Protection about wound rotor motors

330 series relays motor protection of NewElec are particularly planned to fulfil the protection needs of wound rotor motors utilized on overhead cranes and winches. The information required to get sufficient protection may be found easily on the nameplates of the manufacturers which are attached with the motor. The manufacturer utilizes these ratings of technical to prepare a thermal type model meant for the motor.

So as to properly prepare a appropriate protection relay for those wound rotor motors it is imperative to obtain the machine’s thermal rating, number of per hour starts and factor of cyclic duration. While operators of crane are suppose to move their cranes within the ratings of the motor an extra “Trip State Indicator” might be attached within the cabin. The benefit which is obtained that the operator can get a visual display for the main crane hoist motor thermal uses and extract much from the functioning of the crane based on the availability of thermal capacity.

A system of excitation in respect of a synchronous generator contains traditional brushless exciters for offering base excitation in respect of a controlled source of DC and rated power output interconnected with traditional system of brushless excitation to substitute the excitation of the base. The substitute excitation is provided to the winding of synchronous generator to offer forcing excitation for permitting operation of the machine at levels of power that go beyond the base power of the generator output level. The brushless exciters engage a full-wave hybrid rectified in which few of the correcting diodes are manageable as per the external signal for the total exciter power adjustment provided to the rotor. A technique of optical transmission is applied for communicating sync and gate signals in between rotating and stationary components. This is the main functioning of the exciters.

Brushless Exciter Is Used In Synchronous Motors

Brushless exciter is mostly used in the synchronous motors. This gives the base excitation for the motor. The motor requires kilo watts of power to excite field winding. But if the brushless exciter is used then there is no requirement of so much power. The field winding is stationery and the armature is rotating. The rectifier assembly and the exciter are placed on the machine shaft which is synchronous. This will avoid the usage of the bearings. The collector ring is not at all necessary as there are no sliding contacts. The motor can be used for the hazardous places. A few hundred watts are enough for operating this motor. So the cost of the maintenance of the equipment is completely reduced.

By using the brushless exciter the efficiency is increased. The power can be regulated automatically. The brushless exciter will provide the base excitation. This excitation is given to the rotating field winding. Maintenance was always a problem in the olden type of motors but now the brushless exciter has made the maintenance cheaper and easier. The generator power will operate in higher levels which will be more than the base power.  The negative DC current will increase the de-excitation of the generator.

Wound rotor motors are three phase motors. They start with a higher speed. So the torque will be more in the initial stage itself. These motors will give high torque loads. These motors have a long acceleration time.  The speed and the torque can be controlled. These controls will be either manual or automatic. With the resistance the starting of the round wound motors can be changed. Thus the speed control applications are using these motors. The resistance can be controlled by reducing for controlling the speed. The speed can be select according to the load.

The wound rotor motors are mostly with the wire wound resistors. The resistor used may be of different variety. The resisters will be used according to the usage of the motor. In some motors thyristers are used to control the resistance. A continuous constant torque is maintained for the operation of the load. This is possible by controlling the resistance value. The speed control operations are done with this wound rotor motor. The resistance selected will select the speed also. So the resistance is directly dependent on the operation speed.  If the output is needed in variable speeds then the automatic resistance control is used.

More about Brushless Exciter

In order to control the excitation of the synchronous motor, a Brushless Exciter is used. The circuit of an exciter comprises of a discharge resistor, a rotating transformer which is centre tapped and a field winding. One end of the winding and the one end of rotating transformer are coupled together. The other end of the winding is coupled along with the center tap of the transformer and the end of discharge resistor. The exciter uses four powers SCR. The circuitry of the Brushless Exciter controls the excitation level and the application of field current. The time for application and removal of field is determined by the circuitry by monitoring the frequency, phase angle and presence of discharge current.

There are wide ranges of motors available in the market. Each motor is dedicated for a particular application. Here is a brief discussion on few types of Wound Rotor Motors:

Brushed: The different components of this type of motors are: an armature, split ring commutator and a magnet. The rotor windings are fed the electrical power source with the help of commutator and its neighboring brushes. This magnetizes the core of the rotor temporarily. It is the function on the commutator to see that the magnetic poles of the rotor and that of the stator never aligns with each other. This will ensure the continuous motion of the motor.

Brushless: There are some problems with the brushed design which are being eliminated in the brushless one. An external electronic switch replaces the commutator. These switches work in synchronization with the position of the rotor.

Ironless/Coreless: It is a modified version of the brushless or brush Wound Rotor Motors. It is generally applied to enhance acceleration. The core of the rotor has no iron component in it. The rotor can fit inside the stator magnets taking the form of a self supporting structure or a winding filled cylinder.
Printed Armature/Pancake: Here the windings are disc shaped running between the arrays of a magnet with higher flux density. The magnets are arranged facing the rotor in a circular form. The extremely flat profile is the reason for its name.

Now with a bit of understanding of the above different types of motor, you can easily pick the one that fulfills your requirements. There are some disadvantages of each type as well as advantages too because of which they are still in use.

The essential features of the brushless exciter and wound rotor motor

The generators need the direct current for the purpose of energizing the magnetic field it has got. The DC field current that it has is acquired from a completely separate source which is known as an exciter. Either of the both, static-type or rotating exciters are mostly used in the AC power generation process. There are commonly two different types of these rotating exciters and they are the brush exciter and the brushless exciter. The basic difference between the brush exciter and the brushless exciter is that the process that is used for transferring of the DC current to generators. The Static excitation due to these generator fields are offered in various forms including the field-flash voltage that is from the storage voltage and batteries from a large system of the solid-state components that are there. The DC generators which are available are either self-excited or separately excited. The excitation process in the use of the current includes the directly connected or may be gear- connected shaft driven DC generators.

The special thing about the Wound Rotor Motors has got three phase induction motor which has got high starting torque and it makes that ideal for the applications in which the standard NEMA design motors lag behind. The wound rotor motor is specifically effective and efficient for the purpose of application in which the use of a squirrel cage motor might be resulting in an initial current that is too high in comparison to the potential of the power system. The Wound rotor motor is good for the high inertia loads for a long accelerating time. This is the reason why you are able to control the torque, speed and also the resulting heating of the motor. This particular control over the motor could be manual or automatic. It is also quite effective and efficient with the high slip loads along with the adjustable speed that can be installed and not needing any precise speed regulation or control. The construction and the configuration of the wound rotor motor is quite different from the above mentioned squirrel caged motors, particularly in the design of the rotor. They also put the slip rings and the brushes. The stationary brushes are made to ride on each and every slip ring that is forming an external circuit and in that you can insert any desired amount of resistance. These work very good that way.