Selecting electric motors for severe duty applications

Electric motors are used in a multitude of applications within every conceivable industry. They are the workhorses of industry. Many applications require special consideration when selecting an appropriate motor. This article is written to qualify and assist the specifier in selecting an appropriate motor for a severe duty application. This excludes motors used in hazardous areas.

What are severe duty applications?
This is typically an application that subjects the motor to a number of environmental or application issues that a normal motor would not be subject to. These include:

• Motors that are 'hosed' down for cleanliness and hygiene reasons in food processing plants.
• Motors that are subject to chemical spills or splashing of corrosive materials.
• Motors that are subject to dusty and harsh conditions in mines, quarries and processing plants such as flour mills.
• Motors that are installed in high humidity areas where a significant change in temperature occurs.
• Motors that are installed outside with no protection from direct sunlight and heavy rain.
• Motors that are operated 1000 metres or more above sea level.
• Motors that are frequently stopped and started.
• Motors that are plug-reversed for rapid deceleration/stop.
• Motors that are subject to large mains voltage variations.
• Motors that are used on crushers, hammer mills, and the like, and are therefore subject to shock loads.

What are the causes of premature motor failure?
Before we can select an appropriate motor for a given application it is important to understand the types of failures that occur and the reasons for them. Some typical causes of motor failure include:

• Bearings fail due to contamination or excessive load, vibration and misalignment. Also improper greasing, incompatible grease and grease contamination.
• Insulation or rotor fails due to frequent stopping and starting of the motor.
• Condensation and ingress of liquids into the motor that damage the winding insulation, causing premature winding failure.
• Shaft breakage due to poor alignment or excessive shock loading, or not accounting for heavy belted loads when specifying the motor.
• Winding insulation breaks down due to excessive current during acceleration (motor overloaded and overheating).
• Low mains-voltage causing excess current draw.
• Phase unbalanced voltage causing excessive temperature rise.
• Bad connection on motor terminals, corrosion or electrolysis.
• Fans on motors damaged or restricted causing lack of cooling.
• Insufficient cooling on motors due to high ambient temperatures, or a build-up of dirt and dust on the body of the motor.
• Motor overloaded frequently.

What do IP (international protection) ratings represent?
The IP ratings are made up of two figures. The first figure is a protection against solid bodies; the second figure is a protection against liquids. Below is a summary chart.

The difficulty with IP ratings is the fact that the ratings are primarily designed for an 'enclosure for electrical equipment'. When they are applied to rotating machinery the fact that the shaft is rotating presents additional complications with respect to sealing the enclosure from the various solid bodies and liquids.

How do manufacturers of electric motors interpret IP ratings?

Although there are criteria for testing within the IP ratings, there are no definitions as to how to manufacture an electric motor. So the issue becomes very subjective and open to interpretation by the designer and manufacturer.

Suppose a manufacturer uses a single lip contact seal on the motor shaft. This would meet the requirements of IP 55 and possibly IP56, but if this motor was operated in a dry dusty environment, the lip seal would be ineffective after a short period of running, due to the lip contact simply grinding a groove in the motor shaft.

Another example would be trying to build a motor to 'completely protect against dust' as well as protect against jets of water from all directions. In other words IP65. For example, talcum powder is an extremely difficult dust to keep out of any enclosure; it is abrasive and will destroy contact lip type seals. It will enter the motor terminal box through the gasket material and any imperfections in the machining surfaces. It will also enter the motor through the condensation drain holes unless they have special one-way drain features. Also the powder is usually present all the time, whereas jets of water are usually intermittent.

It is therefore important for the motor supplier to understand the application in order to supply a suitable motor.

Washdown (hosedown) applications
These are very common applications in the food processing and pharmaceutical industries. The obvious reason is cleanliness and health and safety standards. Many plants use chemicals to kill bacteria along with a hosedown and sometimes steam. These washdowns usually take place between or after shifts.

