Motor Heating Effect Of Different Starting Methods

I was recently in another forum discussion on the use of soft starters, where someone made reference to the (I believe) myth of soft starters reducing motor heating during startup. Some years ago I was in a discussion of the exact opposite claim, that soft starters WILL cause more heating in the motor, and I was directed to a fantastic article written by an Allen Bradley engineer on the various methods of starting high inertia loads. In there, he made a wonderful case for the fact that any fixed frequency starting method has essentially the same net effect on raising the motor temperature, regardless of current limit etc. His main point was that the net slip losses from stand-still to full speed are always the same, only the amount of time can be varied. We all know that it takes a specific amount of energy to accelerate a motor, regardless of how that energy is delivered. So since heat represents energy and energy is a product of power and time, the net heat gain in a motor is going to be the same.

I happen to subscribe to this view, but I cannot find any corroborating information. Marke's paper on starting high inertia loads is good, but does not specifically address this issue, being something rather unique to soft starter applications. Unfortunately, that A-B engineer's paper appears to no longer be available.

Anyone else have any thoughts on this and/or documents I can point to on this subject?

Selecting A Pre-charge Resistor Contactor

I need some design help. I am trying to help someone design in a contactor for use in a pre-charge resistor circuit, where the contactor will only close AFTER the pre-charge resistor has been in the circuit for 1.2 seconds. The question is, does the contactor still need a capacitive duty rating? I say yes, because when it opens it still has to contend with the extra arcing as the capacitors discharge. One of my cohorts says no, because the inrush has already been taken care of by the resistors, so the contactor is just acting to bypass the resistor and when opening, the arc will transfer back to flowing through the resistors. I have searched long and hard for information on selection criteria for pre-charge resistor contactors, but can't find anything useful. Anyone have an opinion or suggestion?

Plagiarization

Hey Mark, check this out! The 1st response should look familiar eh?

http://cr4.globalspec.com/thread/11182/Motor-Starter

2 Contactor Star-delta Starter.

OK, this is a new one on me.
I recently was involved in a proposal to supply contactors for star-delta motor starting on elevators. In the proposal, the starter samples were a pair of mechanically linked contactors with bridging bars on the load side, and a shorting bar across 3 terminals on the line side of one of them. I was under the impression that these were simply 2 of the 3 contactors and that the user just wanted to see how we accomplished the mechanical interlocking and didn't need to see the entire starter, but when they showed me their finished product (using another brand), I saw that these were the ONLY contactors used! I wasn't allowed to take pictures or look at a schematic, but for the life of me I cannot figure out how they are able to do star delta starting with only 2 contactors! When I asked someone to explain it, I was told " Don't bother trying to figure it out, that is how the elevator industry has done it for years". Anyone familiar with this?

Energy Savers On Submersible Pumps?

I just got wind of the fact that one of our favorite "energy saver" companies is pursuing a Medium Voltage power assembly for the purpose of marrying their "energy saver" controller to it. The intended market is supposedly submersible deep well pumps. I don't want to get into the specific problems of doing this in MV (I already know them and don't want to help them by discussing them here), but the curious thing I heard in this scenario is that they seem to think that they can save energy on submersibles! Apparently they have "proof" of the concept with their low voltage products.

Can anyone think of how a submersible pump could be wasting energy? The way I see it, it is either running or not. There is no "unloaded" condition on a submersible because if that were the case, the water would not be getting lifted up the hole. The only possibility is that the pump manufacturers routinely oversize the motors for the task, which I seriously doubt. Am I all wet with this thinking (pun intended), or is this just another example of their bending the truth to fit their marketing goals of attracting new investors (which is how I heard of it)?