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Motor Starting Study

Motor Starting Study

Motor Starting Study


I have started to learn about the motor starting study by ETAP software. I have some questions,

Why Parameter Estimation Module is used in ETAP Motor starting study? though the the manufacturer provides machine performance characteristic data (i.e. Motor Speed Vs Torque, Current, and Power Factor curves)?
Why to derive equivalent circuit of a Motor? Don't the manufacturer provide all the data?
What it has to do with motor models such as single cage with/without deep bars, double cube with/without rotor cages?

RE: Motor Starting Study

I always make a library entry for the motor using it's actual data and curves. Same for the load. Otherwise, the curves don't match the motor data. You don't need the equivalent circuit to do it.

RE: Motor Starting Study


Deriving equivalent circuit of a motor arms you very well to appreciate motor performance, and why the performance is so. It may look a little bit complex, but it makes you very grounded and thorough in being able to analyze motor performance. And one thing with understanding from first principles is that, you become more creative with what you know. My advice, if you can get someone to put you through that, it is well worth it. And not really a big deal if you already understand transformer equivalent circuit.

All the best.

RE: Motor Starting Study

> Why to derive equivalent circuit of a Motor? Don't the manufacturer provide all the data?

Ordinarily the manufacturer would provide the performance data and test data. Sometimes things like torque speed curve, current speed curve. Often the estimated equivalent circuit parameters are not included.

And if you're doing a motor starting study on existing motors purchased long ago, the manufacturer's data is not always available. You can still estimate equivalent circuit paramters from nameplate and whatever performance data is available. (The more data, the better the quality of the estimate, but sometimes you have to work with what you have rather than what you want)

> What it has to do with motor models such as single cage with/without deep bars, double cube with/without rotor cages?

When you estimate L1, R1, those are constant and not dependent on slip / speed. However L2 and R2 will in general vary with slip / speed for due to things like deep bar effect (except for very small motors and shallow bars). The manner in which they vary with frequency depends on the rotor bar construction / shape so you have to take that into account in some way. Some programs simply take as input a value for L2 & R2 at full speed and another value for L2 & R2 at starting/LR condition, and simply draw a straight line between them... that's a crude approach but you can improve it if you know something about the shape of the rotor bars. If you knew the geometry of a rectanglar bar you can get a pretty good estimate of the frequency dependence curve from first principles. For double cage rotors sometimes the rotor is modeled with two rotor branches in the equivalent circuit.

(2B)+(2B)' ?

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