Electric motors for dry and wet areas

Motors used in food production areas are primarily selected for either ‘dry’ areas or ‘wet’ areas. These are entirely different situations with very different demands. We assume we are looking at safe areas, where there is no hazard from combustible gas or liquids.

When selecting motors, the IP rating is an important consideration. There are other factors, of course, but the IP rating is very important.

Dry areas - IP55 and IP65

Typical dry areas are found in industries such as bakeries, dry pet food processing and cereal processing. Standard industrial motors having a rating of IP55, normally considered ‘weatherproof’, are suitable for dry areas.

The term ‘IP55’ is worth explaining. In simplified terms, the first number ‘5’ means ‘protection against dust’, and the second number ‘5’ means ‘protection against water’. This is commonly understood as water from steady rain or water from a low-pressure spray. So, for a dry area, IP55 is commonly used. In fact, a clean, dry area indoors is one of the most ideal places to install an electric motor.

In dry areas, most industrial electric motors have been found to be very reliable. The main consideration is dust and powder - there is the possibility that, over time, an accumulation of dust may restrict airflow around the fan and the motor cooling fins. A regular cleaning program using compressed air is commonly used to keep the motors clean and guarantee their reliability.

In some applications where dust is extreme, a motor with a rating of IP65 might be nominated. In this case, the first number, ‘6’, means ‘dust tight - no ingress of dust’ and is a stronger barrier against dust. Again, the second number, ‘5’, is protection from rain or low-pressure water spray. So an IP65-rated motor is a dust-tight motor protected from water.

Wet areas - IP56 and IP66

Wet areas, meaning areas of a factory that are hosed down regularly, pose a special problem because the most serious threat to an electric motor is water. Typical wet areas are obviously the chicken, turkey, meat and fish processing industries, but also others such as salad, milk and cheese processing.

Considerations

Typically, at the end of a shift, the machine is turned off and then cleaned with a high-pressure cleaner using a caustic solution. This is great for cleaning machinery, but poses a constant potential problem with electric motors, as water entering a motor will inevitably lead to failure - and downtime.

The majority of standard motors are rated IP55, as discussed above. These motors are totally unsuitable if they are the target of a high-pressure jet of water. If used in wet areas, industrial motors should have an IP56 or IP66 classification. To explain ‘IP56’, the second number, ‘6’, means protection against high-pressure water jets, commonly termed as ‘hose-proof’. Most standard motors can be modified to a rating of IP56 with simple modifications. Generally, stainless steel motors for the food industry have an IP rating of IP66 as standard - so they are dust tight and high-pressure hose proof.

For many years there has been a tendency in the food industry to make one of two choices for wet areas:

1. Modify standard motors to IP56 to withstand wet area conditions.
2. Cover standard motors with stainless steel shrouds or covers, to protect them from the harmful effects of high-pressure cleaning jets.

Until the last 10 years when purpose-designed stainless steel motors started to enter the market from several suppliers, there had not been any other option but to use shrouds. When a stainless shroud is used, the motor is completely hidden from view. An OHS inspection may not identify any safety risk - simply because it cannot be seen. If a shroud system is used, it is essential the shrouds are routinely removed so the motor can be inspected.

Stainless steel motors are especially designed with wet areas of the food industry in mind. Apart from far better sealing, they are smooth all over, with no fins, making cleaning easier - and some are ‘tropic proofed’ internally. With stainless steel motors, the motors are out in the open and easy to inspect. Stainless steel is also more resistant to caustic solutions.

Cost comparison

The cost of a standard motor fitted with a stainless steel shroud, compared to the cost of a stainless steel motor, is often asked about.

There are many variables. The actual cost depends on the actual quantity required, the brand of the standard motor, the brand of the stainless steel motor and the size of the motor in kW. Another point to consider is if the motor is mounted on the floor, or suspended off a machine. Shrouds for motors suspended off a machine are more expensive, as they are of a circular, hinged design, and far more expensive than a simple half-circle shroud. However, a simple cost analysis is possible, to give an indication. The analysis outlined in Table 1 is based on a standard aluminium motor manufactured in Europe and fitted with a locally made stainless steel shroud, versus a stainless steel motor commonly available on the market.

Table 1: Comparison of costs of aluminium motor plus shroud compared to stainless steel motor.

Size (kW)	Aluminium motor			Stainless steel motor
	Motor +	Shroud =	Total
0.18	$151*	$280	$431	$398
0.37	$230	$301	$531	$489
0.75	$272	$326	$598	$684
3.0	$487	$347	$834	$1092

The following should be noted about the table:

1. The cost of the shroud is based on a standard semicircular design, with a 50 mm wide flange for mounting onto a bedplate.
2. The cost of the shroud would increase by approximately a miniumum of 50% for a full circular ‘wraparound’ or hinged design, for motors suspended off machinery.
3. The shroud quotation is based on 1.2 mm thick 304 grade stainless steel sheet.
4. The price of the stainless steel motor is based on a quantity of more than five motors.

It can be seen that, for ratings less than 0.75 kW, there is actually a saving in initial cost by specifying a stainless steel motor, compared to a standard aluminium motor with a shroud. Above 0.75 kW, there is an increasing difference in initial cost.

Motor longevity

With wet areas, there is convincing evidence that no matter how well a motor is sealed, if there is an air pocket inside there will inevitably be condensation. This is because the motor is heating and cooling constantly. From speaking to many maintenance staff and production managers over many years, there is compelling evidence that if there is a drain or weep location on the motor, motor life is extended. This can be anything from a simple open drain hole to a porous plug. Some staff insist on having a drain point on every electrical device to ensure maximum life. Merely as an observation, this appears to be the case, providing water is not allowed to enter the motor during cleaning periods.

Conclusion

Dry areas

Standard industrial electric motors of proven quality with an IP55 rating have proved to be very reliable, provided excess dust is not allowed to collect on the motor.

Wet areas

For wet areas, there are now several choices:

• Modify standard industrial motors to a minimum IP56 rating.
• Fit stainless steel shrouds over standard motors.
• Use stainless steel motors with a standard IP rating of IP56 or IP66.