Why isn't there a sensor for Pool Pumps motors running dry?
Most Swimming Pool Pumps We Repair are pre-designed by manufacturers to be self priming. The trap and basket in front of the pump motor stays full of water when the pump isn't running, allowing the pump to have water at the impeller right when it starts.
In most cases, when a pump runs dry, it is due to the basked not being cleaned out during routine maintenance, or the lack of routine maintenance all together. When the basket becomes full of leaves, grass, and other deposits, it begins to block the water flow to the basket trap. In this case, the pump continues running, and in essence sucks the basket trap dry.
Another common case involves the hoses to your cleaner. Over time, they age, crack, and break. If these problems aren’t handled, your hoses begin to allow air into them, which feeds directly to your pump. If enough air is allowed into the pump when it shuts off, then there won't be enough water in the trap to keep the pump primed.
I believe the reason for not installing a sensor to detect if a pump is running dry, is because most swimming pool pumps run a little dry, especially after cleaning out the basket or backwashing the filter. Due to the filter housing holding so much water, the first 30 seconds of priming after backwashing involves a lot of escaping airflow.
If a sensor was installed in this scenario, it would cause the pump to shut off. This sounds handy, until the trap refills, and the pump turns back on, only to notice that the filter housing is still dry, which would kill the pump again. This cycle would repeat multiple times before the pump was fully primed again, causing stress on the motor.