Motor Plate Markings

[Chestnut]

I have an old 2hp motor. I'd like to rewire it to 240 but the markings on the name plate have me a bit confused.

I'm familiar with the changes in the electrical grid from 110-115 and eventually to today's standard of 120 with the matching 220-230 & 240 respectively. My concern is I've never seen 208 before. If i wired this bad boy to it's 208-230 markings would it be ok to run it on today's 240? The 208 is my concern, i swear I've only ever seen that on 3 phase stuff. Interesting that it's reverse able as well.

My other question is i know to be careful around Capacitors. Do the caps in motors hold a charge that I'd have to discharge or let discharge before I rewire it to it's higher voltage setting?

[drzaius]

240V is what you (and most detached residentials) have. It is derived from a single phase 120/240V, or Edison 3-wire, service. 208V comes from a 120/208V 3 phase service. Many large multi-family buildings have a 3 phase service to the building & then send a single phase 120/208V sub-panel in the individual units, using just 2 of the 3 lines, and a neutral.

Motors have nameplate ratings 115V, or 208-230V. The 115V & 230V numbers date back to ancient times (in motor years) and I can't remember exactly how.

The motor is engineered to run on anything from 208V - 240V with no problem. That way, the same motor can be used in a home or commercial building.

Capacitors will usually have a high resistance bleed resistor across the terminals that will gradually bleed off the charge after power is removed. But it is good practice to short out the terminals with a screwdriver or piece of wire before working on it.

[wtnhighlander]

@drzaius is correct, except perhaps how 'ancient' the 115 / 230 volt ratings are. The US did not standardize on 120v as 'nominal' for household appliance use until 1967, and it took a couple of decades for manufacturers to catch up. 

Nikola Tesla determined that 60hz was the optimal frequency for AC power generation, and recommended 240 volts as the best compromise between safety and efficiency for distribution. Europe chose to use 50hz because it fit the metric sequence of 1, 2, 5 multipliers. Apparently, the US chose 110v nominal originally to meet the demands of Edison's electric lights, already in use and designed for 110 volts DC. The nominal was later raised to 115v to accomodate increasing power requirements over existing wiring, and was briefly bumped to 117 volts after WW II.

My employer sold off a facility a while back, that had originally operated as a cannon ball factory for the CSA. There was still a large amout of gear in that place, rated for 110 or 220 or 230 volts. Ran fine at 120 / 240 volts. I still see motors and other simple electrical machines like that in plant where I work.

[Tom King]

In short, don't worry about it.  It will run fine.