Maintenance of propulsion motors

A rotating electrical machine often forms an important part of a larger installation and if it is

supervised and maintained properly, it will be reliable in operation and guarantee a normal

lifetime.

The purpose of maintenance is therefore to:

• Secure that the machine will function reliably without any unforeseen actions or interventions

• Estimate and plan service actions in order to minimize down time.

Figure 93. Measuring the insulation resistance of a sleeve bearing

Figure 94. Measuring the insulation resistance of a roller bearing

Maintenance of stator and rotor windings.

The windings of rotating electrical machines are subjected to electrical, mechanical and thermal stresses. The windings and insulation gradually age and deteriorate due to these stresses. Therefore, the service life of the machine often depends on the insulation durability.

Insulation resistance test.

The insulation resistance measurement provides information about the humidity and dirtiness of the insulation. Based upon this information, correct cleaning and drying actions can be determined.

Criteria for windings in a normal condition:

Generally, the insulation resistance values for dry windings should exceed the minimum values significantly. Definite values are impossible to give, because resistance varies depending on the machine type and local conditions. In addition, the insulation resistance is affected by the age and usage of the machine. Therefore, the following values can only be considered as guidelines.

The insulation resistance limits, which are given below, are valid at 40 °C, and when the test voltage has been applied for 1 minute or longer.

• Rotor

For induction machines with wound rotors: R(1-10 min at 40 °C) > 5 MΩ

• Stator

For new stators: R(1-10 min at 40 °C) > 1000 MΩ. If the measuring conditions are extremely warm and humid, R(1-10 min at 40 °C) values above 100 MΩ can be accepted

For used stators: R(1-10 min at 40 °C) > 100 MΩ

Figure 95. Connections of the stator windings for insulation resistance measurements

a) Insulation resistance measurement for star connected winding

b) Insulation resistance measurement for delta connected winding

c) Insulation resistance measurement for one phase of the winding. The 'MΩ' represents the

insulation resistance meter.

Figure 96. Insulation resistance measurement of the rotor winding

The polarization index.

For the polarization index test the insulation resistance is measured after the voltage has been

applied for 15 seconds and 1 minute (or 1 minute and 10 minutes). The polarization index test is less dependent on the temperature than the insulation resistance. When the winding temperature is below 50ºC (122ºF), it may be considered independent of temperature. High temperatures can cause unpredictable changes in the polarization index; therefore the test should not be used at temperatures above 50ºC (122ºF).

Dirt and humidity accumulating in the winding normally reduces the insulation resistance, and the polarization index, as well as their dependence on temperature.

Correlation between the insulation resistance and the temperature becomes less steep.

Windings with open creepage distances are very sensitive to the effects of dirt and humidity.

There are several rules for determining the lowest acceptable value with which the machine can be safely started. For the polarization index (PI), the values usually range between 1 and 4.

Values close to 1 indicate that the windings are humid and dirty.

The minimum PI value for class F stator windings is more than 2.

NOTE: If the insulation resistance of the winding is in the range of several thousands of MΩ, the polarization index is not a meaningful criterion of the condition of the insulation, and it can be disregarded.