Originally posted by Big John T:
What is an FOC or DTC AC drive?
FOC - Field Oriented Control
This seems to be the favorite control algorithm these days for both servo motors and 'vector' controlled AC induction motors. The reason volts/hertz inverters don't perform very well is they don't specifically control the strenght or relative angle of the magnetic flux in the motor. 'Vector' control does this for you. FOC is just one of the 'vector' control methods. There is a ton of information on FOC on the Internet. Just search on 'Field Oriented Control'.
DTC - Direct Torque Control
This is another vector control methodology that is not used as much. The only company to use this that I am aware of ABB. There is a pretty decent body of information on the Internet on this also. The ABB-provided info is pretty good but be a little careful. It is in ABBs best interest to make DTC look good against the other control algorithms since ABB is one of the few if not the only manufacturer doing this. Having said that I have heard nothing but good things about the actual output performance of ABB drives using DTC.
Originally posted by Big John T:
Do any AC motors have a NEMA style frame? This looks like a NEMA 42... Is there a chart that shows the NEMA frame dimensions?
At that size I would stay servo. I know they are available but I have personally never seen an AC induction motor that small. Also, you won't be able to get a closed loop AC drive rated that low so you will be paying for capacity you can't use. I start to seriously look at AC induction versus PM servo right around 1HP.
Originally posted by TWControls:
The big thing is speed, not accuracy. If it is running at low RPM then the AC motor is not going to have the power. Also it will have trouble jogging into position
This isn't an issue if you properly select the drive and motor. The drive needs to use a control algorithm that can develop full torque at zero speed. The motor needs to be a TENV or ##BC (##-insert your favorite frame type) in order to stay cool down there.
Peak speed depends on the motor winding. While there are alot of servos out there wound to get to 5000 RPM, that's not an automatic. There are also servos wound to get to only 1500 RPM.
The big differences are torque to inertia ratio, peak torque and torque density. Servomotors have higher torque to inertia ratios. This is due mostly to the use of permanent magnets on the rotor. These produce higher magnetic field strengths than an induction motor rotor of comparable mass. Higher torque to inertia ratios mean higher acceleration rates.
Servomotors generally have a higher peak torque capacity relative to their continuous torque than do AC induction motors. Induction motor rotors go into magnetic saturation at about 250% of rated torque (I believe). After that pumping more current into the motor just results in more heat, not more torque. In contrast a PM servo will continue to produce increasing torque with increasing current (more or less) up until you de-magnetize the rotor. This is often 400% or more above continuous torque levels. Again, this means higher acceleration rates.
Torque density is torque per frame size. Again, because of the PM rotor, PM servos have higher torque density than a comparably cooled AC induction motor.
Originally posted by TWControls:
Price isn't that much different for a servo motor and drive compared to a AC motor and drive that can handle servo appications
I need to find out who you are buying from. In the sizes I typically work with (5 - 20HP) AC induction is much less expensive.
Keep in mind I'm just trying to be the regulating opinion here. We simply don't know enough about Big John T's application to know if it really requires a servo or not. However, I do know that '
because we have always done it that way' is not a valid design rationale. I've lost count of the number of hours I've spent getting out from under problems caused by that rationale. A servo may be the right answer. Just make sure you know
WHY it is the right answer.
Keith