I have 2 4160VAC 4500hp Sync motors that power reciprocating compressors. My issue is starting the electric motors, this is very inconsistent sometimes they start and sync, and sometimes they don't sync. I am trying to make the issue with management that the reason they are inconsistent is because the motors are tying to start with the compressor loaded with anywhere from 650 to 900 psi pressure. Management says that because the pressure on the compressor is equal on both sides of the compressor (suction and discharge), and a bypass valve is open tying both sides together that this is the same as having zero pressure on the compressor. I know this is wrong, because when I had to work on the motor I couldn't rotate it if there was any pressure on the compressor. If I unloaded the compressor the motor would spin freely. What I'm asking for is a basic law of physics that describes what I'm trying to prove, yes I could do the homework myself, but I'm hoping someone knows off the top of their head. Thanks
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4160VAC Sync Motor and Physics
- Thread starter mkees
- Start date
Is this a new installation or a problem that has developed with an existing application that worked well at one time?
The bypass or unloader is malfunctioning.
I have had similar problems with compressors failing to start.
Causes ranged from stuck unloader valves, leaking unloader valves, to circuit problems with electrically controlled unloader systems.
With a conventional compressor where the case is at atmospheric pressure, you will have up to 900 LBS. pressure pushing the pistons down.
Refrigeration type compressors have the suction applied to the crank case. When the compressor is stopped, the pressure on the top of the pistons is balanced by the pressure in the crankcase acting on the bottom of the pistons.
A conventional compressor will have line pressure (650 psi to 900 psi) on the top of the pistons and atmospheric pressure acting on the bottom of the pistons.
If these are air compressors, then waste the air in the compressor prior to starting.
If this is a gas that can not be vented, I am sure that someone from the oil and gas industry will be able to provide advice.
I have to admit that I may have misunderstood the problem.
If the compressors always start but sometimes fail to sync, then the problem may be electrical or mechanical. We will need some more information about the system.
Bill
--------------------
"Why not the best?"
Jimmy Carter
The bypass or unloader is malfunctioning.
I have had similar problems with compressors failing to start.
Causes ranged from stuck unloader valves, leaking unloader valves, to circuit problems with electrically controlled unloader systems.
Compressor manufacturers don't design equipment that will not start reliably, but compressor skids may be misapplied. Hard to accept that a machine of that size may have been misapplied but stuff happens.OP. said:
This is only true with a pressurised crank case.Management said:
With a conventional compressor where the case is at atmospheric pressure, you will have up to 900 LBS. pressure pushing the pistons down.
Refrigeration type compressors have the suction applied to the crank case. When the compressor is stopped, the pressure on the top of the pistons is balanced by the pressure in the crankcase acting on the bottom of the pistons.
A conventional compressor will have line pressure (650 psi to 900 psi) on the top of the pistons and atmospheric pressure acting on the bottom of the pistons.
If these are air compressors, then waste the air in the compressor prior to starting.
If this is a gas that can not be vented, I am sure that someone from the oil and gas industry will be able to provide advice.
I have to admit that I may have misunderstood the problem.
If the compressors always start but sometimes fail to sync, then the problem may be electrical or mechanical. We will need some more information about the system.
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Thread starter
- #3
Better description of this compressor. This compressor compresses on both strokes, pressure on both sides. This is a natural gas compressor (Cooper). The crankcase is not pressurized. I have recently upgraded the sync controller, because the original, was nearly prehistoric, parts were becoming hard to find, and I was frequently blowing up diodes on the motors' rotating assembly. Bypass valve and compressor work properly. Please ask more questions. I approved a lot of money for this supposed improvement and i hate thinking I didn't improve the operation. Good news is I'm not blowing diodes anymore.
Do I understand, an old machine with a new upgrade?
New sync controller?
The sync controller looks at the frequency of the induced voltage in the field winding. As the motor accelerates the field frequency drops. (The original control was called a "Polarized Field Frequency Relay".)
I suspect that the new controller wants to see a lower frequency and higher speed before sync'ing than the original control.
