Assoc. Prof.. PhD, Nikola Vaptsarov Naval Academy,

Varna, Bulgaria, e-mail:



Assist. Prof., PhD, Technical University,

Varna, Bulgaria, e-mail:



The high-voltage induction motors are widely used in ship electric propulsion drives, like AZIPOD and in ship bow thrusters.

The stability of insulation resistance of the electrical machines is closely connected to the methods of control of the process of drying during the periods of operation and repair.

We propose an experimental method that enables to control the process of drying of ship high-voltage electrical machines by programmable logic controller.


Keywords: drying of electrical machines, insulation resistance, process control with PLC






The insulation of the electrical machines can be moistured from the environmental conditions during transportation, storage, installation or operation. This requires systematic checking of the insulation resistance and taking necessary measures in deterioration of the moisture.

It was found that the critical value of the constant absolute humidity, which changes the electrical properties of insulating materials is not less than 15 g/m3. Of all the climatic factors affecting the electrical insulation, the impact of high humidity (tropical climate) is the most severe and most often causes malfunction of electrical products and burn them. It is known that abrupt changes in temperature cause condensation on electrical equipment that in the presence of salts and contaminants can cause deterioration of the electrical properties of the materials. In the manufacture of ship's electrical equipment the nominal calculated environment temperatures for open decks are accepted from +45C to -30C. Air humidity is sufficiently high (932) % at a temperature (253) C. Under the influence of moisture may occur changes in the electrical, physical, mechanical and chemical properties of insulating materials. The water has a rather low electric resistance and connect to salts forming electrolytes and becoming not only a conductor of electric current, but initiator of electrochemical reactions that accelerate the destruction of the insulation. Thus, the temperature and the humidity are factors that trigger the accelerated aging and destruction of insulation and are the reasons for failure of responsible devices [8].

Passing current through windings is one of the most effective means of drying motors. These internal heating methods apply heat at the winding conductors where it is most effective in driving out the moisture. However, care must be used not to apply too much heat by this method as vapor pressure can be built up within the damp insulation which may rupture it.



Description: loher3



Figure 1: A LOHER Bow Thruster Drive for shuttle tanker, output power up to 2200 kW, voltage up to 11kV [1] and Siemens Schottel Propulsion (SSP) Drive System with high-voltage permanently excited synchronous motor [2].



This danger can be avoided by starting with a low value of current and increasing it gradually until a maximum temperature of 85 C is observable on the surface of the coils. The motor must be sufficiently open so that moisture can easily escape outside the enclosing frame. A forced-air draft by electric fan or blower also reduces the time needed for dryout, which may require hours or days. The current should be reduced or cut off intermittently, if necessary, to prevent excessive temperature so that no scorching could occur.

The progress of dryout should be watched by taking insulation resistance readings using a low-voltage instrument to avoid puncturing the wet insulation. If previous measurements of the dry insulation resistance are available, they will afford a "benchmark" of the normal insulation to show by comparison when dryout has been completed. This often shortens considerably the drying time and helps to prevent false interpretation of the dryout curves when these curves tend to level off at values below normal due to entrapped moisture or local wet spots. Any sudden downward drop in the resistance readings not accounted for by a rise in temperature indicates that trapped moisture is still present and dryout should continue. Each motor will have its own final top reading and Fig. 2 illustrates how the resistances should be plotted against time [3].

The proposed system is designed for drying of high-voltage asynchronous and synchronous electrical machines. It is applicable to work with ship motors with nominal voltage up to Un = 15 kV, widely used in ship electric propulsion drives, like AZIPOD and in ship bow thrusters Fig.1.

The main objective of the process is ensuring acceptable insulation resistance between the three phases [6], [7]. The applied method of drying is through a high power electric three-phase voltage Uz = 380V. Moreover, the removal of moisture is carried out by heating the stator coil by the flowing current.



Description: clip_image002


Figure 2: A typical dryout curve of 150 kW electric motor [3].



The article proposes a drying system, allowing control by programmable logic controller (PLC). Following functions are provided:


         Automatic control of the drying process carried out through the implementation of the PLC software. This lapses the need of human involvement. Practically, the control of insulation in the drying process is carried out in the recommended interval of 1-2 hours, which in standard conditions is imposed by the measurement operator.

         Suspension of the drying process after reaching the nominal parameters of insulation. In terms of energy efficiency, this system reduces the possibility of excess consumption of electricity.

         Option to record measured values and determining the absorption coefficient used for analysis of the drying process, respectively assess the state of the motor.

The proposed system is built on a programmable controller of the company Moeller [4], Easy Series 700, 800, MFD [5]. Practically, the development of the device can be performed with the use of any PLC, providing the necessary functions. Therefore, the article examines the general algorithm of the sequence of compiling and execution of the program.






We propose an experimental method that enables to control the process of drying of ship high-voltage electrical machines, widely used in ship electric propulsion drives, like AZIPOD and in ship bow thrusters.

Drying of the motor is taken when a low insulation resistance is registered. The determination of the required insulation resistance Ri of high-voltage electric machines can be made trough equation:






where Un ,V is the rated voltage of the electric machine, Sn , kW or kVA is the rated output power and kp correction factor, considering the dependence of insulation resistance on its temperature.

The main elements of the proposed system for drying of high-voltage electric motors are shown in Fig. 3.



Description: clip_image003


Figure 3: Wiring diagram of a PLC controlled system for drying of high-voltage induction motors.



