How would an induction motor (6000 hp, 6600 rpm, 4160V) compare to a synchronous motor (6000 hp, 1200 rpm, 4160V) in terms of power from the line to shaft power. I understand that there would be some slip with the induction motor but some rough numbers would help. Also, how much, if any, does power factor play into things. Would the answers be any different if their were a VFD in front of this motor?
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Efficiency of motors (synchronous vs induction) 3
- Thread starter Zoobie
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You are leaving out a lot of detail, such as:
Why the extreme difference in speed, and is that necessary?
A 6600RPM induction motor would be a custom design, are you sure that is not a typo for 3600RPM?
Does the load need to turn at the higher speed or the lower speed?
Are you planning on using a gear reduction to get it down to 1200RPM? or...
Are you planning on gearing the 1200RPM synch motor up to match the speed of the Induction Motor?
Are you penalized for power factor? Not everyone is.
Is there some other advantage to having variable speed in your process?
Which begs the question, what is the machine and what is it doing?
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Why the extreme difference in speed, and is that necessary?
A 6600RPM induction motor would be a custom design, are you sure that is not a typo for 3600RPM?
Does the load need to turn at the higher speed or the lower speed?
Are you planning on using a gear reduction to get it down to 1200RPM? or...
Are you planning on gearing the 1200RPM synch motor up to match the speed of the Induction Motor?
Are you penalized for power factor? Not everyone is.
Is there some other advantage to having variable speed in your process?
Which begs the question, what is the machine and what is it doing?
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electricpete
Electrical
Typically the load determines the speed and you select from among available motors at that speed. Comparing motors at different speeds doesn't seem reasonable to me.
The question of sync vs induction motor for a given speed has been discussed a lot before. Sync has an advantage in being able to adjust power factor, and a disadvantage in my opinion in higher complexity and lower reliability.
If there are any applications where sync motors give an efficiency payback it is mostly among large slow speed motors. Some people in the past used a thumbrule that a sync motor would be worthwhile if power in horsepower was greater than speed in rpm. I'm not sure how true that is.
GE has a great publication on their website on the subject of sync vs induction motors. I have posted the link before. Maybe you can find it.
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The question of sync vs induction motor for a given speed has been discussed a lot before. Sync has an advantage in being able to adjust power factor, and a disadvantage in my opinion in higher complexity and lower reliability.
If there are any applications where sync motors give an efficiency payback it is mostly among large slow speed motors. Some people in the past used a thumbrule that a sync motor would be worthwhile if power in horsepower was greater than speed in rpm. I'm not sure how true that is.
GE has a great publication on their website on the subject of sync vs induction motors. I have posted the link before. Maybe you can find it.
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The motor will be driving a recompressor (high speed load) in an ethane extraction plant. With the sync motor it will go through a gearbox. With the induction motor it would be direct coupled. The speed is not a typo. There are tremendous advantages to having variable speed for this process....this project would not even work without it.
We do have power factor penalties here.
Forgive me if this is a stupid question, but how does power factor relate to power that one would be billed for. I was always under the impression that if your power factor was less than unity that you would have to purchase more power to drive the same load...is that totally incorrect?
We do have power factor penalties here.
Forgive me if this is a stupid question, but how does power factor relate to power that one would be billed for. I was always under the impression that if your power factor was less than unity that you would have to purchase more power to drive the same load...is that totally incorrect?
electricpete
Electrical
If power factor goes down, there is a small increase in total current and small increase in the I^2*R losses of any cables or transformers between your metering device and your tranformer.
Here is the ge document that I was talking about which describes differences between sync and induction motors
You already know you will have to consider efficiency of the gearbox in the equation. In my mind gearbox on the induction motor means more complexity and maintenance headaches which kinds of offsets my previously-mentioned concern about complexity of the sync motor.
I'm sure you already know to account for losses in the gearbox as well.
If you are starting from scratch, I would think a 1200 rpm induction motor driving the load directly (no gearbox) would be an option to consider as well. Less complex than the other options and in my mind less maintence headaches and higher reliability. Unless your paying a lot for power and very little for maintenance and downtime I would think you wouldn't get enough efficiency gains from the other options to make them more attractive then then 1200 rpm no-gearbox option. Just my opinion and as you can tell I am biased toward placing highest consideration on reliability and maintainability over efficiency.
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Here is the ge document that I was talking about which describes differences between sync and induction motors
You already know you will have to consider efficiency of the gearbox in the equation. In my mind gearbox on the induction motor means more complexity and maintenance headaches which kinds of offsets my previously-mentioned concern about complexity of the sync motor.
I'm sure you already know to account for losses in the gearbox as well.
