Hi Alex,
Try this:
Integral action will set the output of the regulator for exact setpoint speed and reduce the possibility of error that could arise with proportional action only.


Integral Gain: Ensures that under steady state conditions that the motor speed (almost) exactly matches the setpoint speed. A low gain can make the controller slow to push the speed to the setpoint but excessive gain can cause hunting around the setpoint speed. In less extreme cases, it can cause overshoot whereby the speed passes through the setpoint and then approaches the required speed from the opposite direction. Unfortunately, sufficient gain to quickly achieve the setpoint speed can cause overshoot and even oscillation but the other terms can be used to damp this out.
Proportional Gain: Gives fast response to sudden load changes and can reduce instability caused by high integral gain. This gain is typically many times higher than the integral gain so that relatively small deviations in speed are corrected while the integral gain slowly moves the speed to the sepoint. Like integral gain, when set too high, proportional gain can cause a "hard" oscillation of a few Hertz in motor speed.
http://www.daytronic.com/products/10K/10APID.htm


PROPORTIONAL ACTION
Proportional Action is referred to as (P) or gain. With proportional action, the controlled object no longer switches as a direct result of the set value (SV). The SYSCON-RKC instrument compares the difference between the set value and the process variable and controls the output proportional to the deviation. This proportional action is active within a user-definable zone around the set point called the proportional band (P). When the temperature rises into the proportional band, the output becomes gradually smaller and the temperature stabilizes somewhere within the proportional band. Proper adjustment of the proportional band will result in smooth control. However, it is seldom that the actual temperature stabilizes exactly on the set point. It usually stabilizes with some deviation called offset. SYSCON-RKC instruments have an adjustable proportional band to meet your process requirements.
INTEGRAL ACTION
The degree of integral action referred to as (1) or reset is expressed as integral time in seconds. The purpose of the integral action is to automatically compensate for any steady state offset inherent with a proportional controller. The integral action moves or resets the proportional band up or down depending on the offset. The integral time on SYSCON-RKC instruments is adjustable and determines how fast the proportional band is moved.