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machine tool spindle motor/drive design engineering questions 1
- Thread starter cobra1234
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- #2
Sorry for that guys! wrote up a post earlier today only to see it dumped by the Inet gremmies!
I have a question regarding machine tool spindle design. It has come to my attention that some premium designers actually specifically use under voltage motors or have them made that way so they can feed them more voltage and keep a nice constant HP graph. I heard that a big company has done this for years using a dual voltage motor, wiring them low volts, then using a high voltage drive.
In operation, I guess the motor wires could care less what voltage they are at, as long as the amperage is within design parameters. I heard also that one of our low voltage systems might use a spindle motor designed for 165VAC but they run it at 240V. IIRC, that spindle is rated at constant HP from 500-6000rpm.
I am trying to learn more about this wiring or trickery in design. Could this be how some companies are getting such a large and flat curve on the HP?
I guess if you think about it, if a motor wired for 15A, 240V, and you feed it 15A at 480V, you have doubled the power from that motor!
I have a question regarding machine tool spindle design. It has come to my attention that some premium designers actually specifically use under voltage motors or have them made that way so they can feed them more voltage and keep a nice constant HP graph. I heard that a big company has done this for years using a dual voltage motor, wiring them low volts, then using a high voltage drive.
In operation, I guess the motor wires could care less what voltage they are at, as long as the amperage is within design parameters. I heard also that one of our low voltage systems might use a spindle motor designed for 165VAC but they run it at 240V. IIRC, that spindle is rated at constant HP from 500-6000rpm.
I am trying to learn more about this wiring or trickery in design. Could this be how some companies are getting such a large and flat curve on the HP?
I guess if you think about it, if a motor wired for 15A, 240V, and you feed it 15A at 480V, you have doubled the power from that motor!
MikeHalloran
Mechanical
AC motors are not that simple.
Read up on variable speed drives, which are really variable frequency drives.
It might be educational to study the handbook for a VFD that you have on premises. I am not suggesting that you change even one tiny setting, but that you can observe the behavior of the VFD while the spindle is running at a few representative speeds, and then go to your desk and figure out what's going on.
Mike Halloran
Pembroke Pines, FL, USA
Read up on variable speed drives, which are really variable frequency drives.
It might be educational to study the handbook for a VFD that you have on premises. I am not suggesting that you change even one tiny setting, but that you can observe the behavior of the VFD while the spindle is running at a few representative speeds, and then go to your desk and figure out what's going on.
Mike Halloran
Pembroke Pines, FL, USA
- Thread starter
- #4
I certainly don't design drives but I am more familiar with them than most and specify settings on some.
I am not quite sure what you are getting at though. Typically even in scalar modes and open loop, a drive would have a pretty linear V/hz ratio but might push the V for torque boost when needed.
I am pretty sure there is some rather specific drive tuning going on and I would like to learn more. I am not sure what looking at a drive when running is going to tell me though. Maybe if I could graph the spindle speed, load, and scope graphs of the of the IGBT outputs.
I guess that is why I am here!
I am not quite sure what you are getting at though. Typically even in scalar modes and open loop, a drive would have a pretty linear V/hz ratio but might push the V for torque boost when needed.
I am pretty sure there is some rather specific drive tuning going on and I would like to learn more. I am not sure what looking at a drive when running is going to tell me though. Maybe if I could graph the spindle speed, load, and scope graphs of the of the IGBT outputs.
I guess that is why I am here!
MikeHalloran
Mechanical
Sorry, you just showed up yesterday, so we know nothing about you beyond what can be inferred from your first nontrivial message. ... and now your second.
No insult was intended.
Mike Halloran
Pembroke Pines, FL, USA
No insult was intended.
Mike Halloran
Pembroke Pines, FL, USA
- Thread starter
- #6
No worries Mike, just did not want to start from the "understanding AC power" handbook....lol I am really trying to understand what special parameters are used in the machine tool spindle systems. Something I have noticed is these motors typically are smooth body so they must have some type of internal cooling situation. Never seen one liquid cooled either.
I am really not sure where these motors perform relative to their base speed. Or even what their base speed really is. Some give little to no specs but most that I see are 4 pole I guess.
I am really not sure where these motors perform relative to their base speed. Or even what their base speed really is. Some give little to no specs but most that I see are 4 pole I guess.
