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hopelessnoob (Electrical)
26 Oct 07 21:49
Hello,

I'm new to 3 phase AC sinusoidal system and my question may sound clueless, I would appreciate your patience.

Say controller(drive) is sending out three phase sine waves and the controller side is delta connected and motor side wye connected. Then my understanding is, the motor side sine wave will be phase-shifted by 30 degrees. Am I correct?

Then my question is, how to make it +30 degrees shifted and how to make it -30 degrees shifted? In other words, is there a way to control the shifted-phase to be ahead(+30) or behind(-30)?


My next question is,
Now let's say phase shift is +30 and now I reverse the motor rotation direction i.e. any two wires out of three wires are switched, what happens to the phase shift?

Does it remain +30 degrees or does it become -30 degrees?


My last question may really sound clueless but...
A 3 phase AC sinusoidal motor driven by 3 phase AC sinusoidal drive will be somewhat similar to stepper motor in that, the 3 phase sine waves will specify a single rotor position(angle) at any given moment and can even stop the rotor at any angle with tremendous zero-speed torque... Then why would a 3 phase AC sinusoidal system need feedback? I.E. Why should it run closed-loop? Why can't it be run open loop without feedback circuit just like a stepper motor? Give enough RMS to the sine wave signal, then the sinusoidal motor will never miss a step (step is not a right word here but I do not know how else to describe it)...


Thank you for your time on this newbie.
gepman (Electrical)
26 Oct 07 23:46
hopelesssnoob
Please clarify what you mean by controller.  Do you mean a VFD, a contactor, a transformer, or something else?
hopelessnoob (Electrical)
27 Oct 07 0:26
I am talking about PM servo motor.
In cnc world, there is controller that generates the sine wave, then a drive amplifies the signal (hence the drive is also called "amplifier").

I am talking about genuine sinusoidal drive, not PWM drives.
davidbeach (Electrical)
27 Oct 07 1:02
No phase shift.  Phase shift happens across a delta-wye transformer but not in the situation you describe.  Think of your controller as a black box and your motor as another black box.  With only three wires out of each of the two black boxes there is no way to determine whether what is in the box is connected in wye or in delta.  No test can differential between a wye connected system and its delta equivalent nor between a delta connected system and its wye equivalent.  Changing the leads would only change the rotation, nothing else.
Marke (Electrical)
27 Oct 07 3:24
Hello hopelessnoob

There is no phase shift in the controller because the input frequency and output frequency are not the same. Internally, the controller converts the incoming AC to DC and back to AC at a different voltage and frequency. Phase shift is only relevant where the input and output are at exactly the same frequency. If this was the case, you would not need a controller.

The star/delta theory is only relevent if you are connecting the motor via a star delta starter. If you are not, then there is no difference between the star connected motor and the delta connected motor in terms of interface to the controller unless you are using the star point for connection as well.

Best regards,

Mark Empson
http://www.lmphotonics.com

waross (Electrical)
27 Oct 07 8:15
By the way, the phase shift IS an issue in parallel transformer situations and transformer protection. A topic usually addressed in the power enginering forum, where davidbeach is a 168 star expert. David works with the 30 deg. phase shift everyday dealing with transformer protection issues. David has given you an answer, not a tutorial. This is engineering tips, not engineering college.
Star delta starting is an issue with star delta starters, not with PM drives.
waross (Electrical)
27 Oct 07 20:13
A clarification on star delta starters. The issue is not the transfer from star to delta. The problems arise from the pen circuit time. Any large three phase induction motor will suffer current spikes and possible mechanical damage if it is open circuited for a short time and then reconnected to the lines.
This has to do with the spinning motor generating a back EMF at a frequency less than the line frequency that drops in frequency as the motor slows down. It can be reconnected when the decaying voltage is badly out of phase, much more than 30 deg.
A closed transition star delta starter will happily make the 30 deg. shift with very little current surge.
I once encountered a small power house where the synchroscope was improperly wired. They were synchronizing  two 600 KW generators when they were actually 30deg. out of phase. They had been doing it for years. Once or twice a year someone would make a sloppy closing. If it was on the side of the envelope that added to the 30 deg. they would shear a coupling key.
Generators are more akin to synchronous motors and are less forgiving with induction motors.
If you are not synchronizing (such as picking up an already spinning motor) and are not physically changing the connections or de-energizing and rapidly re-energizing (as a star delta starter does) then none of this applies to your application.
sreid (Electrical)
28 Oct 07 11:49
Two things about sine drive PM sero motors.

1)The amplifier output is a three phase CURRENT set, not a voltage set.

2) To develop optimal torque, the rotor and stator magnetic vectors must be kept at +/- 90 electrical degrees.  This is the reason for the rotor position feedback requirement.  The position sensor is often a resolver which gives absolute position.

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