The main elements of the proposed system are:



The algorithm of the work requires the following sequence:


  1. Initial insulation resistance measurement. The megger is controlled from the PLC through the contactor K2, connected to output Q2. In the process of measuring the contactor K1 (output Q1), supplying the voltage UZ to the motor, is off. By the switches V1-V4, the resistance of the insulation between the three phases is consistently measured. According to that shown in Fig. 3, the sequence of switching is: insulation resistance between phases A and B - V1, V2; phases A and C - V1, V3; phases B and C - V2, V3. For V1-V4 are used the controller outputs Q3-Q6.
  2. After completing the test, the contactor K2 is excluded, which excludes the megger. Typically, in the process of measuring of the insulation resistance of high-voltage electrical machines certain electrical charge is accumulated. Normally, its dilution is made by connecting the phases to ground, but the absence of such in a vessel requires certain particularities. After completing the test, it is possible to realize the following option in the program of the PLC: the four contactors V1-V4 switch on, which short-connect the three phases of the motor. Time needed in this scheme must comply with specific machines. The process of dilution will be hampered by the lack of land to which connect the phases of the engine. It should be borne in mind that it is necessary to protect the power supply line by the surge caused by the charge into the engine at the moment of re-submission of the drying voltage Uz.
  3. The controller accounts the measured values of the insulation resistance between the three phases. In case of poor insulation resistance the contactor K1 is turned on for submitting the voltage Uz to the electric motor. The parameters of this apparatus must comply with specific motor and the current flowing through his windings during the drying. If it is provided that the rotor shoud be stationary, a three-phase rectifier (Larionov circuit) and constant power supply must be used.
  4. After a certain time, set by the timer setting in the controller, the insulation resistance measurement is repeated.
  5. The process ends when acceptable insulation resistance between the phases is registered. A signal for completion of the drying process is done by the controller.


The functional blocks of the built experimental model of the PLC controlled drying system, are shown in Fig. 4. Their setup and use, regardless of the selected controller type, requires the implementation of functions used to manage the system and the process of drying.

Fig.4 A - timer. It is necessary to use three timers T01, T02 and T03 for time recording: T01 - between two consecutive measurements (e.g. 30 minutes), T02 by the beginning of the running measurement up to 15s and T03 by the beginning of the running measurement up to 30s. The coefficient of absorption is determined by the last two timers.

Fig.4 B - comparator. Compares the measured analog value coming from the megger with a constant value. By the result the state of the insulation is determined and the comparator decide after starting of K1 or termination of the drying.

Fig.4 C - counter. The use of multiple counting units allows the realization of some additional options. By counting the cycles of drying can be estimated the running time. In some cases, it stipulates that the drying process continue for some time after reaching the allowable insulation resistance, as a normative calls that may be 1-2 hours. Implementation can be done with a timer set at the required time without interrupting the process or by counting the necessary cycles of drying by control measurements.

The used PLC have sufficient resources for implementation of additional features: temperature control of the motor, indication of information on the display and more.


Description: Fig 2


Figure 4: Functional blocks of the PLC family Moeller Easy 822 DC-TC:
A - timer;
B - comparator; C counter.





Basically, the proposed system for drying of electric machines with PLC control is applicable to high-voltage machines regardless of their type - synchronous or asynchronous. Experimental studies were conducted with four high-voltage induction motors with a nominal voltage Un = 6kV, 50Hz. Technical parameters are shown in more detail in Table 1.










cos φ























































Table 1: Rated technical parameters of the motors, used in the experiments.



The drying time depends on many parameters: degree of hydration - respectively the primary insulation resistance, type of insulation used, ambient temperature, voltage of drying, the riched temperature of the coil caused by the ongoing current and others. Should be expected that the process will have a different scope and duration. Experimental results are shown in Fig. 5 and Fig. 6, respectively, beginning - the first 3hours and end (about 10 -13 hours) after reaching the allowable insulation resistance. The charts reflect the drying process of four different machines, operating in similar conditions. Measured unacceptably low insulation resistance is the result of condensed moisture in the stator coil after prolonged stay of the engines turned off. Before starting the drying process the functionality of the terminal box is proved and the power cord is disconnected. After reaching the limit value, the process of drying is terminated from the PLC.


Description: Fig%203


Figure 5: Insulation resistance (R) as a function of time (t) at the beginning of the drying process of the four electric motors.


Description: Fig%204


Figure 6: Insulation resistance (R) as a function of time (t) at the end of the drying process of the same four electric motors.


Additional experiments were conducted with other types of electrical machines (rated voltage Un = 15kV), operating under different conditions. The results are shown in Fig. 7. Whereby different type of graphs were obtained.




The main conclusions of our work are:


Description: Fig%205


Figure 7: Insulation resistance (R) as a function of time (t) at the beginning of the drying process of two electric motors with 15kV rated voltage.







[2] The SSP Propulsor, An Ingenious Podded Drive System, Consortium SSP, Siemens AG & Schottel GmbH, brochure, 2006

[3] Facilities Instructions, Standarts and Techniques, Keeping Motor Windings Dry, Volume 3-4, Facilities Engineering Branch Denver Office, Denver, Colorado, USA, September 2000


[5] Display, Operate, Switch, Control, Regulate and Communicate, Moeller GmbH, brochure, 2004

[6] Lloyds Register, Rules and Regulations for the Classification of Ships, Electrical Engineering, Part 6, Chapter 2, July 2004

[7] Russian Register, Rules for the Classification and Construction of Ships (in Russian), vol.2, 2003

[8] Vlasov A.B., Issledovanie izolyatsii sudovih elektricheskih machin v protsesse eksploatatsii i sudoremonta (in Russian), MGTU Journal, Volume 11, 3, 2008, p.475-482