High Effiency Motors and Reliability

High efficiency motors are probably slightly more reliable since they operate at higher efficiencies, less heat will be generated internally.

Be sure you order "wash down" grade motors so they can be kept clean.

I have always found the high efficiency motors to be better overall in every aspect....The ability to match power available to load is an overall improvement in design...This is a general statement, failures can occur for many reasons....The end result achieved will be dependent on several factors, including quality of motors and components, suitability for application, occurrence of abnormal circumstances, overall maintenance and reliability of system design....

In North America, there is a high reliability motor spec that is specifically for the chemical and petroleum processing industry, it's called an IEEE 841 (2009) motor spec. They are by design Premium Efficiency, then also chemical corrosion resistant, inverter rated and often rated for hazardous locations.

The high efficiency motors I understand also have larger starting currents. Every Starting event puts substantial stress on the motor, especially on the rotor of he motor. Does this any way mean these motors shall not be preferred where frequent starting (DOL) is a requirement! Appreciate if you could clarify.

Higher efficiency in AC motor designs is attained in a number of ways, some of which have the side effect of causing the INRUSH current to be higher. But there is a difference between Inrush Current and Starting Current, even though the terms are very often mistakenly used as synonymous. Inrush Current is the VERY BRIEF spike in current that occurs when the motor windings are being magnetically excited. This is because the motor windings are in essence a short circuit, but the current flow is "impeded" (hence the term Impedance) by the interaction of the magnetic fields that are established around the windings once the current starts to flow. But in that instant BEFORE the magnetic fields are established, there is nothing but the wire resistance to resist the flow of current. Because one method of reducing losses in the motor is to reduce copper losses, i.e. that very same wire resistance, that means that the inrush current is higher than it was for older motor designs. There are also differences in the magnetic permeability properties of the motor steel laminations that affect how long it takes to establish the fields, which contributes to allowing the peak to get higher.

All of that takes place however is about the first 2 cycles of the AC power application, and is relatively unimportant in the duty cycle rating of the motor. Where this can cause problems is in protective devices that might see that high inrush and interpret it as a true short circuit condition. For example, many motor protection circuit breakers are designed to allow adjustment of the instantaneous magnetic trips to be up to 10X the breaker rating to allow for this, but the newer EE motors are able to allow 15-20X inrush and it causes the breakers to trip, meaning you must use breakers with higher trip ratings than we used to.

Hi JRaelf, very intersting your write about the inrush current and its effect. My doub is in the reliability of the high efficiency motors due to their high normal running operation temperature awing to more active material in the stator and reduced air gap developing, high temperature affecting the winding and cause grease degradation. This really is necessary to take into accout when this kind of motors are intended to be operate in chemical continuous process.

Thanks

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There can not be any match to high energy efficient motors and they are certainly more reliable for any set up and chemical industry is no exception. There should be world wide legislation in my opinion for use of only such motors as other wise inefficient ones are drain on global energy resources and use of energy efficient and more reliable motors reduces down time leading to productive use of man power.

Explosions, failures and burn-outs can be prevented by selecting, installing and maintaining the correct equipment, having first previously carried out a potentially explosive atmosphere zoning study on the facility. The zoning should be carried out by a suitably qualified Chemical Engineer with full access to details of the materials and processes carried out in the plant. Once this has been done, a drawing needs to be prepared to record the outcome of that study as a baseline for the installation of the correct equipment.

Once that task is complete, an appropriately-trained Electrical Engineer should select the most appropriate motors, cables and glanding for the facility, detailing the equipment in various schedules as a record, and should review its satisfactory installation before sanctioning energisation of individual circuits. Non-compliant circuits should be locked-off until the appropriate compliance can be demonstrated.

If these personnel are not readily available within the facility's own resources, then appropriately-qualified consultants should be hired.

An overview of the completeness of this activity may prove useful in reducing fire/explosion liability insurance premiums for the facility, in reducing personal loss/death liability insurance premiums for the facility, in reducing the risk of prosecution of the senior management at the facility, and the value of these reductions may prove to be an attractive business case in applying for the necessary investment funds.

There is an excellent article in Wikipedia on electrical equipment in potentially explosive atmospheres that will prove useful for further background reading.

Do not select, install, test, maintain or alter electrical equipment in potentially explosive atmospheres without having completed appropriate training beforehand.