Running as an induction motor the motor would reach a speed sufficient to trigger the old sync control.
That speed may not be enough to trigger the new sync control.
There may be a simple adjustment to allow sync at a slightly lower speed.
RTFI
Failing that, contact the control manufacturer for advice on changing the sync speed setting.
Bill
--------------------
"Why not the best?"
Jimmy Carter
New sync controller?
The sync controller looks at the frequency of the induced voltage in the field winding. As the motor accelerates the field frequency drops. (The original control was called a "Polarized Field Frequency Relay".)
I suspect that the new controller wants to see a lower frequency and higher speed before sync'ing than the original control.
Running as an induction motor the motor would reach a speed sufficient to trigger the old sync control.
That speed may not be enough to trigger the new sync control.
There may be a simple adjustment to allow sync at a slightly lower speed.
RTFI
Failing that, contact the control manufacturer for advice on changing the sync speed setting.
Bill
--------------------
"Why not the best?"
Jimmy Carter
electricpete
Electrical
I'm not a compressor guy. I'll just throw out some thoughts on original question for consideration:
1 - is the effective area the same on both sides of the piston? (Piston rod may occupy some of that area). If area is different than the gas pressure can apply a force on the piston even without any differential pressure (Force = P1*A1-P2*A2)
2 - how much dp is required to open the valves. They may be spring loaded. Spring loaded valve with small dp across it may mean the same small dp appears across the piston. Maybe enough to explain the difference you feel by hand.
3 - does gas pressure affect oil to bearing in any way? (doubt that it would but just asking)
4 - does the compressor vent any gas to atmosphere during normal operation or is it entirely contained between suction and discharge. If there is a vent path then would want to explore it to understand if it can somehow bleed air on one side while shutdown.
=====================================
(2B)+(2B)' ?
1 - is the effective area the same on both sides of the piston? (Piston rod may occupy some of that area). If area is different than the gas pressure can apply a force on the piston even without any differential pressure (Force = P1*A1-P2*A2)
2 - how much dp is required to open the valves. They may be spring loaded. Spring loaded valve with small dp across it may mean the same small dp appears across the piston. Maybe enough to explain the difference you feel by hand.
3 - does gas pressure affect oil to bearing in any way? (doubt that it would but just asking)
4 - does the compressor vent any gas to atmosphere during normal operation or is it entirely contained between suction and discharge. If there is a vent path then would want to explore it to understand if it can somehow bleed air on one side while shutdown.
=====================================
(2B)+(2B)' ?
- Thread starter
- #6
Waross, yes it is a new sync controller. I know it is different from the old because I had to pull wires from the CT's and PT's to connect to the controller. I thought the controllers were Power Factor based and not frequency. I don't know of anything on these controllers that controls frequency, I thought frequency was set by the service provider. I will ask the manufacturer about an adjustment for lower speed sync. thanks for the advice.
Electricpete some of your questions I don't have answers, but I'll answer what I know
1 - The effective area on both sides is the same and there is a piston rod on one side.
2 - I don't know about the DP required to open the valves I'll ask.
3 - Gas pressure doesn't affect oil to bearing
4 - the compressor doesn't vent to atmosphere during normal operation
Thanks
Electricpete some of your questions I don't have answers, but I'll answer what I know
1 - The effective area on both sides is the same and there is a piston rod on one side.
2 - I don't know about the DP required to open the valves I'll ask.
3 - Gas pressure doesn't affect oil to bearing
4 - the compressor doesn't vent to atmosphere during normal operation
Thanks
LionelHutz
Electrical
With the mention of replacing diodes in the rotating rectifier, I doubt this is a brush type motor which means the controller does not monitor the field frequency as the motor accelerates.
You need to get management to try and turn the motor with the compressor loaded and with it unloaded. An actual physical demonstration works way better than theoretical equations.
You need to provide details of the mechanical design so others can tell how the compressor is loaded, or you have to look at the compressor and determine how the pressure in the cylinders is still resisting rotation. You posted it was double sided compression or pressure on both sides of the piston but the crankcase was not pressurized but I'm not sure that tells the whole story.