Here's the Reader's Digest version.
Concept 1) AC induction motor torque is directly related to the ratio of voltage and frequency as it was originally designed. As long as you maintain that V/Hz ratio, the motor produces design torque.
Concept 2) A VFD creates voltage and frequency at whatever you tell it to, with only one major limitation: it cannot produce voltage that was not there to begin with. So if you have a 240V drive and feed it 240V, the maximum output of that drive can be 240V.
Concept 3) You can make the frequency be whatever you like within the VFD design range, so for example 30Hz,50Hz, 60Hz, 100Hz, 120Hz or 500Hz.
Combined concepts) If I have 460V available and a 460V drive running a motor designed for 460V 60Hz, the V/Hz ratio that gives me full torque at any speed is 7.67 H/Hz. As long as I maintain that ratio (within tolerances), the motor performs as designed. But say I want to motor to run at 2X speed? I can turn the Hz up to 120, but I still only have 460V. So at 120Hz, my V/Hz ratio is now 3,833, half of what the motor is designed for, so my motor produces LESS torque once I go above the base design speed. At 500Hz, that motor might end up producing so little torque that I could stop it with my hand. (Don't try this at home...).
But if I take a 230V 60Hz motor, it is ALREADY designed to give me full torque at a V/Hz ratio of 3.833. So if I give that 230V motor 460V at 120Hz, I can still get full torque at 2X speed.
"Will work for (the memory of) salami"
Concept 1) AC induction motor torque is directly related to the ratio of voltage and frequency as it was originally designed. As long as you maintain that V/Hz ratio, the motor produces design torque.
Concept 2) A VFD creates voltage and frequency at whatever you tell it to, with only one major limitation: it cannot produce voltage that was not there to begin with. So if you have a 240V drive and feed it 240V, the maximum output of that drive can be 240V.
Concept 3) You can make the frequency be whatever you like within the VFD design range, so for example 30Hz,50Hz, 60Hz, 100Hz, 120Hz or 500Hz.
Combined concepts) If I have 460V available and a 460V drive running a motor designed for 460V 60Hz, the V/Hz ratio that gives me full torque at any speed is 7.67 H/Hz. As long as I maintain that ratio (within tolerances), the motor performs as designed. But say I want to motor to run at 2X speed? I can turn the Hz up to 120, but I still only have 460V. So at 120Hz, my V/Hz ratio is now 3,833, half of what the motor is designed for, so my motor produces LESS torque once I go above the base design speed. At 500Hz, that motor might end up producing so little torque that I could stop it with my hand. (Don't try this at home...).
But if I take a 230V 60Hz motor, it is ALREADY designed to give me full torque at a V/Hz ratio of 3.833. So if I give that 230V motor 460V at 120Hz, I can still get full torque at 2X speed.
"Will work for (the memory of) salami"
- Thread starter
- #8
This seems to point at a "constant torque" configuration as opposed to a "constant HP" which I think is more common.
I have some questions maybe you can answer.
1. Breakdown torque is an increase OVER max rated torque in which slip would increase in the motor to achieve that. Spindles are commonly rated for continuous and 30min ratings. The 30 min rating is usually about 35% over cont. There is also usually another 1min rating that is at about 150% of cont. Could we assume they are trying to tap into breakdown torque here or is this just a situation of monitoring heat from the motor to determine rating?
2. Assuming a motor is operating at a constant rpm but with variable load. As the motor is loaded, it will want to slow down or slip. A closed vector motor does a really good job of holding that speed rather than slowing down. How and why? What would the vector system do different than scalar in terms of helping the motor maintain speed? Would it be safe to assume the vector system would actually increase the hz to where the slip is increased to increase torque WITHOUT reducing the speed of the motor?
3. When looking at a motor configuration we are discussing, when you double the voltage that the motor is wired for, you double the HP but the torque remains constant. So could we probably call this spindle configuration constant torque?
4. In examining this a bit more, drives are rates for volts and amps. If amps stays the same and would have to for drive protection, you are saying that a motor would produce its rating torque as long as the V/Hz relationship is held. At what minimum speed would the motor torque fall off or could we hold this all the way down to say 1-2Hz?