If you are starting from scratch, I would think a 1200 rpm induction motor driving the load directly (no gearbox) would be an option to consider as well. Less complex than the other options and in my mind less maintence headaches and higher reliability. Unless your paying a lot for power and very little for maintenance and downtime I would think you wouldn't get enough efficiency gains from the other options to make them more attractive then then 1200 rpm no-gearbox option. Just my opinion and as you can tell I am biased toward placing highest consideration on reliability and maintainability over efficiency.
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electricpete
Electrical
Please ignore all my comments in last paragraph of my last post. I was confused about your options. Now I reread your post and my last paragraph doesn't apply at all.
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Is there such thing as a 6600 rpm induction motor? Normally induction motors operating at 50hz or 60hz are limited to 3000 rpm or 3600 rpm. I would think you would need a special high-frequency power supply (another effect on efficiency) in addition to a special design considerations for the motor.
I would think 1200 rpm induction motor driving through gearbox should be thrown in as alternative to 1200 rpm sync motor driving through gearbox.
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Is there such thing as a 6600 rpm induction motor? Normally induction motors operating at 50hz or 60hz are limited to 3000 rpm or 3600 rpm. I would think you would need a special high-frequency power supply (another effect on efficiency) in addition to a special design considerations for the motor.
I would think 1200 rpm induction motor driving through gearbox should be thrown in as alternative to 1200 rpm sync motor driving through gearbox.
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electricpete
Electrical
You need a vfd in front of the motor to get it to run at 6600rpm, right?
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electricpete
Electrical
My previous statement:
"If power factor goes down, there is a small increase in total current and small increase in the I^2*R losses of any cables or transformers between your metering device and your transformer."
should have read
"If power factor goes down, there is a small increase in total current and small increase in the I^2*R losses of any cables or transformers between your metering device and your motor."
Maybe I need to get my morning coffee now!
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"If power factor goes down, there is a small increase in total current and small increase in the I^2*R losses of any cables or transformers between your metering device and your transformer."
should have read
"If power factor goes down, there is a small increase in total current and small increase in the I^2*R losses of any cables or transformers between your metering device and your motor."
Maybe I need to get my morning coffee now!
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LionelHutz
Electrical
"You need a vfd in front of the motor to get it to run at 6600rpm, right?"
I'd say so. As already stated 3000rpm max for 50hz or 36000rpm max for 60hz when talking squirrel cage induction motors.
It seems you're talking that you will for certain be using a VFD for this application. I can't think of any advantage in running a synchronous motor with a VFD over an induction motor with a VFD. Add in the fact that the induction motor eliminates the gearbox and the synchronous motor brush maintenance and the answer seems obvious.
The power factor is the ratio of total kVA draw vs kW draw. Since the voltage remains the same between VA and W this implies it's the also the ratio of real current vs reactive current. So, if you do not have a unity power factor you're not using any more power but, rather, you're drawing current from the utility which they can't bill you for. This means the utility needs to be capable of handling this extra current they get no money for and actually lose I^2xR losses to (and the I^2R losses in a transmission system can be large). So, they will penalize you to keep the power factor close to unity and minimize their system oversizing and I^2R losses.
I'd say so. As already stated 3000rpm max for 50hz or 36000rpm max for 60hz when talking squirrel cage induction motors.
It seems you're talking that you will for certain be using a VFD for this application. I can't think of any advantage in running a synchronous motor with a VFD over an induction motor with a VFD. Add in the fact that the induction motor eliminates the gearbox and the synchronous motor brush maintenance and the answer seems obvious.
The power factor is the ratio of total kVA draw vs kW draw. Since the voltage remains the same between VA and W this implies it's the also the ratio of real current vs reactive current. So, if you do not have a unity power factor you're not using any more power but, rather, you're drawing current from the utility which they can't bill you for. This means the utility needs to be capable of handling this extra current they get no money for and actually lose I^2xR losses to (and the I^2R losses in a transmission system can be large). So, they will penalize you to keep the power factor close to unity and minimize their system oversizing and I^2R losses.
electricpete
Electrical
So the alternatives are:
vfd => 2-pole induction motor @ 6600rpm =>compressor
1200rpm Sync motor =>gearbox w 6600 rpm output=>compressor
and I suggest also to consider:
1200rpm ind. motor =>gearbox w 6600 rpm output=>compressor
Now another thought comes to mind. Why not use a 3600rpm motor in front of a 3600/6600 gearbox? I think that would be cheaper and more efficient than 1200 in front of 1200/6600 gearbox.
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vfd => 2-pole induction motor @ 6600rpm =>compressor
1200rpm Sync motor =>gearbox w 6600 rpm output=>compressor
and I suggest also to consider:
1200rpm ind. motor =>gearbox w 6600 rpm output=>compressor
Now another thought comes to mind. Why not use a 3600rpm motor in front of a 3600/6600 gearbox? I think that would be cheaper and more efficient than 1200 in front of 1200/6600 gearbox.
