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VFD easy question 3
- Thread starter lukin1977
- Start date
Hi rockman7892
You are on the right line.
If we assume that the equivilent circuit of the magnetizing component of the induction motor was purely inductive, then we could use V/Hz to maintain constant flux in the iron and this would give us maximum torque at all speeds. NB the reason for the constant flux in the iron is that if we increase this, the iron will saturate and all sorts of problems occur.
So constant flux in the iron is the requirement.
In reality, the magnetizing circuit of the IM comprises an inductor with series resistance.
At rated frequency, the reactance of the magnetizing inductance is much higher than the resistance and it can be reasonably considered as pure inductance.
As the frequency is reduced, the inductance stays the same, but the reactance (jwL) reduces and at low frequencies, the resistance becomes significant and eventually dominates.
If the ratio to the inductance to resistance was the same for all motors, it would be possible to modify the V/Hz curve to still provide for constant flux, but every motor is different so we need to do this dynamically. V/Hz drives usually have a low frequency voltage boost to modify the V/Hz curve and tweaking this will give more torque at low speeds. Essentially, you need to provide the voltage for the V/Hz which changes with frequency plus the voltage across the resistance which is fixed except for temperature changes.
Once we have dynamic control of the flux, we can do more than just operate with a constant flux, we can make the flux density follow the load torque.
This will reduce the flux at light load and reduce the iron losses, and will also enable a temporary flux boost to increase the torque on peak demands.
This is done by adjusting the flux for a constant slip. So now we target the slip and control the voltage to keep the slip constant. If the load torque reduces, the slip will reduce. We reduce the voltage and the slip will increase. If we increase the load torque, the slip increases so we increase the voltage to reduce the slip.
Provided that you have an accurate measurement of the slip, you will always have full torque available. Using a shaft encoder gives you this information. Using a mathematical model is OK once the model is tuned to match the motor, but at very low speeds, the information comes in so slow that it is difficult to predict what the actual slip is. Rather you get an indication of what the slip was, hence the sensor less vector are less effective at speeds close to zero.
The DTC drives (Direct Torque Control) use a totally different approach which is predictive and looks at each point on the waveform and makes adjustments to bring it into line with where it should be. These drives provide much higher torque down to much lower frequencies.
Best regards,
Mark Empson
L M Photonics Ltd
You are on the right line.
If we assume that the equivilent circuit of the magnetizing component of the induction motor was purely inductive, then we could use V/Hz to maintain constant flux in the iron and this would give us maximum torque at all speeds. NB the reason for the constant flux in the iron is that if we increase this, the iron will saturate and all sorts of problems occur.
So constant flux in the iron is the requirement.
In reality, the magnetizing circuit of the IM comprises an inductor with series resistance.
At rated frequency, the reactance of the magnetizing inductance is much higher than the resistance and it can be reasonably considered as pure inductance.
As the frequency is reduced, the inductance stays the same, but the reactance (jwL) reduces and at low frequencies, the resistance becomes significant and eventually dominates.
If the ratio to the inductance to resistance was the same for all motors, it would be possible to modify the V/Hz curve to still provide for constant flux, but every motor is different so we need to do this dynamically. V/Hz drives usually have a low frequency voltage boost to modify the V/Hz curve and tweaking this will give more torque at low speeds. Essentially, you need to provide the voltage for the V/Hz which changes with frequency plus the voltage across the resistance which is fixed except for temperature changes.
Once we have dynamic control of the flux, we can do more than just operate with a constant flux, we can make the flux density follow the load torque.
This will reduce the flux at light load and reduce the iron losses, and will also enable a temporary flux boost to increase the torque on peak demands.
This is done by adjusting the flux for a constant slip. So now we target the slip and control the voltage to keep the slip constant. If the load torque reduces, the slip will reduce. We reduce the voltage and the slip will increase. If we increase the load torque, the slip increases so we increase the voltage to reduce the slip.
