadvantages of a VFD turned down..... without the variable aspect to counterbalance it.



Well, my Holbrook Model C's 3-speed is a huge BTH, with 3 HP in each speed, the current does vary a bit (so I assume the efficiency's down a bit on the lowest speed range), but it's on the nameplate as a "constant HP" motor! Along with the 8:1 backgear and a (roughly 5:1) 2-speed clutch/gearbox arrangement, it gives a standard range of 22 to 1500 RPM with 3HP at all of the 12 speeds (and with the VFD, a 7.5:1 range in each speed, although the power does drop off away form the designed 50-60 Hz). The backgear does mean I have to stop the spindle though, as the ranges don't really overlap (unless the VFD control's twiddled!) - so with backgear it's six speeds roughly 22- 190 RPM, then direct's 170 - 1500 RPM - then from roughly a fifth of the speed on the plate, up to 1.5x (both extremes for really short periods, though!)

John S, I assume the 4-speed motor was in a Holbrook Model H No.-whatever, as the Minor / Major etc. had (as far as i know) DC motors with electronic speed control?

Dave H. (the other one)