To go off on a tangent raised by JBMcB; 50 Hz and 60 Hz were just a couple of the frequencies used early in the development of power systems. A footnote from a history of the Consumers Power Company will explain it better than I can:

When alternating current came into use in the early days of the electric
business, there were no standards of alternation. Consequently, a wide variety
of frequencies developed. Since many early frequencies were based on
alternations per minute, they resulted in fractional cycles per second. This is
shown in the following tabulation:

Alternations per minute --> cycles per second

2,000 = 16 2/3
3,000 = 25
3,300 = 27 1/2
3,500 = 29 1/5
3,600 = 30
4,000 = 33 1/3
4,800 = 40
6,000 = 50
7,200 = 60
8,000 = 66 2/3
10,000 = 83 1/3
15,000 = 125
16,000 = 133

Michigan had its share of these frequencies. The early 125 and 133 cycles of
Jackson were soon changed to 60 cycles. The territory in Grand Rapids had
several frequencies. The Edison d.c.-system gave way to 60-cycle alternating
current; the hydro plant on the Flat River, however, generated at 25 cycles.
Interurban lines between Grand Rapids and Muskegon, as well as between Grand
Rapids and Holland, operated at 3,500 alternations per minute, or approximately
29 1/5 cycles per second. When Rogers Dam was built in 1905, it was decided to
use 27 1/2 cycles so that a 25-cycle load could be taken on by speeding up the
motors by 10 percent, and yet the railway load could be readily taken on by
slowing down the rotary converters by 6 percent. When Croton [Dam] was added
two years later, it was decided to increase the frequency to 30 cycles, which
could be readily converted to the 60 cycles existing in Grand Rapids and
Muskegon and would cause less lamp flicker than either 27 1/2 cycles or 25
cycles.

After several years, the 60-cycle service in Grand Rapids and Muskegon was
changed to 30 cycles. When Stronach Dam was built in 1914, and 44,000-volt
lines were run to Cadillac and Manistee, it was necessary to change those cities
to 30 cycles since Stronach and later Junction [[Tippy Dam) were to tie into the
western 30-cycle system.

Direct current resulted in fewer transmission losses, and the fewer cycles of
alternating current, the lower the losses. So 30 cycles had some advantage in
that respect over 60 cycles.But, the 30-cycle fluctuation became objectionable
with incandescent lighting. Also, in the course of time, there was more
standardization on 60 cycles throughout the country. Thus 30 cycles became a
nonconforming standard for equipment, and at times customers had difficulty in
providing equipment for 30 cycles.

So, every now and then, the idea of changing the 30-cycle system to a 60-cycle
system was presented. Finally, the 1928 budget included an item for such a
changeover.

In a press release, C. W. Tippy pointed out that the existing 30-cycle equipment
meant superior operating economies for the company and that the changeover would
not increase capacity, but actually reduce it. Tippy pegged the cost of the
changeover at $60 per 100,000 customers.

When the project commenced in 1929, a great deal of preliminary work was
necessary: surveying customers' requirements, providing 60-cycle feeds, etc.
One thing that brought the changeover to a head at this particular time was the
contemplated construction of Oxbow [[later called Hardy) Dam, which was to be the
world's largest earth dam. The question arose as to whether it should be a
30-cycle or a 60-cycle operation and whether the transmission line from it
should connect into Grand Rapids or should go cross-country to Lansing so as to
become part of the then existing 60-cycle system."