Wednesday, 18 February 2015

Dave Hawley, ABB’s General Manager for Motors & Generators writes: Overheating is one of the most common causes of premature motor failure. So if your motor is running a temperature then it might be prudent to take evasive action now.

The trick is knowing where to look for the cause of overheating. Here are the four top causes… there may be other reasons for overheating but this is a good place to start:

1. Insulation
Most industrial electric motors have an insulation class B or F, the latter of which can operate up to a maximum temperature of 155°C. Check the motor’s insulation class to ensure that the designed temperature rise is not being exceeded in the application’s environment. If it does, then a higher insulation class should be specified for the motor to be installed.

2. Load and duty cycles
The running time and load on the drive-end shaft can cause a motor to overheat. Motors that are running continuously need to be fitted with bearings able to cope with high operating hours, and will have to be re-lubricated in-line with the motor manufacturer’s recommendations.

High load on the drive-end shaft can put unnecessary stresses through the shaft bearings, and this can cause excessive friction, and eventual overheating to the point of failure.

The running hours and anticipated drive-end shaft load should both be taken into account in motor specifications, with the internal bearings and lubrication type used within the motor matched to fit within these parameters.  Failure to take these into account will increase failure risk and more importantly see breakdown and maintenance intervals become more frequent.

3. Operating environment
Excessive heat and humidity outside the body of the motor have a direct impact on an electric motors ability to dissipate internal heat, produced during operation, away from its windings, rotor and other internal components. This can see overall motor temperatures rise and can lead to failure.

The correct IP rating, should be used on the motor to ensure that any external contaminants are kept out of the motor.  Electric motors use a two-digit IP rating based on protection against solid objects (the first digit) and liquids (the second digit).
  • IP67 is one of the highest ratings making the motor dust tight and protected against the effects of immersion in a liquid.
  • IP11 is the lowest rating and protects the motor against ingress of solid objects greater than 50mm in diameter and against dripping water
A more detailed explanation of motor IP ratings can be found in the ABB low voltage (LV) motor guide.

4. Cooling
An electric motor relies on two cooling methods to remove the heat generated during operation:

1. Convection through motor body vanes.  This is done through vanes cast into the body of the
    motor that allow air to pass between them and carry away the heat.

2. Air cooling through a fan.  This is a plastic or aluminium fan that is connected to the shaft of the
    motor and rotates to draw air into the body or over the body of the motor of the motor to provide
    cooling.

Good maintenance of these cooling methods is the secret to keeping your motors cool.  Keeping the vanes on the motor clear of dust and debris will ensure effective cooling, as will regular inspection of cooling fans. Ensure that the air flow of the fan is not obstructed and make sure that if cooling fans are damaged they are repaired or replaced immediately.

To find out more about how to tackle your overheating motors, download ABB’s motor tips e-book


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