Hot Tub Pump Start Capacitor Guide
Capacitors are rated in microfarads (µF or mfd)
All hot tub pumps are started by a capacitor and if the pump has more than 1 speed it will have more than 1 capacitor inside it.
They are usually housed on the pump motor body either in a box on top, in the back or strapped to the side.
When a hot tub pump fails via its capacitor it is always for a reason, which is usually a restriction that's stopping the motor flow easily, this could be the bearings but in hot tubs it is usually a blockage in the water pipe or a dirty filter. a top tip when replacing the capacitor is to check your filters, if they are more than a year old or not in the best condition replace them at the same time. (Hot Tub Filters)
Note: Do not attempt to change a hot tub capacitor if the pump is still plugged in or the tub is still switched on. Capacitors generate very high voltage which can be dangerous.
The simplest way to explain the mechanics of a hot tub capacitor would be to compare it to a battery. Both store and release electricity. Capacitors are charged with electricity, then releases its stored energy at a rate of sixty times per second in a 60 cycle alternating current system. The sizing is critical to motor efficiency just as sizing of batteries is critical to a radio. A radio that requires a 9V battery will not work with a 1.5V size battery. Thus, as the battery becomes weaker the radio will not play properly. A motor that requires a 7.5 mfd hot tub capacitor will not work with a 4.0 mfd capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination.
For more detail on capacitors please got to Wikipedia
Hot Tub DIY - Hot Tub Parts and Support