Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Nicklas Karlsson]

On Wed, 25 Jan 2017 23:22:29 -0500
Stephen Dubovsky  wrote:

> On Wed, Jan 25, 2017 at 10:25 PM, Jon Elson  wrote:
> 
> > Field weakening is not possible on permanent magnet
> > brushless motors.
> >
> >
> It actually is.  Apply current in the D axis will oppose and reduce the net
> flux in the gap.  Move a coil in between two N poles permanent magnets
> facing each other and you will get no voltage as the field is zero at the
> center.  You risk demagnetizing the magnets at high temp.  You also risk a
> fault in the drive shutting off all the fets/igbts while running near top
> speed, loosing the field weakening, back emf goes to full strength, and
> rectified voltage back through the bridge becomes MUCH higher than the DC
> bus can handle.

Did not try that but have blown up many inverters for other reasons.

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Nicklas Karlsson]

> On 01/25/2017 11:22 AM, Nicklas Karlsson wrote:
> > For permanent magnet motors there is a rather linear correlation between 
> > RPM and voltage although field weakening might be possible to lower voltage 
> > then loaded below rated torque.
> >
> >
> Field weakening is not possible on permanent magnet 
> brushless motors.

Never thought to much about it. I think some configurations of the magnets are 
rather hard to demagnetize but are not sure.

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Gene Heskett]

On Thursday 26 January 2017 10:11:05 Stephen Dubovsky wrote:

> Eric, Did you lock the rotor?  If you don't and just apply a field the
> motor will align to a stable N-S.  Even at VERY high temps & fields
> you will not damage the magnets (you are actually trying to magnetize
> them even more - its how magnets are made afterall.)  You need to lock
> the rotor (or use active field/orientation control) if using opposing
> forces just sitting still N-N & S-S as this is a highly unstable
> condition.  Any tiny offset will generate Q axis forces and the rotor
> will *RAPIDLY* move to the stable N-S state.  We never ruined a motor
> but were instructed by the motor designers that it can happen.  No
> idea what the coercivity of a modern neodymium magnet is at typical
> motor operating temp.
>
In useing Jon's pwm-servo amp to run 1 hp spindle motors, that concern 
has allways entered into my thinking when setting the current limit of 
that amplifier. Those motors have an FLA of about 9.5 amps, so the amps 
were setup to hold to 15 max. I ran an ER32 nut down onto a clamp and 
stalled it for 3 or 4 seconds 2 days ago, but when it had moved off the 
clamp, everything fired right back up. I can hear the motor iron singing 
at about 14 kilohertz as it limits. But I don't believe I have damaged 
the magnets.  Bearings in the mills headstock, now those we won't 
discuss.  They were crap from the gitgo and I have got to replace them 
with quality bearings by spring if not sooner.

> On Thu, Jan 26, 2017 at 7:09 AM, Eric Keller  wrote:
> > On Wed, Jan 25, 2017 at 11:22 PM, Stephen Dubovsky
> > 
> >
> > wrote:
> > > On Wed, Jan 25, 2017 at 10:25 PM, Jon Elson
> > > 
> >
> > wrote:
> > >> Field weakening is not possible on permanent magnet
> > >> brushless motors.
> > >
> > > It actually is.  Apply current in the D axis will oppose and
> > > reduce the
> >
> > net
> >
> > > flux in the gap.  Move a coil in between two N poles permanent
> > > magnets facing each other and you will get no voltage as the field
> > > is zero at the center.  You risk demagnetizing the magnets at high
> > > temp.
> >
> > We tried to do this for some fault detection research.  We set the
> > winding currents so that the motor didn't move, but just heated
> > itself.  No failures.  There is enough change in the magnets with
> > heat that we could detect the changes, but they went right back to
> > normal when the temperature went back down. We were hoping to get
> > some degradation in the magnets over time, but it never happened. It
> > was disappointing and reassuring at the same time. I suppose we
> > should have gone higher with the current, because we never had any
> > winding failures either. The drive was made for a lot more current
> > than the motors could take.
> >
> > 
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Cheers, Gene Heskett
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-Ed Howdershelt (Author)
Genes Web page 

