On farms, motors of 10 hp, 25 hp and even larger are required. But many
farms have only a single-phase, three-wire, 120/240-volt service. That
provides only two high voltage lines to the farm and only one transformer.
Single-phase motors 5 hp and larger require an unusually high number of
amperes while starting, and the line and transformer often are too small
to start such a motor.
Before buying even a 5-hp single-phase motor, check with your power supplier
to see whether the line and the transformer serving your farm are large
enough to operate such a motor. If you operate the motor only a comparatively
few hours per year, your power supplier will object to installing a heavier
line and transformer. In a few localities, at least some of the farms are
served by a three-phase line, providing three transformers and three wires
to the farm. If you are fortunate enough to have three-phase service, your
problems are solved. Use three-phase motors, which are simpler in construction
so they cost considerably less than single- phase motors and rarely have
a service problem. Note: If you have three-phase service, you will have
three-phase power at 240 volts available in addition to the usual 120/240-volt,
single-phase for lighting, appliances and other small loads.
Today’s three-phase distribution practices are increasingly moving in the
direction of 208Y/120-volt systems, in which the voltages between the neutral
and all three ungrounded conductors are equally 120 volts. The older 240-volt
systems have 120 volts to two of the ungrounded conductors from the grounded
conductor, and 208 volts to the third. This makes it more difficult for the
power suppliers to balance the 120-volt loads on their systems, and it creates
a potential hazard if the property owner inadvertently connects a 120-volt
load incorrectly at 208 volts.