As the proud new owner of the 3520-B I am really enjoying the lathe. Quite a step up from the Jet 1236. One question The owners manual says to turn the lathe speed control down before cutting the switch off. When turning the power off for short periods for various reasons, should I always cut off the main switch, or is it ok to just cut the speed control to zero and leave the main push button switch on? Or is there a time limit where it would be better to go ahead and cut the main switch. I dont want to do anything to damage the variable speed motor and inverter. Thanks for your replys. By the way, I have been reading posts here for a while and this is the first question that I have not been able to answer from previos posts, so thanks for the imformation that I have already gleaned from all your knowledge.
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question about Powermatic 3520-B
- Thread starter Gerald
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Richard Baker
RIP
Ditto re. Jet 1642 EVS
Same questions. My old dvd player would not play the dvd about the speed control unit.
And Gerald, welcome to this forum!
Same questions. My old dvd player would not play the dvd about the speed control unit.
And Gerald, welcome to this forum!
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Hi Gerald - welcome to the forum. I'm pretty sure this question has never been asked also.
In short just hit the stop button.
Longer answer:
There is a built-in lag in the powerdown cycle. When you hit the stop button the lathe will not immediately stop. It has a lag of 3-4 seconds during which time the lathe will slow down under controlled braking power.
I don't know the RPMs under which this lag is activated. If the lathe is only going 6 RPM it pretty much just stops.
One thing to consider for pieces with more mass is that you may want to slow it down with the controller. I have turned more than one piece that was over 100 pounds. If you just hit the stop button it will use the built-in lag to brake the piece. However that is only for 3-4 seconds. The piece will probably still be spinning freely at that point. To get around this situation you could do one of two things:
- use the controller to initially slow down the piece
- reprogram the lag in the controller
It's usually not a problem. Most people probably just hit the stop button. It is good practice to stop the lathe occasionally when turning to inspect your work and also to look for signs of trouble (wall too thin, loosening of the faceplate, or checking for loose bark, etc).
If you have not seen the Tips subforum, there is a post about PM 3520 which lists a lot of the tips posted in this forum.
In short just hit the stop button.
Longer answer:
There is a built-in lag in the powerdown cycle. When you hit the stop button the lathe will not immediately stop. It has a lag of 3-4 seconds during which time the lathe will slow down under controlled braking power.
I don't know the RPMs under which this lag is activated. If the lathe is only going 6 RPM it pretty much just stops.
One thing to consider for pieces with more mass is that you may want to slow it down with the controller. I have turned more than one piece that was over 100 pounds. If you just hit the stop button it will use the built-in lag to brake the piece. However that is only for 3-4 seconds. The piece will probably still be spinning freely at that point. To get around this situation you could do one of two things:
- use the controller to initially slow down the piece
- reprogram the lag in the controller
It's usually not a problem. Most people probably just hit the stop button. It is good practice to stop the lathe occasionally when turning to inspect your work and also to look for signs of trouble (wall too thin, loosening of the faceplate, or checking for loose bark, etc).
If you have not seen the Tips subforum, there is a post about PM 3520 which lists a lot of the tips posted in this forum.
Richard Baker
RIP
Jeff, The Jet (and one assumes the PM is similar) apparently switches to "braking mode" almost instantly when one hits the "Off" switch. The motor becomes a generator and its output is dissipated through a resistor to slow the lathe motor and spindle. The "braking mode" disengages when the motor and spindle are about to stop, and they coast to a stop. You can tell when braking "disengages", but I'd have to go do it to describe how you know.
Anyhow, that's one data point from one lathe. The braking resistor is in a separate heat sink mounted below the control unit on back of the headstock.
Still intuitively seems like a good idea when turning a BIG chunk to turn down the speed before hitting "off".
Anyhow, that's one data point from one lathe. The braking resistor is in a separate heat sink mounted below the control unit on back of the headstock.
Still intuitively seems like a good idea when turning a BIG chunk to turn down the speed before hitting "off".
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DC Injection Braking
Most likely these controllers use what is known as DC injection braking. A DC voltage of around 30 V is applied to the windings through the braking resistor. The peak current will be as much as 1.5 times the peak rated current of the motor so this type of braking (and most other types of electrical braking, as well) can't be applied for more than a few seconds or the motor will overheat. Also, because the effectiveness of the electrical braking becomes less as the motor slows down, there is no point in maintaining this type of braking below a certain speed threshold and that is why it switches off. DC injection braking produces a stationary magnetic field in the stator winding rather than the rotating magnetic field that is produced when the motor is running. Under normal use, there is not a thermal problem, but just don't get overly fascinated "playing" with the dynamic braking or the motor may begin getting too hot.