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Stephen Dubovsky]

Eric, Did you lock the rotor?  If you don't and just apply a field the
motor will align to a stable N-S.  Even at VERY high temps & fields you
will not damage the magnets (you are actually trying to magnetize them even
more - its how magnets are made afterall.)  You need to lock the rotor (or
use active field/orientation control) if using opposing forces just sitting
still N-N & S-S as this is a highly unstable condition.  Any tiny offset
will generate Q axis forces and the rotor will *RAPIDLY* move to the stable
N-S state.  We never ruined a motor but were instructed by the motor
designers that it can happen.  No idea what the coercivity of a modern
neodymium magnet is at typical motor operating temp.

On Thu, Jan 26, 2017 at 7:09 AM, Eric Keller  wrote:

> On Wed, Jan 25, 2017 at 11:22 PM, Stephen Dubovsky 
> wrote:
> > On Wed, Jan 25, 2017 at 10:25 PM, Jon Elson 
> wrote:
> >
> >> Field weakening is not possible on permanent magnet
> >> brushless motors.
> >>
> >>
> > It actually is.  Apply current in the D axis will oppose and reduce the
> net
> > flux in the gap.  Move a coil in between two N poles permanent magnets
> > facing each other and you will get no voltage as the field is zero at the
> > center.  You risk demagnetizing the magnets at high temp.
>
> We tried to do this for some fault detection research.  We set the
> winding currents so that the motor didn't move, but just heated
> itself.  No failures.  There is enough change in the magnets with heat
> that we could detect the changes, but they went right back to normal
> when the temperature went back down. We were hoping to get some
> degradation in the magnets over time, but it never happened. It was
> disappointing and reassuring at the same time. I suppose we should
> have gone higher with the current, because we never had any winding
> failures either. The drive was made for a lot more current than the
> motors could take.
>
> 
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> engaging tech sites, SlashDot.org! http://sdm.link/slashdot
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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Eric Keller]

On Wed, Jan 25, 2017 at 11:22 PM, Stephen Dubovsky  wrote:
> On Wed, Jan 25, 2017 at 10:25 PM, Jon Elson  wrote:
>
>> Field weakening is not possible on permanent magnet
>> brushless motors.
>>
>>
> It actually is.  Apply current in the D axis will oppose and reduce the net
> flux in the gap.  Move a coil in between two N poles permanent magnets
> facing each other and you will get no voltage as the field is zero at the
> center.  You risk demagnetizing the magnets at high temp.

We tried to do this for some fault detection research.  We set the
winding currents so that the motor didn't move, but just heated
itself.  No failures.  There is enough change in the magnets with heat
that we could detect the changes, but they went right back to normal
when the temperature went back down. We were hoping to get some
degradation in the magnets over time, but it never happened. It was
disappointing and reassuring at the same time. I suppose we should
have gone higher with the current, because we never had any winding
failures either. The drive was made for a lot more current than the
motors could take.

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Stephen Dubovsky]

On Wed, Jan 25, 2017 at 10:25 PM, Jon Elson  wrote:

> Field weakening is not possible on permanent magnet
> brushless motors.
>
>
It actually is.  Apply current in the D axis will oppose and reduce the net
flux in the gap.  Move a coil in between two N poles permanent magnets
facing each other and you will get no voltage as the field is zero at the
center.  You risk demagnetizing the magnets at high temp.  You also risk a
fault in the drive shutting off all the fets/igbts while running near top
speed, loosing the field weakening, back emf goes to full strength, and
rectified voltage back through the bridge becomes MUCH higher than the DC
bus can handle.  Been there done that w/ a eurocopter APU starter we
designed w/ Kollmorgen. They had to add an output contactor that opens for
a drive fault.
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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Jon Elson]

On 01/25/2017 11:22 AM, Nicklas Karlsson wrote:
> For permanent magnet motors there is a rather linear correlation between RPM 
> and voltage although field weakening might be possible to lower voltage then 
> loaded below rated torque.
>
>
Field weakening is not possible on permanent magnet 
brushless motors.

