Harmonics and VFD Driven WYE Motor

[EmagSamurai]

I have a simple question that I've been struggling to come up with a simple answer for. I was presented with the following statement today:

"Any motor larger than 5 hp, driven by a VFD, should be delta wound"

The explanation for this was that the harmonics resulting from the VFD will not have a neutral to flow down on a WYE wound motor, so the imbalance currents will be carried by the other phases. A WYE wound motor will see imbalance current 1.73 times higher than a delta motor, which will result in reduced life expectancy. DELTA wound motors typically have larger conductor windings, so they can handle the added heat.

I've never heard this before, and I can't even remember a time where a neutral was brought out of a motor regardless of the size.

Is this a rule of VFDs that I just haven't been let in on?

The system in question is ungrounded delta/delta (4160/480) supplying the VFDs, which feed a 7.5 hp YY/Y motor.

 

[Jraef]

Bunk. Someone is confusing Wye power systems with Wye wound motors. Apples and oranges. How the motor is wound is completely irrelevant to harmonics.

And the part about an unbalanced load? Remember, the motor IS the load, so how is the motor going to have an unbalanced load in the first place? The only way is if something is wrong with it, in which case harmonics is not the thing to be worried about.

 

[Besoeker]

I've never heard this before, and I can't even remember a time where a neutral was brought out of a motor regardless of the size.

Nor can I. I'm with Jraef on this. It should be filed in the round filing cabinet.

 

[EmagSamurai]

I spent as much time as I could reading up on this yesterday. If harmonics are the convent, then they can be considered in three groups. Triplens, which are the only harmonics that flow on the neutral. 3rd order harmonics don't seem to be an issue with VFDs from what I've read. The other two groups are positive and negative sequence harmonics. These VFDs will primarily produce 5th and 7th order harmonics which are positive and negative sequence. So these won't flow in the neutral anyway.

So yeah, I agree. Sounds like bunk . unfortubately, I have to figure out how to convince him of this, which I don't think will be simple.

 

[Jraef]

How about this?

If you were in the any other part of the world besides North America, and bought a "dual voltage" motor, the motor is connected, INTERNALLY, in Delta or Wye, because for them, the two voltages they have available are different by the square root of 3; 230/380V, 230/400V or 240/415V; the same 1.732 relationship. So if you have 400V available to you, you connect it in Wye. if But if you only have 230V 3 phase available to you at your location, the motor MUST BE connected in Delta. You have no choice. Either way, the motor works the same. But in his scenario, all of the millions of motors connected to VFDs around the world being connected in 230V Delta because that's what they have, are ALL burning up because of added harmonics? Hmmm... That's not happening.

 

[EmagSamurai]

Excellent point!

 

[Besoeker]

How about this?

If you were in the any other part of the world besides North America, and bought a "dual voltage" motor, the motor is connected, INTERNALLY, in Delta or Wye, because for them, the two voltages they have available are different by the square root of 3; 230/380V, 230/400V or 240/415V; the same 1.732 relationship. So if you have 400V available to you, you connect it in Wye. if But if you only have 230V 3 phase available to you at your location, the motor MUST BE connected in Delta. You have no choice. Either way, the motor works the same. But in his scenario, all of the millions of motors connected to VFDs around the world being connected in 230V Delta because that's what they have, are ALL burning up because of added harmonics? Hmmm... That's not happening.

We have did a few that were 400/690V.
The six ends were brought out so either connection could be made.

 

[Besoeker]

So yeah, I agree. Sounds like bunk . unfortubately, I have to figure out how to convince him of this, which I don't think will be simple.

It doesn't sound like bunk. It IS bunk. Might I suggest that you direct him to this very forum?

 

[EmagSamurai]

This brings to mind a questions I've wondered about in the past.

With a wye wound motor, if we have a voltage unbalance I assume the "center point" voltage will move around since there is no neutral for the unbalance current to flow in?

 

[GoldDigger]

This brings to mind a questions I've wondered about in the past.

With a wye wound motor, if we have a voltage unbalance I assume the "center point" voltage will move around since there is no neutral for the unbalance current to flow in?

Yes, and that is essential since unbalanced voltages on a single or three phase motor cause much higher (percentage) current imbalances which can lead to overheating and damage.

 

[EmagSamurai]

Similar question, but cast a little differently. Suppose we have a balanced system with high 3rd order harmonic content. What happens in the motor? There is no neutral for the zero sequence current to flow. These circulate in the coil of a delta, but there seems to be no where for the current to go in a wye connected motor with no neutral.

 

[GoldDigger]

Similar question, but cast a little differently. Suppose we have a balanced system with high 3rd order harmonic content. What happens in the motor? There is no neutral for the zero sequence current to flow. These circulate in the coil of a delta, but there seems to be no where for the current to go in a wye connected motor with no neutral.

The relatively simple answer is that in a wye wound motor with no connection to the neutral point of the wye there can be no symmetric third harmonic currents present on any of the supply lines. The motor presents a high (nearly infinite) impedance to triplen harmonic currents.
You could look at it as having a tuned third harmonic filter present.

 

[EmagSamurai]

I was circling around that conclusion, but this seems like I would be getting something for nothing (which is always cause for concern ). That is, I'm completely blocking 3rd order harmonics at this load with no resulting losses. This seems too good to be true. :?

 

[GoldDigger]

I was circling around that conclusion, but this seems like I would be getting something for nothing (which is always cause for concern ). That is, I'm completely blocking 3rd order harmonics at this load with no resulting losses. This seems too good to be true. :?

Well, it is possible that as a result the total effective current through the motor windings for the same applied voltage might be lower. That would be a tradeoff if true. (Note: I have NOT done the math, so I might even have it backwards....)

 

[winnie]

Third harmonic voltages from the supply are _in phase_ in a balanced three phase system. Thus there is no third harmonic voltage between the terminals and third harmonic current doesn't get driven into the motor.

In a delta wound motor, if 3rd harmonic currents were somehow generated (say because of the shape of the magnetic field) then they could circulate. This happens in delta-wye transformers where 3rd harmonic secondary currents cause circulating currents on on the primary.

It is very common for VFDs to intentionally add 3rd harmonic to the output. This 3rd harmonic does not cause motor current, but does allow a higher fundamental output voltage for a given DC rail voltage. Look up '3rd harmonic boost' for the details.

-Jon

 

[EmagSamurai]

Thanks for all the input! I think the pieces are slowly coming together for me.

So for an unbalanced supply voltage, I've always been told that the zero sequence currents circulate in the delta winding. I assume the same is true for a delta wound motor. Based on the previous replies, an unbalanced supply to a wye wound motor will cause the "center" voltage to move. Does this mean the zero sequence currents will return along the phases like a positive or negative sequence current? There's something to this I'm missing.

 

[Jraef]

Thanks for all the input! I think the pieces are slowly coming together for me.

So for an unbalanced supply voltage, I've always been told that the zero sequence currents circulate in the delta winding. I assume the same is true for a delta wound motor. Based on the previous replies, an unbalanced supply to a wye wound motor will cause the "center" voltage to move. Does this mean the zero sequence currents will return along the phases like a positive or negative sequence current? There's something to this I'm missing.

You have somehow jumped from harmonics to current imbalance.... Apples and oranges.

Damage to a motor from unbalanced current is the result of heating from excess NEGATIVE sequence currents, not Zero sequence. It will not matter if the motor is Wye or Delta wound.

 

[EmagSamurai]

Sorry about that :ashamed1: I got a little greedy. The opportunity to have all those lingering questions from college answered was just more than I could take . I started A new thread with my new questions.

Thanks again!