5. Everything seems to hover around "base speed". What determines base speed or is this just designed in with the number of poles? Reason I ask is an inverter duty motor (spindle motor) would always be connected to an inverter. Just curious how they determine "base speed" when that speed is so variable?
I have some questions maybe you can answer.
1. Breakdown torque is an increase OVER max rated torque in which slip would increase in the motor to achieve that. Spindles are commonly rated for continuous and 30min ratings. The 30 min rating is usually about 35% over cont. There is also usually another 1min rating that is at about 150% of cont. Could we assume they are trying to tap into breakdown torque here or is this just a situation of monitoring heat from the motor to determine rating?
2. Assuming a motor is operating at a constant rpm but with variable load. As the motor is loaded, it will want to slow down or slip. A closed vector motor does a really good job of holding that speed rather than slowing down. How and why? What would the vector system do different than scalar in terms of helping the motor maintain speed? Would it be safe to assume the vector system would actually increase the hz to where the slip is increased to increase torque WITHOUT reducing the speed of the motor?
3. When looking at a motor configuration we are discussing, when you double the voltage that the motor is wired for, you double the HP but the torque remains constant. So could we probably call this spindle configuration constant torque?
4. In examining this a bit more, drives are rates for volts and amps. If amps stays the same and would have to for drive protection, you are saying that a motor would produce its rating torque as long as the V/Hz relationship is held. At what minimum speed would the motor torque fall off or could we hold this all the way down to say 1-2Hz?
5. Everything seems to hover around "base speed". What determines base speed or is this just designed in with the number of poles? Reason I ask is an inverter duty motor (spindle motor) would always be connected to an inverter. Just curious how they determine "base speed" when that speed is so variable?
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1
- #9
mikekilroy
Electrical
Cobra, you are asking good questions. To your OP, yes, MT spindle motor mfgrs play games with 'base speed' and 'base speed voltage' ratings in order to give wide constant HP ranges; I have been associated with MT mfgrs of motors and drives for over 30 yrs doing just this. For instance, we often had meetings with the end customer (often Cincinnati Milacron, aka lots of different names to date) and our motor & drive designers to discuss just where to bend the rules.
But the bottom line is your original comment is correct: we often made (make) the normal base speed voltage lower than what is common - but for a slightly different reason that you suggest: it is to move breakdown torque out of our way so we CAN offer that wide HP range. I made a spreadsheet showing how this works some time ago but could not put my finger on it right away. It is easy to plot the breakdown torque as well as the rated torque for any motor design configuration - where they meet is the end of usable cosntant hp range. It is not to go the double voltage like using a 230v motor on 460v to 120hz but some compromise in between. We often run '230v' motors with a 160v base speed but allow the constant torque range (constant v/hz) to continue above 160v@1800rpm for instance to 230v@86hz THEN we are out of volts so go into constant hp range. Ditto for 460v systems. Fanuc is good for doing this alsdo but they do not give such details. But no one wants to see 86hz (2500rpm) so we still call it 1800rpm for instance, and then we call 1800-6000 constant hp - you and most folks on this eng forum know that we have MORE hp from 1800 to 2500 rpm but we ignore it and rate it at the motor nameplate hp @ 1800 - so we get a much wider constant hp range. I will search for that example but int he meantime if you google for breakdown torque vs rated torque you will see the breakdown torque goes down by volts squared and rated goes down linearly so you seer they will eventual cross. [/i]
1. Breakdown torque is an increase OVER max rated torque in which slip would increase in the motor to achieve that. Spindles are commonly rated for continuous and 30min ratings. The 30 min rating is usually about 35% over cont. There is also usually another 1min rating that is at about 150% of cont. Could we assume they are trying to tap into breakdown torque here or is this just a situation of monitoring heat from the motor to determine rating?
Since motors are typically designed to have 200-250% breakdown torque (that is amount above nameplate torque) then it is used. But these S3 and S6-30 ratings are simply HEAT thermal limits, nothing more.
2. Assuming a motor is operating at a constant rpm but with variable load. As the motor is loaded, it will want to slow down or slip. A closed vector motor does a really good job of holding that speed rather than slowing down. How and why? What would the vector system do different than scalar in terms of helping the motor maintain speed? Would it be safe to assume the vector system would actually increase the hz to where the slip is increased to increase torque WITHOUT reducing the speed of the motor?