Provided that you have an accurate measurement of the slip, you will always have full torque available. Using a shaft encoder gives you this information. Using a mathematical model is OK once the model is tuned to match the motor, but at very low speeds, the information comes in so slow that it is difficult to predict what the actual slip is. Rather you get an indication of what the slip was, hence the sensor less vector are less effective at speeds close to zero.
The DTC drives (Direct Torque Control) use a totally different approach which is predictive and looks at each point on the waveform and makes adjustments to bring it into line with where it should be. These drives provide much higher torque down to much lower frequencies.
Best regards,
Mark Empson
L M Photonics Ltd
rockman7892
Electrical
Marke
Thanks for the great explanation.
When you referred to the magnetizing circuit of the Im, I'm assuming that you were refering to the jXm component in the equivilent circuit model. It is the Im current through this jXm component that produces the flux in the motor is this correct?
Does the jX1 component in the stator in the equivilent ciruit model have anything to do with the flux, or is this just an inherent inductive property of the stator windings?
Thanks for the great explanation.
When you referred to the magnetizing circuit of the Im, I'm assuming that you were refering to the jXm component in the equivilent circuit model. It is the Im current through this jXm component that produces the flux in the motor is this correct?
Does the jX1 component in the stator in the equivilent ciruit model have anything to do with the flux, or is this just an inherent inductive property of the stator windings?
Yes, that is correct.
The current through the inductive component that creates the flux.
For the purposes of considering the flux in the gap, just look at an inductive element in series with a resistive element with the resistance becoming dominant at a low frequency.
If the resistive element is equal to the reactive impedance at 5 Hz, then at 50 Hz, it would be only 10% of the value and at 90 degrees. In this case, at 50 or 60Hz, you could almost ignore the effect of the resistance on the current through the reactor, but at 5 Hz the current would be down to around 70%.
The actual values varies considerably between motors depending on size and design so it is not practical to have a simple rule of thumb.
Best regards,
Mark Empson
L M Photonics Ltd
The current through the inductive component that creates the flux.
For the purposes of considering the flux in the gap, just look at an inductive element in series with a resistive element with the resistance becoming dominant at a low frequency.
If the resistive element is equal to the reactive impedance at 5 Hz, then at 50 Hz, it would be only 10% of the value and at 90 degrees. In this case, at 50 or 60Hz, you could almost ignore the effect of the resistance on the current through the reactor, but at 5 Hz the current would be down to around 70%.
The actual values varies considerably between motors depending on size and design so it is not practical to have a simple rule of thumb.
Best regards,
Mark Empson
L M Photonics Ltd
- Thread starter
- #25
itsmoked
Control of the AC motors that move the shafts and gears to spin the capstan that pulls the wire trough a die. Drawing machines have one motor for each capstan and each capstan "feeds" the next one with wire.
I´m just asking if anyone has experience with this application and knows if the VFDs are installed: V/Hz, Sensorless Vector or Flux Vector?
thanks,
lukin1977
Control of the AC motors that move the shafts and gears to spin the capstan that pulls the wire trough a die. Drawing machines have one motor for each capstan and each capstan "feeds" the next one with wire.
I´m just asking if anyone has experience with this application and knows if the VFDs are installed: V/Hz, Sensorless Vector or Flux Vector?
thanks,
lukin1977
rockman7892
Electrical
Ok I finally found the equation I was looking for relating Voltage and frequency to flux:
Flux = (Vm/wN)coswt
I can see now with this equation how changing the voltage and frequency in a motor will influence the flux.
The part I am trying to understand now is how flux is related to torque. Is there any direct relation or euqation between the flux and torqe in a motor?
In the attached equivelent circuit diagram it appears that jXm is the flux producing component as mentioned by others. What does the jX1 component represent? Does this just represent the inductive properties of the stator? Does jX1 produce or contribute to the flux?
The other diagram in the attachment shows speed torque curves for a motor operated above and below base speeds. For speeds below base speed, if the voltage and frequency are adjusted proportionally to keep the flux constant then why do these speed torque curves and maximum torqes change for different operating speeds as shown? Although the speed torque curves vary do the full load torques all stay the same as long and the flux is kept the same?
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