The basic math is pressure times piston area gives the force required to move the piston.
You need to get management to try and turn the motor with the compressor loaded and with it unloaded. An actual physical demonstration works way better than theoretical equations.
You need to provide details of the mechanical design so others can tell how the compressor is loaded, or you have to look at the compressor and determine how the pressure in the cylinders is still resisting rotation. You posted it was double sided compression or pressure on both sides of the piston but the crankcase was not pressurized but I'm not sure that tells the whole story.
The basic math is pressure times piston area gives the force required to move the piston.
At locked rotor, a voltage is induced in the motor field winding. The frequency of this voltage is the slip frequency. At locked rotor the slip frequency equals the line frequency. As the motor accelerates, the slip frequency drops. When the motor is close to synchronous speed the slip frequency is very low. The synchronizing gear looks at the slip frequency.
A synchronous motor may have a PF controller to help correct the plant power factor. Is that the controller that was replaced?
Just to clarify: The motor used to start with no problem. The motor still starts but will not always synchronize after the change of controller. Is this correct?
Bill
--------------------
"Why not the best?"
Jimmy Carter
A synchronous motor may have a PF controller to help correct the plant power factor. Is that the controller that was replaced?
Just to clarify: The motor used to start with no problem. The motor still starts but will not always synchronize after the change of controller. Is this correct?
Bill
--------------------
"Why not the best?"
Jimmy Carter
- Thread starter
- #9
Here's what I have, a Basler MVC236, a Basler SCP250m, and a Hyundai Ideal Digital Synchronizer System.
The motor always starts and spins, but doesn't always sync, at that point I get a PF trip on my Multilin 469. I went back and read the book on the Hyundai Ideal DSS, and sure enough it works based on frequency and voltage. I don't know if there is an adjustment to allow synchronization at a slower speed, book didn't mention it. I have ask the Vendor, no response yet.
I found out that the compressor piston is 14" in diameter. I think I did my math right, with 900psi on the compressor I get nearly a force of 40000 pounds to move the compressor.
The motor always starts and spins, but doesn't always sync, at that point I get a PF trip on my Multilin 469. I went back and read the book on the Hyundai Ideal DSS, and sure enough it works based on frequency and voltage. I don't know if there is an adjustment to allow synchronization at a slower speed, book didn't mention it. I have ask the Vendor, no response yet.
I found out that the compressor piston is 14" in diameter. I think I did my math right, with 900psi on the compressor I get nearly a force of 40000 pounds to move the compressor.
Hi mkees,
I have never seen a synch-motor started under a loaded condition, especially when driving a constant displacement compressor.
If I understand your condition; the compressor is provided with a bypass loop (to aid starting), and some form of on-off valve on the compressor discharge to enable gas flow to the plant header. What takes you down is a PF trip on the 469, after the motor starts to accelerate.
Per the 469 manual;
Can you confirm that the (469) PF trip is blocked until such time as the field is applied? Frankly, if the 469 is configured properly and in fact operating correctly, I am surprised that you would see this trip.
I looked thru the 469 manual and could not find any info on how the 469 knows when field has been applied. Does anyone know how the relay determines when the field has been successfully applied? Is it some highfalutin algorithm or does it need a discrete input to be toggled, say from the exciter?
As a heads-up, many years ago I was commissioning 7 x 16,000 synch-motors, each with a 269 (MPR) relay. In those days the MPR determined that the motor was running by monitoring the current, and not by monitoring a 52a contact from the CB. The problem was that since the motors were started unloaded, and the exciter was set to approx unity pf, that the MPR would trip-out on 'too many starts' whenever the motor was unloaded or lightly loaded. GE finally provided us with a firmware update to allow a 52a contact to be brought in the MPR. This feature is now standard on all GE MPR relays, but I have routinely seen switchgear manufacturers not wire a 52a contact into the MPR. This is not a problem with induction motors, but can be an issue with synch motors.