Yes. Remember, a 'skalar' drive is simply a power supply with no feedback. So it puts out a voltage at a given frequency so the motor simply runs down the 'slip' curve. A vector drive is some better; some simply increase the freq along the v/hz curve, others do much better control of the Isq current leaving the Isd magnetizing current constant. But yes, they will maintain speed better, both sensorless and sensored drives.
3. When looking at a motor configuration we are discussing, when you double the voltage that the motor is wired for, you double the HP but the torque remains constant. So could we probably call this spindle configuration constant torque?
of course. Then if you continue above that doubled voltage it will become constant hp.
4. In examining this a bit more, drives are rates for volts and amps. If amps stays the same and would have to for drive protection, you are saying that a motor would produce its rating torque as long as the V/Hz relationship is held. At what minimum speed would the motor torque fall off or could we hold this all the way down to say 1-2Hz?
never if a vector drive; @ 0rpm it has nameplate rated torque.
5. Everything seems to hover around "base speed". What determines base speed or is this just designed in with the number of poles? Reason I ask is an inverter duty motor (spindle motor) would always be connected to an inverter. Just curious how they determine "base speed" when that speed is so variable?
number poles gives simply a speed at a given hertz; there is 'nominal' ratings for this - 50 & 60hz. So you will see 1500, 1800, etc 'typical' base speeds. In the MT spindle world it is not uncommon for us to rate motors where the VOLTAGE reaches max available from a supply such as 230v, & 460v. Hence you will see wierd 'base speed' of that 2500rpm etc. 'Base speed' is just a name.
But the bottom line is your original comment is correct: we often made (make) the normal base speed voltage lower than what is common - but for a slightly different reason that you suggest: it is to move breakdown torque out of our way so we CAN offer that wide HP range. I made a spreadsheet showing how this works some time ago but could not put my finger on it right away. It is easy to plot the breakdown torque as well as the rated torque for any motor design configuration - where they meet is the end of usable cosntant hp range. It is not to go the double voltage like using a 230v motor on 460v to 120hz but some compromise in between. We often run '230v' motors with a 160v base speed but allow the constant torque range (constant v/hz) to continue above 160v@1800rpm for instance to 230v@86hz THEN we are out of volts so go into constant hp range. Ditto for 460v systems. Fanuc is good for doing this alsdo but they do not give such details. But no one wants to see 86hz (2500rpm) so we still call it 1800rpm for instance, and then we call 1800-6000 constant hp - you and most folks on this eng forum know that we have MORE hp from 1800 to 2500 rpm but we ignore it and rate it at the motor nameplate hp @ 1800 - so we get a much wider constant hp range. I will search for that example but int he meantime if you google for breakdown torque vs rated torque you will see the breakdown torque goes down by volts squared and rated goes down linearly so you seer they will eventual cross. [/i]
1. Breakdown torque is an increase OVER max rated torque in which slip would increase in the motor to achieve that. Spindles are commonly rated for continuous and 30min ratings. The 30 min rating is usually about 35% over cont. There is also usually another 1min rating that is at about 150% of cont. Could we assume they are trying to tap into breakdown torque here or is this just a situation of monitoring heat from the motor to determine rating?
Since motors are typically designed to have 200-250% breakdown torque (that is amount above nameplate torque) then it is used. But these S3 and S6-30 ratings are simply HEAT thermal limits, nothing more.
2. Assuming a motor is operating at a constant rpm but with variable load. As the motor is loaded, it will want to slow down or slip. A closed vector motor does a really good job of holding that speed rather than slowing down. How and why? What would the vector system do different than scalar in terms of helping the motor maintain speed? Would it be safe to assume the vector system would actually increase the hz to where the slip is increased to increase torque WITHOUT reducing the speed of the motor?
Yes. Remember, a 'skalar' drive is simply a power supply with no feedback. So it puts out a voltage at a given frequency so the motor simply runs down the 'slip' curve. A vector drive is some better; some simply increase the freq along the v/hz curve, others do much better control of the Isq current leaving the Isd magnetizing current constant. But yes, they will maintain speed better, both sensorless and sensored drives.
3. When looking at a motor configuration we are discussing, when you double the voltage that the motor is wired for, you double the HP but the torque remains constant. So could we probably call this spindle configuration constant torque?
of course. Then if you continue above that doubled voltage it will become constant hp.
