Shop Talk
Electric motors in the US and North America are designed to operate on 60 hertz power where most of the rest of the world uses 50 hertz. As domestic companies purchase machinery overseas it is important to make sure equipment designed to operate on 50 hertz is properly converted to provide good service life at 60 hertz.
For a 3-phase electric motor operating on line power, a change from 50 to 60 hertz means a 20% increase in RPM that cannot be avoided. Properly loaded, the motor will perform well at the increased speed if the appropriate voltage is applied. It is the driven load where problems are likely to develop.
Most equipment manufacturers design production speeds around what is most efficient to produce the product. Maintaining that speed often requires a 20% ratio change in the drive train by re-gearing, changing sheave or sprocket diameters. There are other viable reasons to maintain the designed production speed. Consider an application having a motor driven fan or pump. The horsepower demand of a fan or pump Increases by the cube of the change in RPM. An application designed for 50 horsepower-50 hertz will demand 86.4 horsepower from the motor at 60 hertz. In this case it is critical to change the ratio of the drive train to avoid severe motor overload and eventual winding failure. Motor speed up on a poorly converted machine may result in overload, frequent breakdowns, or incompatibility with associated equipment. Conveyers, feeders, indexing tables, etc. can have timing problems in addition to increased mechanical stress. Higher speeds also increase the possibility of resonance problems that can lead to destructive vibrations in the structure.
Many reputable foreign manufacturers have broadened their market by properly re-designing their products to give good service life at 60 hertz. If the manufacturer does not make the necessary conversions it then can become the problem of the end user. Incompatibility problems with the new machinery are rarely easy to solve and can be very frustrating for persons who are only interested in getting into production. Solving such problems can be difficult for a competent electrician who does not know the manufacturers designed horsepower requirements or characteristics of the various components. A call to the sales rep or importer might find them familiar with the sales aspects of the machinery. , but for application questions you will often be referred to the original manufacturer, who may not know your language or your application. Unfortunately, the burden of the problem usually comes back to the end user to solve.
It is important to know that special transformers or rewinding the motor will not change the frequency back to 50 hertz. Finding a motor with a different number of poles is not a viable option either as the available pole/RPM ranges does not fall within the desired 20% change in speed. Frequency converters such as motor-generators or inverters can provide the desired 50Hz power but these devices add more complexity and considerable extra cost.
There are factors and recommendations beyond the initial cost to consider before buying any electrically powered machinery manufactured outside the US.
- Get a list from the dealer or manufacturer of customers that have purchased similar machinery. Contact those companies to see if they are happy with that product.
- If the machine is identical to the 50 hertz counterpart, find out if the components are oversized to compensate for the increased demand or will the machine operate it at reduced capability
- Replacement parts for critical components should be available and easily obtained. Try to avoid manufacturers that do not inventory such parts in North America.
- Make sure technical support is available and that you can easily communicate with those persons.
- Make sure that manuals and schematics are provided and written in English.
- Avoid used obsolete machinery.
- Get input from persons responsible for the operation and maintenance of the machine.
- Request that the machine be designed for specific standard voltages at 60 Hz with a guarantee from the manufacturer. Make sure the manufacturer is reputable and will honor that guarantee.
- Purchase orders should include the desired hertz-voltage requirement.
Many such problems begin with the idea of saving a buck on the initial cost of a machine. Be careful as saving bucks up front can sometimes turn into an expensive endeavor down the road.
Last Updated ( Wednesday, 01 February 2006 )
