Hi all,
Please help me to understand the implementation of "On the Fly start or Spin start " feature in AC drive. How the speed is sensed that time ,i.e. the motor is still rotating and power is restored. And why it is said that DTC has the inherent ability to catch the running speed.
Please help.

On an asynchronous motor, you've got some remaining flux even when there's no current on the stator.
That give a little voltage on the stator at the same frequency than the rotor (like if you have a very bad synchronous motor).
A VSD can sense this voltage and start at the rotor frequency instead of 0Hz.

What type of motor are you using, and do you have a speed sensor?

It's works on every asynchronous motor and, by consequence, on synchronous motor too.
You need voltage sensors on the VSD output phase.
The speed of the motor is estimated by the VSD thanks to the voltage frequency.

Are we talking about starting from zero speed and sensing position, or are we talking about using the rotor as a flywheel?

Sir,
Thanks for the reply.
I got the concept. My query is how to find the speed of the motor. Are you talking about to find the speed by stator voltage frequency, i,e, let say by using the time span between two zero cross? or some other way is there..
I have seen in drive manufacturer manual, they say about speed search. Is it the same thing..
Please reply.

There are different pattern match schemes within the power electronics
based upon sensor and motor type,

I'm talking about finding the stator voltage frequency.
When there's no current on the stator, there's still a small DC DC magnetic field on the stator.
->In this case the stator voltage frequency and the rotor frequency are the same (modulo the number of pole).
Off course like the magnetic field is very low, this function don't work very well at low speed : the stator voltage is too low.

To Mr Treichel : we are talking about finding the rotor speed of an induction motor when there's no current on the stator.

Thanks for your reply again.
Please let me be sure about the speed sense algorithm. ZCD ( zero cross detection) of supply voltage can be done?
Yes, even it will be also complex to find speed ( as well as zero cross) at that very low speed. Then what can be the other options..
Even I had read somewhere, that intentionally short circuit is made for this ( spin start) implementation.
Please help.

Sorry but I cannot go deeper without explaining some design rules of my company.

Raja, this is the main research topic of sensorless control. There are literally hundreds of related IEEE papers that discuss this topic. Note that the method that Allart mentioned (approximation of rotor position/speed based on stator/rotor flux linkage signiture) is only one of many methods for sensorless control. Other methods include impulse response analysis, torque ripple measurments, current response, etc. Regardless of the method, i would advice you to search the IEEE database and your questions will be answered.