Are they typically inspecting randomly or is this during a scheduled inspection for something else?Originally Posted by Frank Pratt
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Are they typically inspecting randomly or is this during a scheduled inspection for something else?
Contributor
Let me just point out something, in case you're not aware of it. Canada's electrical system is the same as the US. They bring 240 volts into a house with a center tap grounded. So the voltage to ground is only 120 volts, even when you're using 240 volts.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Contributor
That would be at a scheduled inspection where permitted work is being done.
Contributor
Good point Mike, thanks.
Our two low voltage 3 phase systems are 600/347 and 208/120 volts.
The 208 volt system can also be used in a dwelling, Inhave a friend with a 208v 3 phase service at his house….Rod
Contributor
Rod
Why the 208vac, 3 phase for a house. Electric heat or ? Would have to be a sizeable load as locally here the Electric company will keep you on single phase up to 600 amp before they will talk about 3 phase even if three phase runs right past your property
Ron
Contributor
Just a detail, running your motors at 60 hz is not a good idea. If you use a vfd it can probably be set up to deliver a maximum of 50 hz.
Contributor
Actually, running a 50 Hz motor at 60Hz is not a bad thing. A 50 Hz motor has to have more iron in the core because of the lower Hz but that doesn't hurt when you run it at 60 Hz. Going the other way is more of a problem.
A 50 Hz induction motor run at 60 Hz will run faster. A 2 pole 50 Hz motor run at 60 Hz will run about 3450 RPM. At 50 Hz it will run about 2875 RPM.
On airplanes, for example in WWII, they used 400 Hz motors because they were much lighter (didn't require as much iron in the core).
Mike
Last edited by Mike Henderson; 10-10-2021 at 1:30 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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I assume this is similar to what's done in Europe. They bring three phase power into the area and use three transformers to step the voltage down. The secondary is configured in a Y, with 208 volts from line-to-line and 120 volts from line to center. The center is grounded for safety.
In Europe they provide residential power that way, but the line-to-line voltage on the secondary is 385 volts and the line-to-center is 220 volts. The center is grounded for safety so only one wire of the 220 volts is "hot" to ground.
But I suppose that's not what you really asked.
Mike
Last edited by Mike Henderson; 10-10-2021 at 1:39 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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I'm curious: will you have this shipped from Europe to Canada? Isn't that a big issue even before getting the juice right?
Contributor
Huge house, about 25,000 square feet plus a hobby shop.
Backup is a 100Kw Cat diesel with hospital grade silencing, he has lots of money….Rod
Contributor
Thanks Rod & Mike
Contributor
Mike
I am been seeing VFD's advertised that will go to 120hz. Wondered about over speeding 60hz motors but never looked farther into this.
Getting ready to put a VFD on a 18" bandsaw so I can run it on single phase power. (Planed on using a rotary phase converter but ordered and paid for 2 months ago and still not here and no answer to emails.) Would I be able to change drive pulley to slow down the saw then use the VFD to increase the HZ to bring back to original speed when cutting thin stock and slow down to 60 hz when resawing to have more power available to cut with? Also would this work to make the saw capable of cutting metal?
thanks
Ron
Contributor
I don't think I can answer all your questions. When you put a VFD on an Induction motor, and slow it down, the torque generally remains constant. But since HP is torque times RPM times a constant, as you slow down an induction motor, the horsepower decreases. The motor will have a fan for cooling and the volume of air moved is directly related to the RPM. So at 30 Hz (half RPM), the volume of air moved will be half the volume at 60 Hz. But if the motor is getting hot in your application you can put an external fan blowing on the motor.
When you overspeed an induction motor, the torque decreases (above the rated Hz) but the horsepower tends to remain constant. So over-speeding by raising the Hz beyond 60 on a 60 Hz motor will not get you any additional power.
When you under-speed the motor, you will lose some power - the motor will not have nameplate horsepower any more.
A VFD is an excellent way to modify the speed of an Induction motor but you have to understand what's going on to use it properly.
If you need full horsepower at a lower RPM, the best way is to use pullies to slow down the blade. When you use pullies to slow down the RPM, the torque at the work pully is increased sufficiently to keep the rated horsepower. (HP is torque times RPM times a constant)
Remember that torque is just a measure of force and horsepower is a measure of work. How much you can do is dependent on the horsepower, not the torque.
Mike
Last edited by Mike Henderson; 10-10-2021 at 10:56 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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The HP = torque x RPM x a constant explains why induction motors stall so quickly.
As you load an induction motor, it will eventually slow down. As it slows down the torque generally remains constant but the HP decreases. Since there's not as much HP the tool will not be able to keep doing the work and the motor slows down more. This happens very rapidly and the motor stalls.
On old Unisaw table saws, they put in a very small induction motor - maybe 1HP - but used a repulsion/induction motor instead of a plain induction motor. The 1HP induction motor would stall very easily but as it began it's death spiral, the repulsion portion would cut in and provide additional power to keep it turning. You couldn't keep cutting at that RPM - the motor would burn out if you did - but it allowed the operator to recover - to pull the work back and allow the motor to get back up to speed, then push the work back into the blade as a slower speed. Note: the repulsion portion increased the HP but required additional current (more power in to get more power out) and the motor would overheat at that current level.
Could you do more than 1HP of work on one of those old Unisaws? No, it's still just a 1HP motor but it improved the operator experience.
Once you had larger motors the repulsion part of the motor was not of much use, and it added cost to the motor and required maintenance (brushes).
Mike
[In actual motors, the motor doesn't stall as soon as it reaches the nameplate HP. It will actually supply a bit more than nameplate HP (service factor) but at increased current, which will overheat the motor if the loading is continuous. Some slight overloading and overheating , say 10 degrees C, will just decrease the life of the motor. More than that can cause the motor to smoke and completely fail.]
Last edited by Mike Henderson; 10-10-2021 at 10:56 PM.
Go into the world and do well. But more importantly, go into the world and do good.
Contributor
Thanks Mike
I originally wanted to just use a Rotary Phase Converter, however can't seem to get the control panel that I paid for.
Have been advised to buy a VFD to power up and supply 3 phase power and was also told that I could speed up and slow down the saw to make it handier to use. Just getting into resawing once I get this bandsaw working.
I first worked on/with VFDs in 1990 and was trained on setting them up and operating them, 250hp driving 2 125hp motors for HVAC fans. Since then have hooked controls up to VFD's in HVAC applications and have changed out VFD's and installed a few new ones even.
HOWEVER what is available now is totally different than what I trained on, and I just have not taken the time to really understand what can be done with one outside of driving a fan or pump.
Let alone this single phase to three phase conversion and overspeeding to 120hz
Ron
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