if an acs712 current sensor measures ac current , can't it measure frequency of the supply ??? If so howcan it be done or the calculation involved

 

No. The output of the device is a voltage proportional to the current. It has no output for frequency.

 

The ACS712 does not measure AC current on its own, it simply outputs a voltage proportional to the instantaneous magnitude of the input current. You would have to construct some kind of instrument that can take this voltage and convert this to a value.
A digital oscilloscope could be used to see the output voltage and would display the frequency. To determine the value of the current you would have to apply a scaling factor to the oscilloscope reading.

 

if an acs712 current sensor measures ac current , can't it measure frequency of the supply ??? If so howcan it be done or the calculation involved

The output of the acs712 is a voltage linearly proportional to the current, so you will see the frequency of the supply in its output voltage.
You could then measure the frequency with an oscilloscope, a frequency-to-voltage converter, a counter, or a microprocessor.
Frequency, of course is just the inverse of one cycle period time.

Don't know why you mentioned phase angle in your post title, since phase has nothing to do with measuring frequency.