Thread: 50hz versus 60 hz cyclone

Originally Posted by Chipman

From a purely scientific view, you guys have provided some interesting ideas. I wonder if anyone has ever measured the current draw for different flow rates for a given system. Hugh you said that yours was drawing 11 amps... can you block off some gates and measure the current again (SAFELY) and let us all know what happens.

I just did that measurement with my 1 HP DC. All gates open - current was 3.5 A, all gates closed current was 3.4A - what this shows is the bags are clogged and need cleaning. I'll measure it again next time I clean them.

Originally Posted by Chipman

From a purely scientific view, you guys have provided some interesting ideas. I wonder if anyone has ever measured the current draw for different flow rates for a given system. Hugh you said that yours was drawing 11 amps... can you block off some gates and measure the current again (SAFELY) and let us all know what happens.

I actuallly did this test on my blower (3~, 3hp, 400mm/16" impeller - http://woodworkforums.com/showthread.php?t=71557 ... cyclone still not completed). While measuring the current draw on one of the phases I placed a board over the inlet (completely blocked it off) and watched what happened to the current. Sure enough the current droppped by about 20% (if I remember correctly - it was a while ago and I don't know what I did with the figures recorded).

Thanks to those who reported on the measurements you did with your dust extractors open and closed...saves me doing the experiment

So the data in Hugh's table is correct.... for a given system that has less restrictions and back pressure, the current draw is greater.. I guess this is because it is actually lifting more air (instead of beating around in something like a closed box...is this something like cavitation?)

Chrisp, I have seen those relationships before and what is interesting, is that according to Hugh's tables, if the pressure is reduced, the CFM is increased and the power required increased. Actually, your idea of a belt drive gives complete control...you can fine tune the operation by changing the pulley and thefore the fan speed (provided it does not overload the motor and burn it out) Come to think of it, when I have worked in some factories (long time ago now) with blowers of different kinds, they often had a motor with a belt drive. I don't remember seeing a DC like that though.


Cheers,

Chipman

Originally Posted by Chipman

Chrisp, I have seen those relationships before and what is interesting, is that according to Hugh's tables, if the pressure is reduced, the CFM is increased and the power required increased.

I suppose what I was trying to point out is that while there is a relationship between pressure and CFM with various restrictions, the real question is how much effect does reducing the speed (50Hz vs 60Hz) have on the performance. I think Hugh is pointing out the effect of varying the restrictions (for a given speed) where as I thought I'd throw in the effect of speed. I thing both points are valid - bigger ducts will help compensate for some of the loss of speed.

I do often wonder if we in the 50Hz world are being conned with dust collectors at bit. I have looked at the specs of a well known brand that sells in both the 50Hz and 60Hz markets. The horse-powers and CFMs are specified identically on both the 50Hz and 60Hz models - and the part number for the impellers is the same - it just doesn't make sense .

I think belts could be a way of getting the air velocities up - something I understand is essential for good operation (read "fine dust separation") of a cyclone separator.

Chris

" I do often wonder if we in the 50Hz world are being conned with dust collectors at bit"

That's an understatement and a half, but only slightly more than the 60hz world
Airflows quoted are usually without any restrictions, and only acheived for seconds, before the motor overloads, and filtration is measured for blocked up filters with very little airflow.

For example, i've seen claims in adds for 1900 cfm from a 14" fan and a 3hp motor, and another for 2500cfm from a 3 hp (fan unspecified) both at 50hz, which are way higher than the real results for a 15.5" with a 5hp moter at 60hz.
Large grains of salt are required

I guess one advantage of the pulleys is that the motor could be inverted to give an extra 18" of headroom, but i wouldn't go to the trouble unless i needed to

I reckon the differences in CFM are caused because they use Chinese air to measure it instead of American or Australian air.

All this 50 - 60 stuff hertz. I was just thinking and no that didn't hurt.

I thought that almost all consumer induction motors 220 volts and up were 50/50 Hz motors these days.

And then, being even more anal retentive, doesn't the frequency of the AC line determine motor RPM on induction motors? Basically aren't the motor windings the governing factor for RPM? It is possible to wind a motor to operate at half speed.

As I understand it, there is a loss of efficiency and what should be 3600 RPM usually works out to be 3450 on 60 Hz. Wouldn't one expect a 50 Hz motor to operate at 2875 RPM?

I've also been told that a true 60 Hz motor running on 50 Hz power will run hotter due to less inductive reactance. That also implies that the motor operating on 50 Hz will be less efficient.

Originally Posted by rrich

All this 50 - 60 stuff hertz. I was just thinking and no that didn't hurt.

I thought that almost all consumer induction motors 220 volts and up were 50/50 Hz motors these days.

And then, being even more anal retentive, doesn't the frequency of the AC line determine motor RPM on induction motors? Basically aren't the motor windings the governing factor for RPM? It is possible to wind a motor to operate at half speed.

As I understand it, there is a loss of efficiency and what should be 3600 RPM usually works out to be 3450 on 60 Hz. Wouldn't one expect a 50 Hz motor to operate at 2875 RPM?