I have never seen a synch-motor started under a loaded condition, especially when driving a constant displacement compressor.
If I understand your condition; the compressor is provided with a bypass loop (to aid starting), and some form of on-off valve on the compressor discharge to enable gas flow to the plant header. What takes you down is a PF trip on the 469, after the motor starts to accelerate.
Per the 469 manual;
If the 469 is applied on a synchronous motor, it is desirable not to trip or alarm on power
factor until the field has been applied. Therefore, this feature can be blocked until the
motor comes up to speed and the field is applied. From that point forward, the power
factor trip and alarm elements will be active. Once the power factor is less than either the
Lead or Lag level, for the specified delay, a trip or alarm will occur indicating a Lead or Lag
condition. The power factor alarm can be used to detect loss of excitation and out of step.
factor until the field has been applied. Therefore, this feature can be blocked until the
motor comes up to speed and the field is applied. From that point forward, the power
factor trip and alarm elements will be active. Once the power factor is less than either the
Lead or Lag level, for the specified delay, a trip or alarm will occur indicating a Lead or Lag
condition. The power factor alarm can be used to detect loss of excitation and out of step.
Can you confirm that the (469) PF trip is blocked until such time as the field is applied? Frankly, if the 469 is configured properly and in fact operating correctly, I am surprised that you would see this trip.
I looked thru the 469 manual and could not find any info on how the 469 knows when field has been applied. Does anyone know how the relay determines when the field has been successfully applied? Is it some highfalutin algorithm or does it need a discrete input to be toggled, say from the exciter?
As a heads-up, many years ago I was commissioning 7 x 16,000 synch-motors, each with a 269 (MPR) relay. In those days the MPR determined that the motor was running by monitoring the current, and not by monitoring a 52a contact from the CB. The problem was that since the motors were started unloaded, and the exciter was set to approx unity pf, that the MPR would trip-out on 'too many starts' whenever the motor was unloaded or lightly loaded. GE finally provided us with a firmware update to allow a 52a contact to be brought in the MPR. This feature is now standard on all GE MPR relays, but I have routinely seen switchgear manufacturers not wire a 52a contact into the MPR. This is not a problem with induction motors, but can be an issue with synch motors.
LionelHutz
Electrical
Since it didn't get through to anyone.
**** YOU HAVE TO HAVE A BRUSH TYPE MOTOR OR ELSE YOU CAN NOT CHANGE THE SYNCHRONIZING SPEED. ****
Diodes on a rotating assembly tells me this motor is brushless. Your external exciter can not detect the rotor speed of a brushless motor by direct measurement of the field winding because the field winding IS NOT external to the rotor.
**** YOU HAVE TO HAVE A BRUSH TYPE MOTOR OR ELSE YOU CAN NOT CHANGE THE SYNCHRONIZING SPEED. ****
Diodes on a rotating assembly tells me this motor is brushless. Your external exciter can not detect the rotor speed of a brushless motor by direct measurement of the field winding because the field winding IS NOT external to the rotor.
- Thread starter
- #12
I do apologize, these are brushless motors.
I know that when the motor is started the exciter field current is applied, i don't know if there is a contact providing this to the 469, I"ll have to look at the prints on Monday. I also know there is a time (30 seconds) before the 469 really looks at anything. At the end of that 30 seconds I immediately get a PF fault if the motor hasn't sync'd.
Trying to read between the lines from what everyone is telling me, sounds like the compressor needs to be unloaded for the motor to start properly.
I know that when the motor is started the exciter field current is applied, i don't know if there is a contact providing this to the 469, I"ll have to look at the prints on Monday. I also know there is a time (30 seconds) before the 469 really looks at anything. At the end of that 30 seconds I immediately get a PF fault if the motor hasn't sync'd.
Trying to read between the lines from what everyone is telling me, sounds like the compressor needs to be unloaded for the motor to start properly.