When a motor is running, the hot air inside expands and is expelled. When the motor cools, it draws the outside air into the motor. This is particularly bad when the motor is still 'hot' after operation and is washed with cold water. This has an immediate effect of 'sucking' moisture into the motor by means of condensation. It is impossible to keep out this condensation from a motor because it is not hermetically sealed.

Washdown motors are purpose built for this application. They are available in a variety of types. The basic construction is a smooth steel band that allows for easy cleaning of contaminants and does not allow dust and the like to settle on the outside (finned type motors can collect bacteria and dirt easily). Original washdown motors were an 'epoxy painted' motor, with the inside treated to prevent any rusting, a treated rotor and stainless steel shafts and hardware, plus seals and flingers. Lower power ratings are TENV (totally enclosed none ventilated). TENV motors have one less shaft output cause sealing problems and do not rely on external cooling, making them also eminently suitable for 'inverter' variable speed applications.

The latest paint-coated washdown motors use an 'autophoretic' process on the steel band. This process uses a charged primer that penetrates into niches and complex shapes. It provides an excellent surface for the subsequent special US-FDA-approved paint that is applied both externally and internally.

A paint surface, no matter how good, will in time chip and peel away, especially if it is used with harsh chemicals. Thus the reasons to develop 'paint-free' washdown motors. This motor has a stainless steel stator body and specially treated die-cast end plates, along with stainless steel shaft and hardware. Finally, the ultimate in corrosion protection is the 'all-stainless steel motor'. The features of this motor include 300-series stainless steel on all external surfaces, encapsulated windings and a labyrinth seal on both ends of the shaft extension to protect motor bearings by rotating and expelling contaminants.

How does condensation affect motors?
Condensation is a real enemy of motor life. Condensation forms inside the motor when there is a change of temperature between the inside and the outside of the motor.

The greater the variation of temperature, the larger the amount of condensation. This condensation will not cause any harm if it is allowed to drain out of the motor. However, many condensation drains get blocked up with particles from the inside of the motor (such as rust), or from the outside environment. If the drains get plugged or blocked then the condensation sits inside the motor and eventually breaks down the insulation system in the motor, or penetrates the bearings. Many users 'seal' up a motor in the belief that they are preventing moisture getting in, but they forget that they are also preventing condensation from escaping. Space heaters are commonly used in motors that are used in high humidity areas, or where rapid cooling is experienced, such as kilns or hosedown applications. The space heater is a strip heater attached to the motor winding that keeps the motor temperature above the ambient temperature at all times; it is switched on separately when the power is removed from the motor. They are generally 240 volt, low wattage devices.

IEEE841-2001 standard for severe duty motors
IEEE (The Institute of Electrical & Electronic Engineers) was sponsored by the Petroleum & Chemical Industry to establish the criteria for IEEE standard for petroleum & chemical industry - severe duty totally enclosed fan cooled (TEFC) squirrel cage induction motors - up to and including 500 hp (375 kW). Excluded from the scope of this standard are motors with sleeve bearings and additional features required for explosion-proof motors.

The IEEE841-2001 standard applies to high efficiency, totally-enclosed fan cooled (TEFC), horizontal and vertical, single speed, squirrel cage, three-phase induction motors.

The purpose of this standard is to define a specification that deals with mechanical and electrical performance, electrical insulation systems, corrosion protection, and electrical and mechanical testing for severe duty motors. Many of the specified materials and components stem from experience with severely corrosive atmospheres, and the need for safe, quiet, reliable, high efficiency operation of electric motors.

The standard specifies a number of reference standards such as fatigue life of ball and roller bearings, test methods of salt spray (fog) testing, IEEE test for sound measurement, insulation tests, efficiency testing and a number of other requirements.

In summary, there are many applications that require special considerations in the selection of an appropriate motor. The IEEE841-2001 standard helps the user define a severe duty motor, and ensures that they will be installing the best possible motor for these applications.

For further information contact Australian Baldor Pty Ltd
PO Box 33, Kenthurst 2156

Ray Harding