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Old 02-19-2015, 10:31 AM   #1
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Default Synchronus motor question about power factor

I have a quick question that I couldn not find the answer of. Why are the synchronus motors capable of running at any power factor hence used in pfc? Please a logical explanation since I'm no expert.
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Old 02-19-2015, 10:55 AM   #2
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These motors are capable of adjusting their slip, sometimes referred to as a slip-guard relay. Increases or decreases the excitation field voltage..the greater the voltage, the rotor will tend to lead, and lessening of the field voltage, will cause the rotor to lag the stator field. I think in delmars book, it calls this the rubber band effect, cause if the lag is great enough, the out-of step relay will trip the motor.



In a nutshell, the ability for the synchronous motor to lead the stator field ( by using the field adjustment rheostat on the exciter), acts just like a capacitor does, and will improve the overall pF.
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Old 02-23-2015, 10:11 AM   #3
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I know that, I know that when overexcited they will give ractive power and when they are underexcited they will absorb reactive power. But I don't understand the reason why this happens

Is it because when subexcited the rotor tends to catch up with the rotating magnetic field so it will require more reactive power and when it's overexcited it wants to slow down to reach the speed of the rotating magnetic field so it lets some of it's ractive power go??? Is that the explanation?
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Old 02-23-2015, 12:24 PM   #4
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Think of it this way. If it were an induction motor, the current will always lag the voltage, because the rotor excitation must come FROM being induced by the stator fields FIRST. Therefore because torque is created by slip, and slip is the result of that time lag, the PF is always lagging because the active power must be there first in order to create the reactive power.

In a synch motor, the rotor excitation is done separately, no time lag to induce it. So the rotor speed can be made to match the stator frequency, hence the term "synchronous". If you OVER excite the rotor fields, and the rotor is ALREADY spinning, it is now going to try to induce a voltage on the STATOR windings and try to create negative slip. But because the generator, SOMEWHERE up the line, is always going to be more powerful than the synch motor, it cannot make the generator run any faster. So that negative slip energy gets expressed into the weaker reactive power component of the local grid, essentially correcting the poor PF of other inductive loads near by.
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Old 02-25-2015, 09:10 AM   #5
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Very good answer. I'm understanding the motors better now but one question.

When you overexcite the motor(synchronus) it's trying to induce a voltage on the stator field. It does this because it's trying to create that slip difference between rotating magnetic field speed and rotor speed? You said it's trying to create negative slip but if you overexcite the motor and the rotor spins faster than the stator field doesn't that mean the slip is already negative? You also said that the negative slip energy gets expressed into weaker reactive power? Do you mean that the motor becomes of weaker reactive power because it gives some to the generator?

Sorry for my questions I'm a noobie when it comes to electrical machines . Thank you again for answering!
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Old 02-25-2015, 11:43 AM   #6
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Well, it's not really negative SLIP energy, because there isn't really any slip, I was just using that as an analog that might be easier to understand. The concept is way more complicated than that and was an entire weeks worth of study in an EE class, I was trying to give you the sound bite version. If you want the intricate details, you should do an Internet search on the term "synchronous condenser", which is what we call a synchronous motor being used to correct power factor in a facility. The Wiki article on that term actually does a decent job of touching the highlights, I just didn't want to put everyone else reading this to sleep because I know some of them are driving...
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Old 02-25-2015, 12:43 PM   #7
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To add to what JReaf said, be careful when describing a synchronous motor as having slip.

A synchronous motor rotor is synchronized to the line frequency unlike an asynchronous motor, so there is no 'slip' involved. Unlike an asynchronous motor, a synchronous motor is usually excited by a DC current applied to the rotor. By varying this DC current they can be set up to have a leading power factor to act in a similar way as capacitor banks in a factory. The DC current changes the synchronous angle of the rotor with respect to the field in the stator which affects it's power factor. This is counter-intuitive to most as people generally associate all motors as having lagging power factors; nevertheless, this is a common application for these kinds of motors.

That was the Coles-Notes edition as there is a lot to learn about these machines if you want to.
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