Reading the frequency of an AC signal using an arduino
The best way to do this is to being the signal into a counter/timer channel of the microprocessor. Configure the counter/timer to operate in counter mode. If you can count the pulses for a set interval of say one second would give you a reading of pulses/second from the motor. 

There will sometimes be a situation where waiting one second is intolerable because you need the speed update more often. In some cases you can reduce the interval of pulse counting to a 100 msec period. 

In other cases it may be more suitable to use the timer/counter in timer mode so that it can be used to measure the duration of the input pulses. This would then need to be computationally inverted to turn duration into equivalent pulse speed.