Ac Commutator Variable Speed Motor

Does anyone know how an AC commutator motor actually works?

We have a project where existing variable speed motorshave speed control supplied via a commutator and external reactor it seems very similar to the Schrage motor described briefly on the LMphotonics main website. I have never seem these before.

The units are 400V specified as 1000/100rpm
Amps 203/97
Stator amps 190/108
50Hz

The units were originally supplied by Lawrence Scott and are 1960s vintage. I had assumed they were rotor resistance slip ring but this seems not to be the case.I have found some brief explanations of how these motors work and it seems to relate to variable turns ratio beteen the stator and rotor presumably resulting in a variable voltage / torque but I am not at all clear what is really going on and what the specific characteristics of these devices are. We are replacing them with variable speed drives. It would help considerably if we could understand how these existing devices were meant to operate.

Many thanks for any assistance you can provide

Matt

Use Of Regen Drives For Var Correction

I work as an elecrical engineer in the water and sewage treatment industry. In recent years many of the owners have intallated combined heat and power plants to take advantage of the bio-gas which is a by product of sewage digesters. These machines are mainly spark ignition gas engines and are often run by a third party company under an energy purchasing agreemnt. In the UK we have a scheme called ROC (renewable obligation certificates) which is essentially a carbon trading market. The power generated from these sets can be sold and the offset purchased as if imported onto the site. As the value of the ROC is higher than the imported power these sets are generally run in this offset mode.
Forgive this drawn out description but now I will get to the point. These sets run at a power factor of near unity. They therefore change the balance between kW and kVAr on the site. The tariff does not charge for VAr for a power factor above 0.95 inductive but as these sets effectively reduce the kW import but leave the reactive unaffected they bring down the power factor. Although the VAr imports are about the same the proportion is lower as a proportion of MVA and the site starts paying for them as the power factor <0.95. There is no option to export VAr from the sets as they are part of a third party agreement.
It has been suggested that as a large number of partly loaded regernerative variable speed drives are installed these could be used to export VAr (essentially run them with leading power factors) and this could correct the site power factor the right side of the threshold.
I don't really understand how this works it seems as if the drives operate a bit like 4 quadrant DC systems and I suppose could be used as an alternative to a DC system on a crane i.e. to provide regenerative braking. I have no idea if they could be used for PF correction or if this is worth looking at.
Is this a sensible proposition?

Thanks in advance for your views.

Equivalent Circuit And Data Sheet Consistancy

Hello,

I'm new to this forum and I was hoping someone might be able to help with a design question. I'm currently trying to specify a medium voltage autotransformer (Korndorfer type) for a 1.1MW water pumping system. The pumps are operating on a high static head the torque speed characteristic is therefore heavily loaded at >85% of rated speed. In order to calculate the reduced voltage which will permit the pumps to start whilst minimising the starting current I've modelled the load torque characteristic and the electrical torque characteristic using the manufacturers equivalent circuit. I've modified the rotor resistance and reacance as a straight line fit between a slip of 1 and operating slip.
As a consequece of this calculation I've noticed that the starting current on the data sheet of 600% x FLC actually comes out at 720% x FLC on a 100% starting voltage when calculated from the equivalent circuit. Whereas I have no real issue with the motor manufacturer modifying a design to achieve this starting current I am now concerned that the properties of the motor supplied for evaluation against other manufactuers data is of questionable accuracy. I am looking at fairly small differentiators so approximate or generic data will not provide the necessary resolution.

Does anyone know if it is reasonable to expect the equivalvent circuit data to accurately match the data sheet or is my approach in simply reducing the equivalent circuit too simplistic (I know there are satuation and inrush issues which I may have completely overlooked)
Should I be concerned with the validity of the data sheet. 600% does seem a remarkably rounded figure?

Thanks for your help