If an induction motor is rated at 0.5HP and 370W at 220V single phase, is this just at start up or when under operating load. In this instance it's a submersible water pump designed for continuous duty operating on a power curve with a maximum flow rate of 400Litres/min and maximum dynamic head of 14.5 metres. Apart from general interest my main reason is that I want to get some idea of the operating costs of running this pump to supply my pond waterfall on a 24/7/365 basis.
The rated power is the continuous rating. Start up power is understood to be higher, but of course, short lived. The National Electrical Code has information about different types of motors having start up surges of X times the continuous power. I think induction is 2 times, and household split phase motors tend to be 3 times. The half horsepower rating is impossibly amazing in combination with the power rating. It means the motor is 99.19% efficient. Almost certainly the result of multiple roundings. (Note that efficiencies over 95% only occur in very expensive motors, like 200 or more horsepower. See www.energy.wsu.edu/ftp-ep/pubs/en... )
The watt rating is based on full load current, so 370 watts is the power it consumes when it is running at full capacity. At start-up, the energy consumed is considerably more, as the rotor accelerates but should drop to 370 watts as soon as it reaches full speed. Depending on your electricity costs, it should cost around 2-3 cents per hour to operate, or $15-20/month if you run it 24/7
370 W is the under load power usage rating of the motor. When idling it will be less. If put under load that would require the motor to use more than 370W it risks burning out (there is often a safety margin).
A three phase motor cannot run on pure single phase power. Capacitors can be used to derive the phase delay needed to make such a motor run, but its efficiency will generally be impaired. An induction motor cannot be used as a generator because there is nothing to generate the field magnetism; you need a synchronous motor or other type of motor with a generated (or permanent magnet) field to act as a generator.
In the case of submersible pumps for wells, the motor efficiencies usually are even lower for a given motor size. It may be due to the constraint that the motor has to fit in a small-diameter well casing. It could also be due to the motor manufacturer being able to get away with producing a low efficiency motor since the liquid in which the motor is immersed will conduct away the waste heat. And submersible motors tend to be exempt from efficiency requirements.