I've also been told that a true 60 Hz motor running on 50 Hz power will run hotter due to less inductive reactance. That also implies that the motor operating on 50 Hz will be less efficient.

Rich,
I'm not an expert by any stretch but I believe that most of the inductive reactance of a motor comes from the magnetic field, that the permanent magnet in the armature creates, cutting the field windings. That is a much stronger force than the field of the alternating current in the field windings themselves. Our 5hp motor draws 20.8 FLA but it draws 105 amps under a locked rotor condition. This says that most of the inductive reactance happens when the motor is spinning up to speed. I could be wrong on this.

I thought that almost all consumer induction motors 220 volts and up were 50/50 Hz motors these days. Did you mean 50/60HZ? If you did, the Leeson motors aren't rated at a dual frequency and as I posted before in the other thread, I believe it's because of the centrifugal start switch which opens at 75% of full RPM. 50 Hz motors running 20% slower only give that switch a 5% margin of error which Leeson obviously thinks might be cutting it too close.


Ed

When I went to school (long ago)
all our power was 50hz exactly.The power generators had to be doing 3000 rpm and a few other tech things before joining the power grid otherwise a big bang occured.
Australia supplied nom.240volt single phase 50hz power to houses,
The 50 hz = 50 cycles per second.
If you have a 2 pole motor
50 c/s x 60 seconds (in a minute ) = 3000 rpm minus losses such as friction etc etc.(giving around 2800rpm)

50 c/s x 60 seconds = 1500 rpm - losses (giving around `1400rpm)

There is a difference between a induction motor and a permanent magnet (aka synchronous) motor.

The idea that we have a rotating or changing magnetic field at a speed controlled by the mains frequency is correct. A permanent magnet motor will run exactly at the mains frequency = synchronous speed (3000 RPM, 1500 RPM, etc, depending upon the number of poles in the motor).

Induction motors reply on induction to produce the magnetic field in the rotor. These motors are also called "squirrel cage motors" as the rotor consists of two copper rings with copper bars strung between them (hence the squirrel cage name). The whole lot is often cast in aluminum. The changing magnetic field in the stator induces a current into the rotor (think of it as a transformer). The induced current produces a magnetic field in the rotor. This field then serves much the same function as that in a permanent magnet motor - forces the rotor to turn and try to keep up with the moving field.

However, if an induction motor actually reaches the synchronous speed, the induction ceases as it needs a difference between the rotating (mains generated magnetic field) and the motors rotor speed for the induction to occur. Therefore, in an induction motor, the rotor has to "slip" (relative to the mains synchronous speed) in order for there the be a magnetic field in the rotor. This "slip" is often of the order of 4% at full load on an induction motor. Hence, induction motors run at about 1440 RPM on a 50 Hz supply.

Chrisp has got it right

By the way, I did a simple experiment with my Physics students at school today... we measured the current drawn by a 12V computer fan.. Completely unobstructed, the fan drew 0.27A and when completely blocked off on both sides, the current fell to 0.24A. Blocking off on one side had almost no effect. The voltage also rose slightly as expected when blocked off. This is consistent with the DC's drawing less current with smaller ducting and or finer or blocked filters.


Cheers,

Chipman

Originally Posted by hugh reid2

For example, i've seen claims in adds for 1900 cfm from a 14" fan and a 3hp motor, and another for 2500cfm from a 3 hp (fan unspecified) both at 50hz, which are way higher than the real results for a 15.5" with a 5hp moter at 60hz.
Large grains of salt are required

Are those figures with a cyclone inline?

Blade depth and angle is another thing to consider about fan design, I would imagine a well designed 14" fan with bigger blades(more surface area)and a more aggressive angle could potentially flow more CFM than a 15" given the same RPM's?

They never tell you the circumstances of the CFM claimed, just that they somehow acheived those results.
A few qoutes from Bill Pentz-

"Hobbyist vendors and magazines test our blowers running without cyclones and filters or ducting. This means these blowers are moving at close to maximum airflow at minimum resistance. Since almost all motors are designed to handle four or more times their running loads while starting, vendors can get away with this testing for a short while before the motor overheats and fails. This makes for a mess for small shop woodworkers trying to make an informed purchasing decision. Those test results often show air volumes that will quickly burn up our motors from trying to move too much air. The bottom line here is any hobbyist fan table that does not also include amperage draw at each resistance level is not to be trusted because the blower more likely than not is running the motor well over its rated maximum amperage!

"Because blower technology is mature, blowers of the same type and size provide near identical performance regardless of which commercial vendor we use. This means we can look at any good commercial fan table and use that table to size our impeller"

"Although blower housing, impeller height, and blade angle do come into play a little, the primary performance factor is impeller diameter."

"Although blower housing, impeller height, and blade angle do come into play a little, the primary performance factor is impeller diameter."

I do not agree with you on that one, thats like saying all 5ltr V8s have 500hp... one end of the spectrum you have 5ltr windsors and 5ltr holdens who have around 200hp then at the other end you got BMW/Mercedes 5ltr's who output is 500hp+... chalk and cheese!