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The 6600 RPM motor does not automatically need to be a 2 pole motor. The AFD can operate at 110 Hz for a 2 pole motor, 220 Hz for a 4 pole motor or 330 Hz for a 6 pole motor. The VFD supplier will probably make the selection based on what works best for the quoted VFD - motor combination.
I believe that there are some motor and VFD design subtleties that may influence that decision.
The choice between a synchronous and induction motor will be even more influenced by the VFD supplier.
At the cost level of this kind of equipment, the supplier should provide complete information on the selection criteria and the line-to-shaft efficiency.
ABB and/or Allen Bradley might also be able to supply appropriate equipment.
I believe that there are some motor and VFD design subtleties that may influence that decision.
The choice between a synchronous and induction motor will be even more influenced by the VFD supplier.
At the cost level of this kind of equipment, the supplier should provide complete information on the selection criteria and the line-to-shaft efficiency.
ABB and/or Allen Bradley might also be able to supply appropriate equipment.
electricpete
Electrical
I think that's an excellent recommendation to get quotes from people who can supply you the entire package. I think there are a few of the big guys out there than can offer the entire line. Rockwell may be another one. Make them tell you the expected efficiency with some documentation to support it.
I have a hard time imagining why a 4-pole motor would be a better choice than a 2-pole motor.
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I have a hard time imagining why a 4-pole motor would be a better choice than a 2-pole motor.
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Zoobie might appreciate a quick explanation of power factor.
For 3-phase systems, power = volts X amps X sqrt(3) X power factor
That formula illustrates that if the power factor is less than 1, more current is required to deliver the same power. The main cost to generate the power is determined by the power rather than the current, but the capacity of the generator, wires etc. is limited to a certain maximum current. Also, the losses in the generator, wires etc. are higher proportional to the extra current. For those reasons, the utility may add to the basic power price to obtain compensation for the extra losses and capacity limitation. That addition is often called a power factor penalty.
The same power loss and capacity principles apply to the output of the VFD, but the power factor at the output of the VFD does not effect the input much.
For 3-phase systems, power = volts X amps X sqrt(3) X power factor
That formula illustrates that if the power factor is less than 1, more current is required to deliver the same power. The main cost to generate the power is determined by the power rather than the current, but the capacity of the generator, wires etc. is limited to a certain maximum current. Also, the losses in the generator, wires etc. are higher proportional to the extra current. For those reasons, the utility may add to the basic power price to obtain compensation for the extra losses and capacity limitation. That addition is often called a power factor penalty.
The same power loss and capacity principles apply to the output of the VFD, but the power factor at the output of the VFD does not effect the input much.
OK, now that we have established a portion of the story, here are some more issues to deal with.
Using a 3600RPM motor and driving it at 6600RPM with a VFD poses an entire new set of problems. That is 83% overspeed for that motor design (110Hz), which means that you will have significantly less torque available at 6600RPM than you did at 3600RPM. If you don't understand that concept, learn about over speeding AC induction motors with a VFD. Briefly, once you go over base speed you are in constant HP mode because the voltage can no longer increase along with frequency since you will be at max voltage when you get to 60Hz. The torque will then drop rapidly as you overspeed, and in fact, you will have slightly less than 1/2 of the motor's torque at that speed (taken from a chart, not calculated). There are tricks to overcome this, but it is all in the motor design, so hopefully someone making this proposal to you has that under consideration.
That brings up another question. HP is a function of speed and torque. You cannot possibly need the same HP motor at 2 different speeds like that, so somewhere that issue must be dealt with. If the compressor needs 6000HP @ 6600RPM, you must take that as your primary requirement. Therefore, considering the above VFD issue, to be sure to get that torque at that speed you will need a 12,500HP motor and VFD!
One trick might be to use a 6000HP 2300V motor and feed it with the 4160V drive so that you can continue the voltage increase without losing torque over base speed, but that would be something to be discussed at length with the motor supplier first. The motor would be bigger and might cost more than a 12,500HP 4000V, but either way it will be a custom motor. The VFD however would be smaller and less expensive at 6000HP 4160V.
The gearbox and lower speed issue has some considerations as well. 6000HP @ 6600RPM = 4772 ft. lbs. of torque at that speed. At 1200RPM, a 6000HP motor provides 26,250 ft.-lbs. of torque, and when geared up at 5.5:1 to get to 6600RPM, you will have the required 4772 ft.-lbs. in theory, except for the losses. Gearbox losses are difficult to pin down, but a "rule of thumb" is to estimate 2% losses through each mesh. So a simple input/idler/output gear set would represent 4% losses. At 6000HP that is 240HP, which leaves you with only 5760HP or 4581 ft.-lbs. of torque at the compressor shaft, significant enough to be considered in the prime mover. You may need to go to 6500HP to be sure.
