The temperature of the AC's evaporator coil is lower than the dew point temperature, hence your humidity dropping because it is condensing on the coil.
If you were simply cooling the air without reaching dewpoint, the humidity would be rising not falling even with no additional humidification.
The capacity of your dehumidification exceeds that of your humidification
Only solutions are
1. Decrease the effects of the evaporator coil (by reducing cooling capacity), smaller ac but running longer. (best solution, most efficient and most stable RH and temperature) and reduce cooling requirements by removing sources of temperature rise.
2. Increase the capacity of your humidifier, but this will also become a problem when the ac cycles off
3. Add additional humidification that cycles only when the AC kicks on, but this will also become a problem when the ac cycles off
Last edited by Av8tor1; 07-15-2015 at 07:11 AM.