There is an emergency stop braking mode that VFD's frequently use called "plugging", but hopefully they don't incorporate that feature on wood lathes (at least, I personally would have concerns about using it) because it produces an almost instantaneous stop. This can damage drive components, electrically overstress the motor, and could likely cause a chuck to unscrew itself.
There are various other braking schemes. A vector control motor can have very rapid and smooth braking, but motor heating is still an issue. Dynamic braking is very simple because it uses the VFD drive components to ramp the drive command down using a programmed profile, but once again, the stored energy of the rotating mass is dissipated in the motor. Finally, regenerative braking is the most efficient and puts the energy back on the power line and so it produces the least amount of motor heating, but it is far too expensive to be considered for use on a woodturning lathe.
Bill
Richard,Texian said:
Most likely these controllers use what is known as DC injection braking. A DC voltage of around 30 V is applied to the windings through the braking resistor. The peak current will be as much as 1.5 times the peak rated current of the motor so this type of braking (and most other types of electrical braking, as well) can't be applied for more than a few seconds or the motor will overheat. Also, because the effectiveness of the electrical braking becomes less as the motor slows down, there is no point in maintaining this type of braking below a certain speed threshold and that is why it switches off. DC injection braking produces a stationary magnetic field in the stator winding rather than the rotating magnetic field that is produced when the motor is running. Under normal use, there is not a thermal problem, but just don't get overly fascinated "playing" with the dynamic braking or the motor may begin getting too hot.
There is an emergency stop braking mode that VFD's frequently use called "plugging", but hopefully they don't incorporate that feature on wood lathes (at least, I personally would have concerns about using it) because it produces an almost instantaneous stop. This can damage drive components, electrically overstress the motor, and could likely cause a chuck to unscrew itself.
There are various other braking schemes. A vector control motor can have very rapid and smooth braking, but motor heating is still an issue. Dynamic braking is very simple because it uses the VFD drive components to ramp the drive command down using a programmed profile, but once again, the stored energy of the rotating mass is dissipated in the motor. Finally, regenerative braking is the most efficient and puts the energy back on the power line and so it produces the least amount of motor heating, but it is far too expensive to be considered for use on a woodturning lathe.
Bill
Richard Baker
RIP
Thanks Bill,
Was hoping you would jump in with a more accurate explanation. A lot has changed in the hundred years or so since I took those EE courses, and I've forgotten most of what I learned anyhow.
Was hoping you would jump in with a more accurate explanation. A lot has changed in the hundred years or so since I took those EE courses, and I've forgotten most of what I learned anyhow.
Bill,
I thought the PM units were running on the same controller system as my Delta X5 using a 220 AC single phase contoller running a 220 AC 3 phase motor. How are they using a DC current on these AC motors?
Thanks
Wilford
I thought the PM units were running on the same controller system as my Delta X5 using a 220 AC single phase contoller running a 220 AC 3 phase motor. How are they using a DC current on these AC motors?
Thanks
Wilford
Am I damaging my PM?
I almost never use the stop button on my PM. I just turn that little knob until the machine stops. Is this bad? I've been doing this since I got the machine about 4 years ago.
Bruceyp
I almost never use the stop button on my PM. I just turn that little knob until the machine stops. Is this bad? I've been doing this since I got the machine about 4 years ago.
Bruceyp
I doubt that would hurt it at all. You probably should not leave it at the zero position and powered on overnight (or more than an hour).
If you do a whole lotta turning (like I did the last few years) it is real easy to just hit the stop button for the frequent inspections during turning. Like I said above that is not very effective for the real heavy pieces. The PowerMatic rep at the 2005 symposium was surprised that my stop button had not yet failed after so much usage. Just lucky I guess.
If you do a whole lotta turning (like I did the last few years) it is real easy to just hit the stop button for the frequent inspections during turning. Like I said above that is not very effective for the real heavy pieces. The PowerMatic rep at the 2005 symposium was surprised that my stop button had not yet failed after so much usage. Just lucky I guess.
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Yes, but wait there is more . . . . . . .
So, after waltzing all over the place, to answer your question, there is DC power available for braking in the controller.
Bruce, I think that you have obviously demonstrated that there is no problem in doing things the way you do it. Since the pot output never goes to absolute zero speed command, there will still be a very small voltage going to the motor, but not enough to make it move and not enough to hurt anything as you have shown, but I still think that it would be better to hit the STOP button either after you have run the speed control to a stop or while it is still running. In the long run it might make the driver transistors last a bit longer before they finally die, but again maybe not.
