In a previous post, we offered to present spindle torque and output graphs to assist a user in understanding of a system. Attached is that documentation, which was prepared for, and will also be on our new website.
There are 3 graphs here and a chart.
- Load response
Load response details what happens to the actual operating RPM of a drive, when loaded on a machining center. Remember that axial drives will not compensate for spindle variation without a very advanced system, so any variance to load will affect thrust, axial load/power, surface finish, tooling life, and achieved precision.
You can feel this if you use a drill or saw and cut something with an AC, open loop motor. The motor changes tone, and the speed slows down. Same is true on an open loop spindle drive on a machining center. Servo drives "push back" by changing frequency, voltage, current, etc. If you fully overload an AC system, the drive, generally, does not know about the overload, and the motor stops and starts to melt ("locked rotor").
If you overload a servo drive, it simply shuts down (and in this case also tells our axial drives to shut down within about 1/100 of a second), protecting the machine from significant damage and loading from shoving a non-operating tool into "something".
- RPM vs Power
This is the actual difference between 3 different combinations of drives and motors. If you're wondering how a 1 HP AC motor with a 2 hp VFD drive (think bench top machine) compares to a 2 hp motor with a 3 hp drive (Think knee mill), to a 3 HP servo drive - here's the data.
This also is why AC machines with traditional drives often have narrower speed ranges, 2 speed controlled transmissions, or in manual machines - belts for multiple ratios. See where the torque starts higher up the RPM scale - this is what is being compensated for.
There are other graphs out there that compare BLDC servo and AC open loop motors that don't have data/details and appear theoretical.
It's true you could under-size a BLDC system and scale the axis to get the graph that has been presented. But take 2 systems that are around 20-25 lbs each, test, and you'll get this data. Otherwise the power density and constant torque characteristics make no sense (as a user has pointed out).
A linear power curve / constant torque, also allows the very simple programming (scaling RPM scales power) and is also ideal for machine, bearing, and precision/tooling life - as thrusts end up staying the same as intended.
You can run feed/speed 10/1000 20/2000 30/3000, etc (if the power demand of the application is linear) to prove out tooling, fixtures, etc. Basically less variables are changing at the same time as torque is constant.
These are the "power curves".
This should be well versed information, specific to a tool, for any machine tool operator and programmer, for any automated machine tool. Attempting to program a tool without comprehensive knowledge of a power curve is a hugely frustrating experience, as you're guessing at how much power/torque is available.
However, it's a common point of confusion in the industry. Scale things up, and things get more expensive fast.
Spindle Power Curves
Yes - there are systems that are [B]wildly[/B] expensive that will slow the machine down to compensate for spindle or axial loading. But it's not your typical machining center that will do so - think way exotic.
Torque is how much "grunt" a system has at a given RPM ? Always the same for an advanced servo drive generally, and most at a "center" or "sweet" spot for most open loop systems. This also presents graphically what 5:1 or 20:1 or "linear" refers to (the flat top part).
If you had a transmission, you'd have multiple curves for each system at different ratios.
This is one of the strong suits of a brushless motor, as well as extreme life (no brushes to wear and replace all the time), and constant performance.
These are the "torque curves"
- Power density
More important than you might ever think. Who cares about 30 lbs on a device that weighs 2000 lbs ? Turns out it makes a world of difference to bearing life, selection for precision, and is a key driver of machine systemic design.
Helpful ? Confusing ? Indifferent ? Welcome any and all feedback.
Mikini Mechatronics, llc