Hi mkees,
I have installed dozens of MV drive systems onto gas recip compressors (ie Ariel) over the last several years and every one of them is unloaded for starting. These systems use induction motors and voltage-source PWM drives, so it's not quite an apples-to-apples comparison, but the PWM drive is capable of producing significant more starting torque than a DOL starter driving a synch motor. I'm quite sure that a PWM drive could start an induction-motor / recip-compressor fully loaded, but I have never tried to do so.
However, I would be extremely surprised if your synch-motor / recip-compressor is suited for anything but an unloaded start. Keep in mind, even though a compressor may have a bypass loop (for starting), there is always some load on the motor, due to the inertia of the moving components and the fact that gas is being pushed around the loop. I have had compressor OEMs tell me that it is as high as 25%.
Also I would think that your motor should be able to accelerate and synch within (say) 5s. If you are tripping out at 30s, what is the motor current during this period? Ditto for the voltage (I'm assuming that the power supply is stiff enough so as to not drop the voltage by more than (say) 10%).
Any chance you can provide I & V curves for the motor during the acceleration interval? This data is available from the 469.
GG
I have installed dozens of MV drive systems onto gas recip compressors (ie Ariel) over the last several years and every one of them is unloaded for starting. These systems use induction motors and voltage-source PWM drives, so it's not quite an apples-to-apples comparison, but the PWM drive is capable of producing significant more starting torque than a DOL starter driving a synch motor. I'm quite sure that a PWM drive could start an induction-motor / recip-compressor fully loaded, but I have never tried to do so.
However, I would be extremely surprised if your synch-motor / recip-compressor is suited for anything but an unloaded start. Keep in mind, even though a compressor may have a bypass loop (for starting), there is always some load on the motor, due to the inertia of the moving components and the fact that gas is being pushed around the loop. I have had compressor OEMs tell me that it is as high as 25%.
Also I would think that your motor should be able to accelerate and synch within (say) 5s. If you are tripping out at 30s, what is the motor current during this period? Ditto for the voltage (I'm assuming that the power supply is stiff enough so as to not drop the voltage by more than (say) 10%).
Any chance you can provide I & V curves for the motor during the acceleration interval? This data is available from the 469.
GG
Hi Lionel,
I don't understand your comment wrt brushless vs brush type exciters, and the impact it might have on the problem here.
The last time I dealt with large synchronous motors was many years ago. These were the 7 x 16,000hp (brushless TEWAC) motors that I mentioned above. The motors and exciters were made by GE Canada, and I recall that they were equipped with a digital tachometer (ie AirPax) which could be set to plus/minus 1rpm. This tachometer would sense when the speed was close to synchronous and then command the field to be turned on. I believe that the exciter had some means to ensure that the field was applied properly and the motor was in fact synch'd (but I don't recall the exact details of the latter).
I'm not sure if the motor in question here uses a digital tach or not (for field application); perhaps mkees (OP) could comment on this.
Regards,
GG
I don't understand your comment wrt brushless vs brush type exciters, and the impact it might have on the problem here.
The last time I dealt with large synchronous motors was many years ago. These were the 7 x 16,000hp (brushless TEWAC) motors that I mentioned above. The motors and exciters were made by GE Canada, and I recall that they were equipped with a digital tachometer (ie AirPax) which could be set to plus/minus 1rpm. This tachometer would sense when the speed was close to synchronous and then command the field to be turned on. I believe that the exciter had some means to ensure that the field was applied properly and the motor was in fact synch'd (but I don't recall the exact details of the latter).
I'm not sure if the motor in question here uses a digital tach or not (for field application); perhaps mkees (OP) could comment on this.
Regards,
GG
- Thread starter
- #16
Let me start with the easy question first, there is no tach, the exciter field is applied as soon as the motor contactor is energized. Exciter field current is constant at about 6.5 amps for 30 seconds, then for 5-1/2 minutes the exciter field is held constant at 13 amps while the bypass valve closes, and the compressor loads. Then the exciter field controller goes into automatic control and field excitation typically drops a bit then climbs to between 10 to 15 amps dependent on load.