Using a VFD of course essentially eliminates any discussion of power factor, because the power factor presented by the VFD to the line will be corrected to about .95 anyway. In also introduces harmonics as an issue, and at 12,000HP that can be a BIG issue.
The use of a synchronous motor can also mean running at unity or even over unity PF to correct for other induction loads (if any), so that is a good option from that standpoint.
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Using a 3600RPM motor and driving it at 6600RPM with a VFD poses an entire new set of problems. That is 83% overspeed for that motor design (110Hz), which means that you will have significantly less torque available at 6600RPM than you did at 3600RPM. If you don't understand that concept, learn about over speeding AC induction motors with a VFD. Briefly, once you go over base speed you are in constant HP mode because the voltage can no longer increase along with frequency since you will be at max voltage when you get to 60Hz. The torque will then drop rapidly as you overspeed, and in fact, you will have slightly less than 1/2 of the motor's torque at that speed (taken from a chart, not calculated). There are tricks to overcome this, but it is all in the motor design, so hopefully someone making this proposal to you has that under consideration.
That brings up another question. HP is a function of speed and torque. You cannot possibly need the same HP motor at 2 different speeds like that, so somewhere that issue must be dealt with. If the compressor needs 6000HP @ 6600RPM, you must take that as your primary requirement. Therefore, considering the above VFD issue, to be sure to get that torque at that speed you will need a 12,500HP motor and VFD!
One trick might be to use a 6000HP 2300V motor and feed it with the 4160V drive so that you can continue the voltage increase without losing torque over base speed, but that would be something to be discussed at length with the motor supplier first. The motor would be bigger and might cost more than a 12,500HP 4000V, but either way it will be a custom motor. The VFD however would be smaller and less expensive at 6000HP 4160V.
The gearbox and lower speed issue has some considerations as well. 6000HP @ 6600RPM = 4772 ft. lbs. of torque at that speed. At 1200RPM, a 6000HP motor provides 26,250 ft.-lbs. of torque, and when geared up at 5.5:1 to get to 6600RPM, you will have the required 4772 ft.-lbs. in theory, except for the losses. Gearbox losses are difficult to pin down, but a "rule of thumb" is to estimate 2% losses through each mesh. So a simple input/idler/output gear set would represent 4% losses. At 6000HP that is 240HP, which leaves you with only 5760HP or 4581 ft.-lbs. of torque at the compressor shaft, significant enough to be considered in the prime mover. You may need to go to 6500HP to be sure.
Using a VFD of course essentially eliminates any discussion of power factor, because the power factor presented by the VFD to the line will be corrected to about .95 anyway. In also introduces harmonics as an issue, and at 12,000HP that can be a BIG issue.
The use of a synchronous motor can also mean running at unity or even over unity PF to correct for other induction loads (if any), so that is a good option from that standpoint.
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Member, [blue]P[sup]3[/sup][/blue]1. Power consumed by two equal HP motors
6,000 HP = 6,000 HP whcih in both cases
= HP x .746 Kwatts
2. YOu can correct for power factor with a synchonous motor. If utility charges for power factor at x$ per KW for PF at Y then this can add up real quick. Here in Seattle I know of some industries paying $10K per month for power factor charge.
3. Synch motors often need some assistance to get started.
6,000 HP = 6,000 HP whcih in both cases
= HP x .746 Kwatts
2. YOu can correct for power factor with a synchonous motor. If utility charges for power factor at x$ per KW for PF at Y then this can add up real quick. Here in Seattle I know of some industries paying $10K per month for power factor charge.
3. Synch motors often need some assistance to get started.
jraef,
That was a good closing argument. Like pete, I was also wondering why lower speed motor (costing more) would be preferred until you brought in the torque angle.
* Anyone who goes to see a psychiatrist ought to have his head examined *
That was a good closing argument. Like pete, I was also wondering why lower speed motor (costing more) would be preferred until you brought in the torque angle.
* Anyone who goes to see a psychiatrist ought to have his head examined *
abcd3286 said:
Yes, that is true for consumption, but not for utilization. Motors are used to deliver a given amount of torque to the load at a certain speed. The relationship to torque and speed is expressed as HP (or kW), but if the speed is different, the torque is different at the same HP. He did not specify which one it was what the compressor needs.
BTW, here in California (PG&E territory anyway), there is NO power factor charge. They don't care (actually, EVERYBODY pays for it, but it is hidden in the normal kWH charges, but that's another story).
Please explain what you mean by saying synch motors often need assistance to get started. You mean above and beyond energizing the amortisseur winding?
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