I ought to mention that hitting the STOP button does NOT shut off the electronics -- they are still sitting there powered up and thinking (scheming, goofing off, plotting) -- the only thing that has happened is that the STOP switch cuts off the drive to the motor after the braking cycle ends.
Bill
Wilford Bickel said:
Wilford, the 240 VAC 60 Hz SINGLE PHASE power to your lathe controller needs to be thought of as a device that powers the electronics and not the power that drives the motor itself because the power that is available at the wall outlet is not suitable for driving the motor for the following reasons:Bruceyp said:
- The motor needs three phase power and the power that you have is single phase
- The speed of an AC induction motor is controlled by the AC frequency and so it will need to be something other than 60 Hz except for the times that the motor speed needs to be 1750 RPM.
- The peak voltage to the motor is scaled as a function of driving frequency to prevent the iron core from saturating at lower frequencies.
So, after waltzing all over the place, to answer your question, there is DC power available for braking in the controller.
Bruce, I think that you have obviously demonstrated that there is no problem in doing things the way you do it. Since the pot output never goes to absolute zero speed command, there will still be a very small voltage going to the motor, but not enough to make it move and not enough to hurt anything as you have shown, but I still think that it would be better to hit the STOP button either after you have run the speed control to a stop or while it is still running. In the long run it might make the driver transistors last a bit longer before they finally die, but again maybe not.
I ought to mention that hitting the STOP button does NOT shut off the electronics -- they are still sitting there powered up and thinking (scheming, goofing off, plotting) -- the only thing that has happened is that the STOP switch cuts off the drive to the motor after the braking cycle ends.
Bill
Attachments
Reply from Powermatic tech.
I called the powermatic tech line this morning . His response was that from a safety aspect if you cut the main switch without lowering the speed control, if the speed control were accidentily turned up it could start too fast and throw off the work. Also, if you cut the speed control without turning off the on-off switch, the speed control could possibly{remote possibility} malfunction and start turning while you are touching the work. As far as damaging any components, It should be fine to stop the work for short periods using either the switch or the speed control knob. He said that either way is safe for the electronics and the motor, but for the safety of the operator, the motor should be started with the speed control turned all the way down. Thanks for all the responses. After hearing from all you guys and the tech at PM I will probably use the on-off switch for light work and suplement that by turning down the speed control when turming heavy bowl blanks. Gerald
I called the powermatic tech line this morning . His response was that from a safety aspect if you cut the main switch without lowering the speed control, if the speed control were accidentily turned up it could start too fast and throw off the work. Also, if you cut the speed control without turning off the on-off switch, the speed control could possibly{remote possibility} malfunction and start turning while you are touching the work. As far as damaging any components, It should be fine to stop the work for short periods using either the switch or the speed control knob. He said that either way is safe for the electronics and the motor, but for the safety of the operator, the motor should be started with the speed control turned all the way down. Thanks for all the responses. After hearing from all you guys and the tech at PM I will probably use the on-off switch for light work and suplement that by turning down the speed control when turming heavy bowl blanks. Gerald
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Shut-down note
UPDATE: It would be good procedure to ALWAYS make certain that the speed control pot is fully CCW before starting because someone may have "twiddled" with the knob or it may have been accidentally moved by brushing against it.
Bill
First of all, don't confuse the MAIN power switch with the big red mushroom switch, which is a START-STOP control for the motor only while the electronics remains powered. The tech support person is partly right and partly wrong in his answer. You should NEVER except in an emergency shut off all power to the unit while the motor is running because it could cause the controller to lose its mind and require resetting back to the defaults. The part that the tech support person was incorrect about was the part about lowering the speed control somehow has a bearing on the way that the system recovers following a power-off shutdown. He/she might have been thinking about the start-up conditions following a power-off shutdown where the numeric keypad has been set to control motor speed rather than the potentiometer. If it is software controlled, a power-off shutdown could result in reverting to keypad control of the speed. But, in any event, if you accidentally shut off the main power or there is a power system failure, you should always be safe and assume that when you re-boot, things will start-up at full speed. Remove any turning from the spindle, run the speed pot to zero, and check that everything is clear before applying power and then hit start.Gerald said:
UPDATE: It would be good procedure to ALWAYS make certain that the speed control pot is fully CCW before starting because someone may have "twiddled" with the knob or it may have been accidentally moved by brushing against it.
Bill
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