RK05 spindle pulleys - trade 50Hz vs 60Hz?

[Carlos E Murillo-Sanchez]

Eric Smith via cctalk wrote:
> On Wed, Jul 25, 2018 at 7:54 AM, Paul Koning via cctalk <
> cctalk at classiccmp.org> wrote:
>
>>> On Jul 25, 2018, at 9:50 AM, GerardCJAT via cctech <
>> cctech at classiccmp.org> wrote:
>>> Why don't you simply power it through an inverter that will output 60
>> Hz, eventually even "down to" 120 V , true sine wave, of course ??? They
>> are not that expensive by now.
>>
>> I wouldn't worry about "true sine wave".  That seems more of a marketing
>> thing anyway, and motors don't care.  Just feed them with a variable
>> frequency motor drive and all should be well.
>>
>>> And be carefull : motor designed for 60 Hz, running "under" 50 Hz, OR
>> THE OPPOSITE, I do not recall  !!!, display a significant reduced life time.
>>> I have to check which is which, but I know this is a question of
>> saturated magnetic field. Better check first.
>>
>> That doesn't sound right.  If you run the frequency up high enough you
>> might get into problems with magnetic materials not designed for it.  And
>> much lower probably gives you reduced torque.  But 50 vs. 60 Hz is a
>> trivial difference for a motor, I can't see any reasons for that to cause
>> trouble.  I routinely run my lathe at half frequency if not less, and it
>> doesn't complain.
>>
> I'm not sure about motors, but 60 Hz power transformers can't handle as
> high a maximum power (or current) when used for 50 Hz. The maximum power
> has to be derated. Some transformers are specified/sold with a single power
> specification for both 50 and 60 Hz use, which just means that the vendor
> has built the necessary derating into even the 60 Hz specification.
>
> Some products were built using different transformers for 50 vs 60 Hz
> models, and the 60 Hz models uses a transformer inadequate for 50 Hz
> operation.
>
It has to do with the physics of flux linkages and saturation. Under 
sinusoidal operation, voltage is proportional to the product of maximum 
flux and frequency.  If you fix the voltage, in order to operate at 5/6 
of the nominal frequency you need a flux that is  6/5 the nominal one.  
This might not seem like much more flux, but due to the nonlinear 
magnetization characteristics,  the required magnetization current will 
not be 6/5 times the nominal current, but it could in fact be three 
times higher or even more, and highly distorted.

Under-frequency and over-voltage can kill power transformers easily.

A transformer designed to operate at 50 Hz will therefore have much more 
iron mass in its core, power and voltage being equal.  That's why in 
airplanes power is distributed at 400Hz; the transformers will be much 
lighter.

carlos.