[rowbare]

Matt,

Here is a slightly modified version of something I posted in another thread that sums it up:

HP is a function of torque and speed. It is a measure of power i.e. how much work can be done in a given time. In our case, it is normally rated in how many cubic inches of metal can be removed in a minute. You can find tables listing the Metal Removal Rate for various metals expressed in cubic inches per minute per HP. A 1 HP 1000 rpm motor will remove as much metal as a 1 HP 5000 rpm motor. However the conditions under which it will do so are different.

Changing speed via mechanical reduction is very different than changing speed with a VFD. When changing speed mechanically, the motor stays at its rated speed and will therefore will put out its rated power (minor mechanical losses aside) at all speeds. The metal removal rate remains the same since the power is the same.

Now using a VFD is a different story. An AC motor under VFD control can be treated as a constant torque device under its rated speed and a constant power device above. Thus if you run a 3450 rpm 1HP motor at 1725 rpm, it will be putting out 1/2 HP. Run it at 4500 rpm and it will put out 1HP albeit at reduced torque. At half speed, it can remove half as much metal as it can at its rated speed or higher.

If you use GWizard, there is a really cool feature in the machine profiles. You can create a profile for your Low range and one for your High range. Then you can setup a curve compensated power adjustment for each. Given the following example values:
1.5 HP motor,
VFD range 10-80 Hz,
low gear max rpm 2000, high gear max rpm 5000,
At 10 Hz, you would have .25 HP
At the motor's rated 60 Hz you would have 1.5 HP
At 80 Hz you would still have 1.5 HP

In the Adjust dialog, you would select Curve Compensated and enter the following:
For your low range profile you would set :
Low Range 250 RPM .25 HP
Peak Range 1500 RPM 1.5 HP
High Range 2000 RPM 1.5 HP

For your high range profile you would set :
Low Range 625 RPM .25 HP
Peak Range 3750 RPM 1.5 HP
High Range 5000 RPM 1.5 HP

Then when you do your feed and speed calculations, GWizard will adjust for available power at the calculated RPM and suggest an appropriate feed rate.

bob

[mattmark]

Thanks Bob - didn't know that feature existed. Will definitely take more time to set up my G-Wizard profile and I probably won't have to do too much tweaking.

Matt

[mattmark]

Hey gang,

Been doing renos and not evening living at home, so machining has been on the backburner.. I did however join a welding course and I'm almost finished the stick welding portion and trying to jump to TIG (not sure if I have to take MIG first).

Quick question about a welding machine: has anyone tried the ThermalArc 211i Fabricator? Looks to be a solid 3-in-1 machine with lots of power capability. And does anyone know of a reliable source for welding supplies/machines in Toronto or Ontario/Canada?

Cheers,

Matt