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Member Since 26 Sep 2007
Offline Last Active Nov 02 2007 04:20 PM

Posts I've Made

In Topic: Motor Cooling

02 November 2007 - 04:20 PM

QUOTE(marke @ Oct 27 2007, 04:36 AM) <{POST_SNAPBACK}>
Hi bob

There are two major issues with the motor rating.
  • Maximum or transient torque (or power)
  • Maximum Continuous power.
The peak torque is determined by the flux in the iron. This in turn is limited by the quality and cross section of the iron. Too much flux will lead to saturation and associated problems. ....


I notice that motor saturation is often quoted. Information I have been provided by manufacturers indicates saturated and unsaturated values for equivalent circuit parameters as a function of slip. I know that in a transformer sub-synchronous operation increases the core hysteriesis losses and can lead to vibration and heating resulting in damage. I have also been told by VSD manufacturers that for low speed operation drives are often oversized due to saturation issues. Can you tell me what effects result from saturation in induction motors and under what conditions does this become a problem?

Many Thanks


In Topic: Earthing Resistance.

23 October 2007 - 04:14 PM


Earthing resistances are used to lessen the earth fault current on a fault. I don't know if there is a standard value for these units but I have fitted them on transformers at 3.3kV and on generator star points.
The rule I use (and seems to be generally applied) is to rate the resistor to limit fault current equal to the current rating of the device (remembering that the voltage is neutral to ground (i.e. dont forget the root 3). The idea is to reduce equipment damage on an earth fault. You ned to select a rating for the resistor that will allow it to pass current for enough time to operate the protection (they are usually rated for 3s or 10s for a particular temperature rise). They get very hot if a fault occurs so place them somewhere where nobody might accidentally touch them.
I have never added these units at LV but at 3.3kV and above. I think that the current can be limited to greater extent if necessary for generators and motors which are not very earth fault tolerant. Clearly the fault must be such that the protection can recognise it and on a machine or transformer there is normally some sort of unit protection like restricted earth fault to allow a more sensitive setting.
Often when fitted to a distribution transormer star point in addition to the restricted earth fault protection a standby earth fault protection is also fitted which is a single element overcurrent relay designed as back up protection.
I've attached a file that might be useful



In Topic: Ac Commutator Variable Speed Motor

23 October 2007 - 03:54 PM

QUOTE(marke @ Oct 18 2007, 01:10 AM) <{POST_SNAPBACK}>
Hi Matt

Yes, sounds rather like the Schrage motor. The speed is controlled by rotating the brush assembly around the shaft axis.
I have not found a good simple explanation of how these work, so if you find one, let me know!!
Best regards,


I managed to get hold of someone at Lawrence Scott who was involved in designing these units. He sent me a paper dating from the 1950's Apparently they were designed by a German Jew who fled Europe in the 1940's. This guy is the "S" of "N-S", he also designed something called a tri-slot motor (some sort of double cage induction motor with low starting torque and high efficiency) amongst other things. In short, a very clever engineer who ended up as the director of the company.
Although the operation of these units was described to me they are not simple. From what I can gather they have a commutator and something called an induction regulator. The commutator is used to correct the rotor frequency back to the stator frequency and the induction regulator is used to correct the voltage. Power can then be drawn from the rotor an injected back into the stator circuit. I'm still trying to understand the how this actually works. This gives the capability to run sub and super synchronous with higher efficiencies and the brushes can be modified to correct power factor (this was explained to me but I didn't really understand) I can send you this paper as an 18 page pdf but it is too large to attach, if you send me an email address I can forward it.
I suspect you will have more success understanding it than me.



In Topic: Use Of Regen Drives For Var Correction

05 October 2007 - 01:18 PM

Thanks for all your comments. It seems that the main point here is that the available power at the VSD terminals is limited to the drive load characteristics. This is fundementally different from the description the manufacturer has indicated. They intimated that the rating of the fist stage was equivalent to the final drive stage and that the drive could act as a VAr generator, the only reference to the attached load is the MVA rating of the unit i.e. that the available current to correct the powe factor is the rated current minus current due to the connected motor. I am now even less convinced that this is viable but will investigate further.

Your comments are much appreciated, I'm way past the limits of my knowledge on this issue

In Topic: ---fault Current----

04 October 2007 - 02:24 PM

I believe leakage current is the current passing due to a voltage accross a leakage impedance. The leakage impedance occurs when you get coupled magnetic sysems like transformers or induction motors. Basically if you couple two magnetic fileds only some of the magnetic flux lines link(think of magnets and iron filings) . The ones that link are refered to as mutual lines of flux. Some of them don't link hoever and these are refered to as leakage flux (i.e. they are only associated with a single magnetic source and don't transfer to the other so they 'leak' out)
The best description of this effect can be found in a text on the ideal transformer which you will find at the start of most power electrical engineering text books (generally around the area where they introduce per unit systems)