Jon

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Jon Elson]

On 01/25/2017 06:10 AM, Steve from Tube Gauge wrote:
> Hi:
>   
> I have a CNC mill that I am considering replacing the control with LinuxCNC.  
> The motors are the red cap Fanuc units.  I have enquired on this mailing list 
> about using the existing servo amps and motors.  From all the replies and 
> help (thank you very much)  I haved concluded that a good solution for me is 
> to remove the Fanuc encoders and amps  and replace them with suitable units 
> that can communicate on an industry standard format.  To that end I am 
> considering using some AMT31 programmable encoders by CUI (or other encoder 
> suitable for 8 pole motors) and some BE40A8 AMC amps that I have sitting on 
> my shelf.
>   
> I am aware of the rpm/voltage relationship with brush DC motors.  Generally, 
> as the voltage rises the motor rpm increases a proportional amount.  How does 
> the voltage influence the performance of the brushless motor?  The Fanuc 
> motors are 5S models, 126 V (DC?), 5.8A stall.  The BE40A8 have a maximum 
> running voltage of 80 VDC.  Will running the motors at the lower voltage 
> result in dramatically reduced performance?  Will it cause other electrical 
> problems like an excessive rise in current demand by the motor?
>   
>
Supply voltage limits speed, current limits torque.  So, 
running a 126 V motor on 80 V will limit you to 80/126 of 
the rated speed. So, if the motor is rated for 2000 RPM at 
126 V, you will only get 1250 RPM.  Directly driving a 5 TPI 
leadscrew, you would get 250 IPM linear traverse.

It will not increase the current demand of the motor at 
all.  (That would apply to an induction motor, only.)

Jon

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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Nicklas Karlsson]

For permanent magnet motors there is a rather linear correlation between RPM 
and voltage although field weakening might be possible to lower voltage then 
loaded below rated torque.

On Wed, 25 Jan 2017 07:10:05 -0500
"Steve from Tube Gauge"  wrote:

> Hi:
>  
> I have a CNC mill that I am considering replacing the control with LinuxCNC.  
> The motors are the red cap Fanuc units.  I have enquired on this mailing list 
> about using the existing servo amps and motors.  From all the replies and 
> help (thank you very much)  I haved concluded that a good solution for me is 
> to remove the Fanuc encoders and amps  and replace them with suitable units 
> that can communicate on an industry standard format.  To that end I am 
> considering using some AMT31 programmable encoders by CUI (or other encoder 
> suitable for 8 pole motors) and some BE40A8 AMC amps that I have sitting on 
> my shelf. 
>  
> I am aware of the rpm/voltage relationship with brush DC motors.  Generally, 
> as the voltage rises the motor rpm increases a proportional amount.  How does 
> the voltage influence the performance of the brushless motor?  The Fanuc 
> motors are 5S models, 126 V (DC?), 5.8A stall.  The BE40A8 have a maximum 
> running voltage of 80 VDC.  Will running the motors at the lower voltage 
> result in dramatically reduced performance?  Will it cause other electrical 
> problems like an excessive rise in current demand by the motor?
>  
> Thanks for your help - Steve V
> 
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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[dragon]

Steve,

From my, albeit somewhat limited, experience I don't think that you will
be that happy with the performance of running the drives at that
voltage. I have some experience with running brushed motors at just over
half voltage and then later going to full voltage on the same machines.
Running with a maximum of 80v means a 37% drop in voltage. The biggest
place that I think you would see it is in poor acceleration and
deceleration. You would likely be limited to about 1/3 the IPM that you
would running full voltage because you need to have enough reserve to
decelerate without causing a following error. This might be even worse
with a brushless but I just don't have experience with them.

You could certainly try with those amps while you look around for
different ones. You won't hurt the motors.



