VFD machine tool spindle acceleration pattern and stall prevention
VFD machine tool spindle acceleration pattern and stall prevention
(OP)
Still setting up a Mits drive on a machine tool spindle and have a couple a general questions.
1. Acceleration pattern - Mits is recommending an "S" shaped pattern instead of linear and indicate a reduced accel time. Curious if an "S" pattern is typical of spindle apps?
2. Stall prevention - Mits indicates that after the base frequency (usually the rated motor frequency), the stall prevention boost reduces with increased frequency. Motor base is 60hz and max freq is 250hz but very concerned about reduced stall prevention at higher frequencies. They indicate current is reduced but voltage is not. Is this typical? Base speed is well below most typical operation speeds.
1. Acceleration pattern - Mits is recommending an "S" shaped pattern instead of linear and indicate a reduced accel time. Curious if an "S" pattern is typical of spindle apps?
2. Stall prevention - Mits indicates that after the base frequency (usually the rated motor frequency), the stall prevention boost reduces with increased frequency. Motor base is 60hz and max freq is 250hz but very concerned about reduced stall prevention at higher frequencies. They indicate current is reduced but voltage is not. Is this typical? Base speed is well below most typical operation speeds.





RE: VFD machine tool spindle acceleration pattern and stall prevention
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If we learn from our mistakes I'm getting a great education!
RE: VFD machine tool spindle acceleration pattern and stall prevention
I guess I was wondering if others here have setup machine tool spindles and how they are typically setup?
RE: VFD machine tool spindle acceleration pattern and stall prevention
RE: VFD machine tool spindle acceleration pattern and stall prevention
From what I gather from you, I should indeed be using the rated frequency of the motor for the peak voltage target in the drive?
RE: VFD machine tool spindle acceleration pattern and stall prevention
For the same peak acceleration/torque value, it actually takes longer than constant acceleration, which instantly steps the accel on and off. But many people value the smoothness more.
Curt Wilson
Delta Tau Data Systems
RE: VFD machine tool spindle acceleration pattern and stall prevention
The main reason for the vector system is to improve every aspect of the spindle. Accel time, decel time, and power in a cut. I am starting to question where we will end up here.
Could the S curve be more tailored to larger spindles with big power? This is a 10HP spindle and even the ones a few years newer with vector systems accelerate about 10x faster and use the exact same motor. I am hoping I am on the right path here.
RE: VFD machine tool spindle acceleration pattern and stall prevention
RE: VFD machine tool spindle acceleration pattern and stall prevention
Above base speed, the field is weakened inversely with speed to keep the back EMF below the available voltage so there is still voltage "headroom" to drive torque-producing current through the phases. However, the torque available falls off inversely with speed in this range. Power available and voltage used are constant in this range.
As you surmise, if you were to set your base speed/frequency to 250 Hz instead of 60 Hz, you would be limiting your torque at low speeds to less than 1/4 of what would be available with a more standard base speed setting of 60 Hz.
Curt Wilson
Delta Tau Data Systems
RE: VFD machine tool spindle acceleration pattern and stall prevention
I was told that simply by tuning the slip ratio in v/hz, you can achieve better acceleration and constant rpm. We are going to have to reinstall the old v/hz drive for now due to our time line and would like to tune it better to achieve better accel/decel, and maintain speed better under load.
I have run identical machines with the same spindle motors and only difference being a vector drive system and speed regulation all the way to 150% spindle load was very impressive. Load goes up but spindle does not bog down. Able to make much heavier cuts. I am curious to learn "what" exactly is getting that extra performance.
RE: VFD machine tool spindle acceleration pattern and stall prevention
If you had 480VAC available then you'd set it to 460V and 120Hz and keep the motor connected for 230V. This way, you'd get full capabilities up to 2x the rated speed of the motor.
RE: VFD machine tool spindle acceleration pattern and stall prevention
Traditional V/Hz control is open loop. It outputs an electrical signal of a frequency corresponding to the instantaneous command value and a proportional voltage as determined by the preset V/Hz ratio. (Note that this is not technically a "slip ratio", although it effectively reduces to one in the steady state at constant velocity.) It does this with no knowledge of what the motor is actually doing. The slip in this case is simply the physical reality of the difference between the electrical frequency of drive output frequency and the rotor mechanical frequency (i.e. speed).
The preset V/Hz ratio allows you to trade off torque for speed. A higher ratio can give you more torque at the cost of a lower base speed.
Vector control, by contrast, is a closed-loop technique. The term "sensorless vector control" is somewhat of a misnomer, as it does use voltage and current sensors inside of the drive. Technical papers usually call it "shaft-sensorless" control. Anyway, it is continually measuring/estimating the physical rotor speed and outputs an electrical frequency that creates the slip frequency needed to get the torque it wants to maintain the desired velocity.
In vector control, there is a V/Hz ratio (often implicit), but it is related to the slip frequency rather than the electrical frequency itself, as in open-loop control. Often this ratio derives from the "magnetization current" setting of the drive. If this setting is left constant over the range of operation of the drive, you have the same speed range as an open-loop V/Hz range (but with improved operation within that range due to the controlled slip).
However, a vector drive, because it has knowledge of the actual rotor speed, can vary this setting dynamically as a function of the rotor speed, something an open-loop drive cannot do. So it can be set high for high torque at lower speeds, then reduced at higher speeds to keep the needed voltage within range. This reduces the available torque (there's no free lunch...) but it does permit some torque to be generated.
This "field weakening" technique sets the instantaneous V/Hz ratio for the slip frequency inversely proportional to the rotor speed when it is above base speed. As you note, this has the effect of outputting a full voltage signal above base speed, but varying the characteristics of the signal.
Curt Wilson
Delta Tau Data Systems