The voltage I'm told by the power company is normal, but I'm not buying it. I don't have much of an argument because the power company is always right (so they say). The voltage at the station is typically 4500VAC, I've seen it as high as 4700, when the motor starts voltage drops to 3400-3500VAC, and current on the motor at initial start (locked rotor) 2100-2300 amps for 2-4 seconds, then it begins to drop, if it sync's current will be about 130, if not it hangs out at 400-450 amps. This may change once I get the compressor unloaded at start.
I guess I could disable the Multilin, but I certainly don't want to damage the motor. I'm told by the manufacturer that I shouldn't run it for very long with out being sync'd because this will cause the motor to heat quickly and cause fatal motor damage. I also understand this is a 1 million dollar repair, and I haven't updated my resume lately.
The voltage I'm told by the power company is normal, but I'm not buying it. I don't have much of an argument because the power company is always right (so they say). The voltage at the station is typically 4500VAC, I've seen it as high as 4700, when the motor starts voltage drops to 3400-3500VAC, and current on the motor at initial start (locked rotor) 2100-2300 amps for 2-4 seconds, then it begins to drop, if it sync's current will be about 130, if not it hangs out at 400-450 amps. This may change once I get the compressor unloaded at start.
I guess I could disable the Multilin, but I certainly don't want to damage the motor. I'm told by the manufacturer that I shouldn't run it for very long with out being sync'd because this will cause the motor to heat quickly and cause fatal motor damage. I also understand this is a 1 million dollar repair, and I haven't updated my resume lately.
Hi mkees,
1) Wow, the voltage swings at your facility are a tad extreme. NEMA MG-1 states, that for synch motors, the max voltage swing at the motor terminals should not exceed plus/minus 10%. Your min/max voltage swings are -15% / + 17.5%. Note: the min voltage will be lower at the motor terminals. Allowing for (say) another 2.5% V-drop between the motor-starter and the motor will result in a 32% decrease in starting torque.
This may be a contributing factor to the difficulty in starting your motor.
2) Does anyone else find it odd that "the exciter field is applied as soon as the motor contactor is energized". I didn't think that the field is normally applied until the rotor is near synchronous speed. Am I missing something here?
Regards,
GG
ps My calcs, in 1) above, are based on a motor NP voltage of 4000V.
1) Wow, the voltage swings at your facility are a tad extreme. NEMA MG-1 states, that for synch motors, the max voltage swing at the motor terminals should not exceed plus/minus 10%. Your min/max voltage swings are -15% / + 17.5%. Note: the min voltage will be lower at the motor terminals. Allowing for (say) another 2.5% V-drop between the motor-starter and the motor will result in a 32% decrease in starting torque.
This may be a contributing factor to the difficulty in starting your motor.
2) Does anyone else find it odd that "the exciter field is applied as soon as the motor contactor is energized". I didn't think that the field is normally applied until the rotor is near synchronous speed. Am I missing something here?
Regards,
GG
ps My calcs, in 1) above, are based on a motor NP voltage of 4000V.
Hi GroovyGuy;
The lower current may be minimizing the "cogging" or torque transients of days of old, when it was common for full voltage to be applied to the field.
mkees:
Have you checked the integrity of the amortisseur winding?
Bill
--------------------
"Why not the best?"
Jimmy Carter
The lower current may be minimizing the "cogging" or torque transients of days of old, when it was common for full voltage to be applied to the field.
mkees:
Have you checked the integrity of the amortisseur winding?
Bill
--------------------
"Why not the best?"
Jimmy Carter
LionelHutz
Electrical
15A is the maximum exciter current? Using 6.5A you are limiting the pull-in torque which could be causing your issues.
You have never posted if the unloader has always been the same or not.
GroovyGuy - the whole conversation going on about setting the synchronous speed by measuring the field winding frequency is not relative to a brushless machine. It's also not htat common to see a tachometer used on a brushless motor.
You have never posted if the unloader has always been the same or not.
GroovyGuy - the whole conversation going on about setting the synchronous speed by measuring the field winding frequency is not relative to a brushless machine. It's also not htat common to see a tachometer used on a brushless motor.
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