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

Hello Matt303

Welcome to the forum.

I personally do not attempt to model the motor as to get an accurate result, you need an accurate model and accurate values to put into the model. I find that this sort of information is very difficult to come by.

I use the manufacturers curves for the motor and the nameplate data, LRC LRT FLC etc and find that from this I can get pretty good results.
I use the Electrical Calculations software that you can download from www.LMPhotonics.com and enter in the curves, along with the load curves and inertia, and generally find that my figures for start current and start time are pretty good.

Have a good day,
Best regards,

Marke,

Thanks for the reply. The interesting point is that of the tw manufacturers under consideration one of them has supplied X and R values and a profile of the values against slip indicating the variance of X and R as a fuction of slip which seems to be more or less a straight line fit, these seems consistent with most of the text book descriptions. Using my model on this manufacturers data set does provide an output closely matching their dta sheet for starting current, the other manufacturers data is qute inacurate however. Unfortunately the inaccurate motor has a better stated performance (no matter how dubious the figures quoted are). The other issue is a very different moment of intertia, both are 4 pole motors on a 50Hz system and both inertia values are quoted in terms of radius i.e. WR2 one is 75kgm2 and the other 25kgm2 which is a bit of an issue for the pipeline surge supression. I think this means one motor is long and skinny and the other short and fat.
I'm uncomfortable about taking these figures at face value becuase I don't believe them and to accept them woud place the other manufacturer at a disadvantage for not quoting unrealistic and overly optimistic data.
The electrial model is only a thevenin reduction of the clasical induction motor model with a locked rotor i.e. allowing for current flow through the magnetising branch. Whereas saturation effects over the fulll speed range could easily introduce inacruacy the initial maximum start current is a fairly simple reduction.
The vendor under scrutiny who are a very major european supplier are looking at the figures currently and I'm waitiing for them to respond in the absence of a resolution I'm inclined to give the data sheet a health warning when providing recommendations.

Thanks again for your advice I'll check out the software

Mat303,


you don't wrote the nominal power for such motors, we cannot check.
There is another possibility: in one of two specifications someone put wrongly WD2 instead of WR2 (diameter, not radius).

Regards
Mario

Mario,

The inertia quoted in both cases refers to the radius definition. I also wondered if the diameter had been used. This would give a factor of 4 however i.e. D=2*R, D^2 = 4*R^2. In this case the problematic motor has a quoed figure of 1/3 of the other. I don't know of any way to calculate this information from the data sheet information. We have a metric based on a large number of motors but this is an impirical calcuation and does not relate to any particular enginering properties. I'ts used by the surge analysis people when having to analyse a pumping systems with limited information. This predicts a value of 75kgm^2 for a 4 pole 1.1MW mtor. The most significant factor seems to relate to the number of poles. For instance it predicts a figure of 25kgm^2 for a 2 pole 1.1MW motor. Initially I thought the vendor may have made an error and quoted a 2 pole machine but they say not and that it is due to the geometry of the machine.