I am just speculating here but I think the output of the single phase motor will be peak 1/2 hp but has a cyclical power output due to the way the commutator and coils work. The 3 phase motor works in a completely different way with a smoother steady output. Perhaps the minimum output of a 1/3 hp single phase motor is low enough during the cycle to stall or cause pulsing in the motion of the lathe with consequent poor finish.

Martin

Another thought,  perhaps at the time the cost of the two options was comparable so they used the largest motor that fitted their price requirements. 

Edited By Martin Connelly on 29/12/2016 08:34:40

Not sure, but I think the alternative motors were a similar frame-size and weight.

MichaelG.

The ML7R I once owned came with a 1/3 hp single phase motor and never seemed under-powered.

Andrew,

You will likely find that well engineered, durable, proper motors are rated as output. However the new trend of exaggerating the product's capability is to advertise (something with smaller output) their motors as the full power input, usually at a visibly (only) keen price! Buy cheap, buy twice syndrome!

Precisely the same as compressors – they used to be sized on output volume of free air but are now more usually quoted as 'swept volume' of the compressor, not taking into account that the compression ratio will affect the real volume forced through the valves and into the tank.

The problem with choosing motors is that power output is only one of the parameters that matters. Power is a rough guide to the rate at which a motor can do work, but far from being the whole story.

Torque (turning power), power output, efficiency, and rpm are all interrelated, and they all matter. To understand the actual capability of a motor, it helps to look at a graph that plots these values over a range, power and torque against rpm being revealing. The shape of the graph reveals cruel truths like the difference between average power output and peak power output. You want the former; they quote the latter!

The characteristics of a motor are strongly influenced by it's construction. The engineer selects the type of motor best suited to his application, for example a Universal Motor is a good choice for a sewing machine, and a poor choice for a lathe. And vice versa.

A 3-phase motor copes well with varying loads. When a single phase motor is used on a lathe it is a good idea to compensate for the motors relatively poor performance across a range of loadings by using a bigger than theoretically needed motor. Using a larger motor makes power loss and stalls less likely when the tool starts cutting.

The whole issue is complicated by sales naughtiness. Often it is far from clear whether a specification refers to input or output power, and we are usually left in ignorance about other important factors.

Dave

Edited By SillyOldDuffer on 29/12/2016 12:00:44

Perhaps it has nothing to do with all these technicalities, but is simply a single phase motor for domestic use and a three phase for commercial workshops?

Could it not be that Myford considered that a 1/3 hp three phase motor gave a similar performance to a 1/2 hp single phase motor as observed by the user ?