On 01/25/2017 06:10 AM, Steve from Tube Gauge wrote:
> Hi:
>  
> I have a CNC mill that I am considering replacing the control with LinuxCNC.  
> The motors are the red cap Fanuc units.  I have enquired on this mailing list 
> about using the existing servo amps and motors.  From all the replies and 
> help (thank you very much)  I haved concluded that a good solution for me is 
> to remove the Fanuc encoders and amps  and replace them with suitable units 
> that can communicate on an industry standard format.  To that end I am 
> considering using some AMT31 programmable encoders by CUI (or other encoder 
> suitable for 8 pole motors) and some BE40A8 AMC amps that I have sitting on 
> my shelf. 
>  
> I am aware of the rpm/voltage relationship with brush DC motors.  Generally, 
> as the voltage rises the motor rpm increases a proportional amount.  How does 
> the voltage influence the performance of the brushless motor?  The Fanuc 
> motors are 5S models, 126 V (DC?), 5.8A stall.  The BE40A8 have a maximum 
> running voltage of 80 VDC.  Will running the motors at the lower voltage 
> result in dramatically reduced performance?  Will it cause other electrical 
> problems like an excessive rise in current demand by the motor?
>  
> Thanks for your help - Steve V
> 
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Re: [Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Andrew]

2017-01-25 14:10 GMT+02:00 Steve from Tube Gauge :

> Hi:
>
> I have a CNC mill that I am considering replacing the control with
> LinuxCNC.  The motors are the red cap Fanuc units.  I have enquired on this
> mailing list about using the existing servo amps and motors.  From all the
> replies and help (thank you very much)  I haved concluded that a good
> solution for me is to remove the Fanuc encoders and amps  and replace them
> with suitable units that can communicate on an industry standard format.
> To that end I am considering using some AMT31 programmable encoders by CUI
> (or other encoder suitable for 8 pole motors) and some BE40A8 AMC amps that
> I have sitting on my shelf.
>
> I am aware of the rpm/voltage relationship with brush DC motors.
> Generally, as the voltage rises the motor rpm increases a
> proportional amount.  How does the voltage influence the performance of the
> brushless motor?  The Fanuc motors are 5S models, 126 V (DC?), 5.8A stall.
> The BE40A8 have a maximum running voltage of 80 VDC.  Will running the
> motors at the lower voltage result in dramatically reduced performance?
> Will it cause other electrical problems like an excessive rise in current
> demand by the motor?
>
>
AFAIK motor torque at higher rpm will be seriously decreased at lower
voltage.
I.e. if the nominal is 3000 rpm, you'll have 3000 * 80 / 127 = 1890 rpm max
But probably they require 127V for 4500 rpm, then it's better: 4500 * 80 /
127 = 2830 rpm

Andrew
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[Emc-users] Voltage/Motor RPM Relationship with Brushless Drives

[Steve from Tube Gauge]

Hi:
 
I have a CNC mill that I am considering replacing the control with LinuxCNC.  
The motors are the red cap Fanuc units.  I have enquired on this mailing list 
about using the existing servo amps and motors.  From all the replies and help 
(thank you very much)  I haved concluded that a good solution for me is to 
remove the Fanuc encoders and amps  and replace them with suitable units that 
can communicate on an industry standard format.  To that end I am considering 
using some AMT31 programmable encoders by CUI (or other encoder suitable for 8 
pole motors) and some BE40A8 AMC amps that I have sitting on my shelf. 
 
I am aware of the rpm/voltage relationship with brush DC motors.  Generally, as 
the voltage rises the motor rpm increases a proportional amount.  How does the 
voltage influence the performance of the brushless motor?  The Fanuc motors are 
5S models, 126 V (DC?), 5.8A stall.  The BE40A8 have a maximum running voltage 
of 80 VDC.  Will running the motors at the lower voltage result in dramatically 
reduced performance?  Will it cause other electrical problems like an excessive 
rise in current demand by the motor?
 
Thanks for your help - Steve V

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