New-to-me term: "High Saturation Motor" 6

[jraef]

I've been in this business 30+ years, yet today I heard this for the very first time from a customer wanting me to help him specify a VFD. He has an 18 pole 200HP AC SCIM and all the time we were talking, he kept want me to assure him that I understood that "this is a high saturation motor, it's not like a regular motor. I need you to understand that when you spec the drive." Not being born yesterday, I gave the appropriate nods and grunts, said "No problem" and eventually went away leaving him satisfied that he was understood. I later found out that I was the first person he has not rejected for this task, all of the others said they didn't understand what he meant by "high saturation motor", and he would promptly escort them out the door. Sometimes old age and treachery win out I guess.

The problem is, I of course have no idea what he means by that either, I was just smart enough to let him finish talking. Now I'm curious if this is anything anyone else knows about. I understand what being an 18 pole motor will mean to the VFD, it's just the term "high saturation" I'm curious about.

Anyone?


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[electricpete]

Never heard of that. (I dont’ work with vfd’s). Just thinking of the terminology, if I had to guess, I can guess two meanings (which in itself is not much help since they are the opposite to each other):

Guess 1 - The volts per hz required to get to saturation is high (compared to nameplate base volts/hz).

Guess 2 - The nameplate base volts per hz already puts portions of the iron far into saturation

Thinking about design of slow speed motor, it is more likely to be designed like Guess #1. It is harder to magnetize the larger airgap circumference, and these tend to operate at lower airgap flux densities, generally resulting in lower tooth flux densities. Also the backiron tends to be larger for structural reasons, resulting in lower back iron flux densities.

Just a guess. Could be totally wrong.

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(2B)+(2B)' ?

 

[electricpete]

I did a word search of my references for “high saturation”. It came up the following

2.2 High saturation ferromagnetic alloys
Iron–cobalt (Fe–Co) alloys with Co contents ranging from 15 to 50% have
the highest known saturation magnetic flux density, about 2.4 T at room
temperature. They are the natural choice for applications such as aerospace
(motors, generators, transformers, magnetic bearings) where mass and space
saving are of prime importance.

That could suggest another guess:

Guess 3 – “High saturation motor” = motor made with “high saturation” alloy steel.

I’m not up on cutting edge technology, but from the discussion it seems unlikely to use this unless there is some reason this motor needs to be compact. If guess 3 were correct, I don’t think it would tell us much useful. Knowing the physical flux density limit of the iron doesn't tell us where it lies in relation to operating flux density (presumably machine would be designed to take advantage of the high flux density steel).... so we wouldn't particularly know any more about the volts/hz behavior of a guess-3 type motor than we would any other.


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(2B)+(2B)' ?

 

[jraef]

It was nothing that special looking, just an old Westinghouse motor circa 1960 on an Worthington pump. It was just a 460V 60Hz 3 phase motor, nothing special about the supply V/Hz ratio, but still your thoughts on 1 and 2 are maybe more likely what's behind his use of the term. Did some comparative reading on combating the effects of saturation when using inverter drives on low speed synchronous motors because of the effects it has on torque pulsation. Seems he may have been onto something worth consideration.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[cswilson]

Pete - Your first guess is basically correct. In field-oriented control, the higher your "magnetization current" (Id) setting (which means the higher your Volts/Hz), the higher your rotor magnetic field, up until you drive the rotor into magnetic saturation.

In a "standard" induction motor, you cannot raise this setting much higher than that which would result in the Volts/Hz at regular 50/60 Hz line operation for the motor. This is because achieving higher magnetic saturation levels costs money, and there is no advantage to this in off-the-line or even most VFD applications.

When running these motors under field-oriented control, you can reduce the magnetization current above this base speed (field weakening) to extend the high speed range of the motor by reducing its back EMF constant. But you cannot to any significant degree increase the magnetization current at lower speeds to increase the torque capabilities, due to magnetic saturation of the rotor.

However, if you have a motor with a higher rotor magnetic saturation level, you can increase the magnetization current to much lower speed values, permitting increased torque levels there. This extends the constant power range of the motor so it goes both above and below the "base speed" of the motor. I'm pretty sure that the induction motors used in electric vehicles like the Tesla (and the old EV1) have high rotor saturation levels, because operation in the constant power range provides a continuously variable software transmission.

Curt Wilson
Delta Tau Data Systems

 

[jraef]

Curt,
My day is complete now because I have learned something new. Thank you so much.

I'm taking the rest of the day off.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[SceneryDriver]

Given what Curt said about high-saturation motors, what does that mean in terms of VFD selection? Thank you, Curt, for a great explanation.

-SceneryDriver

 

[jraef]

Yeah, interesting side note to that issue. With the new knowledge I was better able to drill down to the right information and discovered that the VFD that the customer had WANTED me to specify was not going to work, it did not have enough flexibility in the motor vector algorithm. So we were able to move it into a better drive for him.

In a nutshell vector drives, be they "Sensorless" or "Closed Loop" vector drives* , require the development of a complex mathematical motor model and the monitoring of the motor performance via a feedback signal; an encoder if it is "closed loop" vector or Field Oriented Control, a current monitor inside of the drive if it is "Sensorless" Vector Control. They use that model and the feedback signal to determine an error in performance agains what you have commanded the drive to do, then apply the best algorithm in determining the correct vectors of all of the output current components to get the motor to reduce that error. Less expensive drives tend to have less complex vector algorithms, which means you may not have the ability to program in the magnetic current control component separately, or separately enough, to be of use in this situation. That was the case with the drive that the customer had been asking for because he wanted to match existing drives in use elsewhere. So with the info from Curt, I moved him up into a more advanced drive to allow this to take place.

Thanks again Curt and you too E-pete if for nothing more than the signpost.

*_{(None are really without sensors of some kind, all are really closed loop in some way, this is just how the market describes them for simplicity's sake.)}


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Brad1979]

Does anybody know what that means for the motors? Are higher saturation levels achieved by using different materials?

 

[OperaHouse]

So, does running a motor wired for 380V on a 240V inverter make it a high saturation motor (taking in acconut the lower power) if one wanted better low speed performance? I understand high speed would be an issue.

 

[cswilson]

jraef - Glad I could help! (I'm jealous, though. I can't take the rest of the day off.) It's a shame that there are precious few places that explain these things on a conceptual basis. I agree that "sensorless" vector is a misnomer. The technical literature uses the term "shaft sensorless". Some years ago, I told one of my guys that I wanted to use a certain lab drive setup to try out a sensorless algorithm. "You can't", he said. "Why not?" I asked. "It doesn't have enough sensors!", he replied.

Scenery Driver - As jraef explained, the drive must have the smarts to be able to vary the magnetization current level automatically as a function of speed to exploit this feature. Many simpler drives cannot do this.

Brad1979 - I'm not a motor design expert, but I'm pretty sure that the difference is in the material (and maybe the treatment) of the rotor core's steel laminations (more than the geometry, which doesn't. Pete gives an example above of a cobalt steel with a much higher saturation point than typical steels.

OperaHouse - It's the Volts/Hertz ratio actually used, which determines the airgap flux density, that matters, not the available voltage. Remember that a drive will modulate the available voltage at every instant.

Curt Wilson
Delta Tau Data Systems

 

[jraef]

Well, I didn't really take the rest of the day off either... I'm still at the office, it's 6:13PM, probably another 2 hours of work in front of me.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376