4. In examining this a bit more, drives are rates for volts and amps. If amps stays the same and would have to for drive protection, you are saying that a motor would produce its rating torque as long as the V/Hz relationship is held. At what minimum speed would the motor torque fall off or could we hold this all the way down to say 1-2Hz?
never if a vector drive; @ 0rpm it has nameplate rated torque.
5. Everything seems to hover around "base speed". What determines base speed or is this just designed in with the number of poles? Reason I ask is an inverter duty motor (spindle motor) would always be connected to an inverter. Just curious how they determine "base speed" when that speed is so variable?
number poles gives simply a speed at a given hertz; there is 'nominal' ratings for this - 50 & 60hz. So you will see 1500, 1800, etc 'typical' base speeds. In the MT spindle world it is not uncommon for us to rate motors where the VOLTAGE reaches max available from a supply such as 230v, & 460v. Hence you will see wierd 'base speed' of that 2500rpm etc. 'Base speed' is just a name.
A couple of comments on voltage rating.
The old school voltage rating for NON VFD motors was based on magnetic saturation. As motor voltage is increased the strength of the magnetic field and the available torque increase until the iron reaches magnetic saturation. The motors generally are good for about 15%+ above rated voltage before saturation occurs. If a motor is pushed into saturation the inductive reaction no longer limits increases in current. The current increases greatly and the motor burns up.
Another voltage limit is the insulation rating. Dual voltage motors are insulated for the highest voltage. eg A 230:460V motor will be insulated for at least 460 Volts. With the advent of VFD rated motors the insulation is probably good for around 1000 Volts or more.
SO
The limit is saturation or the point where the inductive reactance no longer limits increases in current.
But inductive reaction is frequency dependent. If you double the frequency, you may double the rated voltage without fear of saturation.
When the frequency goes above base frequency the inductive reactance increases and reduces the magnetizing current, lowering the available torque. This is reducing the magnetic field strength further below the saturation point.
Because of the reduced inductive reactance at higher voltages we may safely increase the voltage on most motors and bring the magnetic field strength back to rated strength without saturating.
Is this cheating? I don't think so. I think that this is a heads up economy that was not feasible before the introduction of Variable Frequency Drives.
This is an extension of an effect that some of us were aware of and dealt with for years. That is the conversion of 50 Hz motors to 60 Hz and 60 Hz motors to 50 Hz.
When we used a 50 Hz motor on 60 Hz and had 20% more HP available due to the increased speed and voltage we didn't think we were cheating.
This 50Hz/60Hz conversion is two points on the HP/voltage curve that VFDs make available.
Bill
--------------------
"Why not the best?"
Jimmy Carter
The old school voltage rating for NON VFD motors was based on magnetic saturation. As motor voltage is increased the strength of the magnetic field and the available torque increase until the iron reaches magnetic saturation. The motors generally are good for about 15%+ above rated voltage before saturation occurs. If a motor is pushed into saturation the inductive reaction no longer limits increases in current. The current increases greatly and the motor burns up.
Another voltage limit is the insulation rating. Dual voltage motors are insulated for the highest voltage. eg A 230:460V motor will be insulated for at least 460 Volts. With the advent of VFD rated motors the insulation is probably good for around 1000 Volts or more.
SO
The limit is saturation or the point where the inductive reactance no longer limits increases in current.
But inductive reaction is frequency dependent. If you double the frequency, you may double the rated voltage without fear of saturation.
When the frequency goes above base frequency the inductive reactance increases and reduces the magnetizing current, lowering the available torque. This is reducing the magnetic field strength further below the saturation point.
Because of the reduced inductive reactance at higher voltages we may safely increase the voltage on most motors and bring the magnetic field strength back to rated strength without saturating.
Is this cheating? I don't think so. I think that this is a heads up economy that was not feasible before the introduction of Variable Frequency Drives.
This is an extension of an effect that some of us were aware of and dealt with for years. That is the conversion of 50 Hz motors to 60 Hz and 60 Hz motors to 50 Hz.
When we used a 50 Hz motor on 60 Hz and had 20% more HP available due to the increased speed and voltage we didn't think we were cheating.
This 50Hz/60Hz conversion is two points on the HP/voltage curve that VFDs make available.
Bill
--------------------
"Why not the best?"
Jimmy Carter
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