question

The advantage of AC motor over DC motor is more efficient and more economical. Another thing that must have to take into account is the continuous service of AC motor.. Please contact me at my email: enbr77@yahoo.com for more info. Indicate the name of your company please. Thanks.

Efficiency has nothing to do with AC or DC. It's a design specification. AC machines are generally used because of the availability of AC power. AC is preferred over DC because of ease in stepping up and stepping down of voltage for transmission.

Your oil drilling rigs are using AC motors because you have AC power from a grid or more probably from AC generators. If you were using DC motors then it would be too expensive to convert AC to DC for such huge demands of current.

i agree with the answer no 2 that there is no relation between efficiency of AC & DC motors and the application type ,,,,,

i think that the philippino who put his email want to get a job somewhere " this is their habits"

None of the answers so far has it. The reason that DC motors were used is to do with their torque characteristics at start up. They are expensive to make in comparison to an ac squirrel cage motor, need more maintenance and are less robust / reliable. The cost of rectification is cheap and has no bearing on the issue.

The squirrel cage induction motor is much better on all of these points BUT it only has any appreciable torque at speeds close to its synchronous speed. Thus for most "start under load" applications the DC machines won the day. However along came variable speed drives which vary the supply voltage and frequency to the induction motor and hey presto you have characteristics which are similar to those of a DC machine. That's why older rigs had DC winches and newer ones have AC.