Spindle Speed

[HeadScratcher]

Just a general conversation about spindle speeds / VFD settings...

I have a 2.2kW spindle and Huanyang Inverter, and I have slowly been learning how to use all the functionality in it.

I was led to be believe that this was a 24,000 rpm spindle, but I have just learned how to display RPM instead of Frequency on the main display (while spindle is running use the >>> key several times until it displays RPM) and it wasn't running at the speed I thought it was, I also learned few new tricks along the way.

To get maximum speed:
PD003 - Main Frequency needs to be set to 400 from the default * in the book (* = no defined frequency)
PD004 - Base Frequency needs to be set to 400 from the default 50 in the book.
PD005 - Max Operating Frequency, needs to be set to 400 from the default 50 in the book.

What I have found is that as expected PD005 limits any settings set in PD003 and PD004, so PD005 is probably best set at 400Hz regardless of what frequency you plan to run. If you set it too low and go to adjust PD003 or PD004 you may forget you set it low, and scratch your head like I did when you cant get the number to go up on PD003.

PD004 seems to limit the speed of PD003 and takes much longer for the spindle to spin up, so again I recommend setting this to 400Hz as well.

So now on to PD003 to see how fast it is really going:

With PD004 and PD005 both set to 400Hz I got the following RPM readings from setting PD003.
150Hz = 4320 RPM
200Hz = 5760 RPM
250Hz = 7202 RPM
300Hz = 8640 RPM
350Hz = 10080 RPM
400Hz = 11520 RPM

So with everything set to maximum, unless I missed a setting that doubles it, the spindle is only spinning at half the speed I was led to believe it was going to.

[malb]

The spindle motor is a 3phase induction motor. The speed of these motors is nominally related to the frequency of the 3 phase power that they receive. The VFD takes 240V single phase or 415V 3 phase and converts this to DC, then creates a three phase output that is normally nominally similar in voltage to it's input, but variable in frequency as determined by the user pushing buttons on the control panel, rotating a control knob, or setting in a programming script. There are a few special VFD's that can take a 240V input and generate a 415V output, but generally a 240V input generates a 240V output and a 415V input generates a 415V output.

The VFD may be able to display either the actual output frequency, or an estimated spindle RPM, derived from the output frequency and known motor characteristics. Unless the spindle is equipped with some form of tacho system to control the display, the VFD cannot display the actual spindle RPM, just a nominal calculated value close to it.

Induction motors, like your spindle, can be manufactured with different winding arrangements and numbers of pole pairs. Their nominal operating speed is f x 60 (60 secs in a minute)/no of pole pairs. For a motor with 1 pair of poles (2 pole) operating at 50 Hz, the nominal speed is 3000RPM. If the motor was manufactured with 2 pole pairs (4 pole), the nominal speed would be 1500RPM, for 3 pole pairs (6 poles) it would be 1000RPM etc.

Actual speed will always be slightly slower that the nominal speed, as to develop torque to overcome mechanical losses and drive a load, the motor has has to slip and run slightly slower than nominal speed. Motors are generally rated to supply their rated output at between 2-4% slip, at which point the motor is correspondingly 2-4% slower than nominal.

Within reason, the frequency/RPM relationship holds over a reasonable range of supply frequencies, but is limited by the losses in the motor once the frequency range becomes extreme. If a single pole pair motor is used at 400Hz supply, it should have a potential 24000RPM nominal spindle speed, although once slip is factored in, this drops to around 23000RPM actual speed..

I am not an expert with the VFD that you are using, but suspect that one of the parameters used in the system is incorrectly set, and the display calculation is being based on the formula for a motor with 2 pole pairs while the motor is actually a single pole pair motor. Unfortunately I cannot identify which parameter this is, or what it needs to be set to.

[HeadScratcher]

Thanks for sharing that insight malb. I was helping an ex auto electrician friend of mine move some stuff he other day, and on the way there I asked him about the VFD and spindle an he explained pretty much what you said there. I was actually surprised to find out that you can have a single phase in, but output 3 phases. He explained by using 3 phase it is easier to reverse the motor and have greater control over it.

This is the same kit that I bought on eBay http://www.ebay.com.au/itm/CNC-2-2KW...item19fc606cec

It shows a range of 8000 - 24000 but like most Chinese things on eBay needs to be taken with a grain of salt. Using the lower setting I had the motor ticking over at very few RPMs, and definitely now where near 8000.

Interesting point you made about the displayed revs. The inverter would be showing a representative revs, just like a car tacho doesn't show the real revs either, but rather a derived reading. Just wondering if there is a simple cheap and simple way to determine if the revs shown are close, or out by double.

[malb]

Hi, as explained above, the inverter input stage converts the mains voltage to a DC voltage. The output stage then converts the DC to an AC voltage of similar voltage to the input, and at a user defined frequency. The critical thing in that process is that the inverter and voltages are reasonably matched. Whether the input is 240V single phase or 415V 3 phase is immaterial provided that the VFD is selected to suit the voltage being used.

The majority of single phase motor configurations that produce significant power employ a start winding and a run winding, and capacitors to induce a phase shift between the windings. This configuration improves starting toque and starts the motor in the required direction. For most configurations all or part of the start winding and capacitor network is switched out of the circuit once the motor approaches its nominal speed. For these reasons, it is not practical to operate these motors as variable speed as either inappropriate switching of the start circuit or changes in the effect of the capacitor network with supply frequency limit the motors operation.

With a 3 phase motor their is no need for the start winding and capacitors, as the phase shift to determine direction of rotation is determined by the phase relationship of the 3 phase input. The motor can therefore operate over a greater range of supply frequencies and reliably start in the correct direction at any of them. Reversing mechanically is simply a matter of swapping any two of the three power cables to reverse the phase relationship within the motor, or with a VFD, commanding the motor and controller to reverse swaps causes the VFD to assemble the output in a reversed sequence.

With regard to establishing absolutely whether the spindle is running at 12K or 24K, the only truly practical method is to beg or buy a handheld optical tachometer. With these units, you put a white dot or line on the collet nut, run the motor and point the tacho at the collet. The tacho emits a constant light toward the collet, and detects reflected light off the dot or line, and counts the number of reflections over a given time to calculate RPM. Handheld optical tachs can be found on ebay from around $15 for basic units, of course coming from HK or China. The only other way to get the display verified with respect to the spindle speed would be to check with the Huanyang experts on the forums to see if they can identify a parameter that tells the display the pole configuration of the motor, and the appropriate value to set for the parameter.

Hope this helps.

[HeadScratcher]

I just bought one of these from eBay http://www.ebay.com.au/itm/251673653...%3AMEBIDX%3AIT so will know one way or the other shortly.

[HeadScratcher]

Finally the rain stopped and I got chance to test the spindle speed yesterday. Seems the VFD reading is about half of the actual speed.
Running the VFD at 200Hz it the tach showed a reading of 11944, meaning at the max 400Hz 24,000 is pretty much on the money.