Variable speed drives are used at many places in industry. Particularly, they are widely used in textile, rubber and plastic industries. The reliability of the drive is most important factor in all these applications . Any break down in the drive may result, not only in the wastage of raw materials but also in a longer down time due to the time taken to clean the equipment before restarting the machine.
Among all the motors, D.C. shut motors is an excellent device for the variable speed drive . Its speed can be regulated precisely from zero to its rated speed . The output torque is constant throughout its operating range . Its field is separately excited by a fixed D.C. voltage and armature is supplied with a variable D.C. voltage to which the speed of the motor is directly proportional . SEI manufactures D.C. motor speed control panels for last 20 years with enviable track record of reliability . The panels are manufactured for motors of capacity 0.5 H.P. to 100 H.P. and rated at 220 or 440 volts D.C. input supply to panels may be 230 v single phase or 400 v. three phase 50/60 Hz A.C. The speed of the motor is set to any required value simply by adjusting the front panel knob.
The field supply is given by rectifying the supply voltage directly . The stepless variation of the armature voltage is derived rom ' VARIPaC ' continuously variable voltage auto transformer - a proud product of SEI .''VARIPaC " is a transformer of special construction , providing a steplessly variable output from fixed input supply . Being a transformer , it is rugged , efficient & capable to withstand voltage/current surges , overloads of short duration etc . without getting damaged . For panels working on 3 phase supply , 3 phase VARIPaCs are used along with 3 phase bridge, rectifier.
SALIENT FEATURES :-
A liberally designed full wave bridge - 1 phase or 3 phase - using silicon diodes is used to produce armature supply. Diodes are under rated at least 100 % so far their voltage and current capacity is concerned . Diodes are mounted on alluminium die cast heat sinks of appropriate capacity . Drives using VARIPaC , are most economical, as the change in the armature voltage is by transformer action , the power consumed is always proportional to the output.
An ammeter on the front panel indicates the armature current, the most important parameter that needs monitoring . Any malfunctioning within the panel or outside it ,will affect the armature current.
All components are carefully designed , manufactured ,tested and used to produce a very reliable panel . All components are fully treated for tropical atmosphere .
With the advances in power electronics , active components like thyristors are used to provide variable armature voltage . No doubt , electronicaly controlled units are more sophisticated and light weight , but at the cost of simplicity and reliability.
By nature electronic components are delicate , susceptable to failure due to small overloads or surge voltages . Probability of failure is more with more components increasing the down time .The repairs and maintainance of such panels call for personnel with higher technical skills which may not be available at all places . On the other hand , panels using VARIPaC are most rugged , capable to withstand short time overloads ,Voltage/current surges and rough handling to a certain extent . These are more simpler to maintain and repair . An electrician with some experience can attend and maintain these panels.
With a small addition of a simple control circuit, SEI speed Control panels can achieve the performance at par to many complicated electronic circuits . A voltage sensing relay, with motorised VARIPaC will maintain the output voltage within a closed tolerance , irrespective of fluctuating input or varying load . A potentiometer in the control circuit permits the operator to set the speed as per requirement. Control circuit uses industrial grade silicon devices ,ensuring reliable performanc.
When the power fails or machine is switched off ,first thing that the control circuit does after the return of the power is it takes the brush arm of the VARIPaC to zero position and then only the supply to machine is restored and the voltage raised to the set value , automatically.
PROTECTIONS :-
HRC fuses provided in armature rectifier supply lines and output lines protect the panel from accidental short circuits and sustained overloads. Field output is provided with glass cartridge of HRC fuse.
Zero interlocking arrangement has been provided to ensure the gradual application of armature voltage from zero . Sudden application of armature voltage will produce a heavy inrush current resulting in the damage to the rectifier stack.
All diodes are provided with R.C. network across them of appropriate value, recommended by the manufactures of diodes to protect them from whole storage effect and voltage surges.
A surge suppressor capacitor is provided on A.C. side of field rectifier.
OPTIONS :-
Following optional features are provided if required , at extra cost.
A voltmeter to monitor the armature voltage . Field failure relay to monitor the field current.
Motor operated VARIPaC using a reversible drive motor working on 230 Volts single phase A.C. supply . This enables the remote controlled operation and closed loop automatic operation . The drive motor used is low speed , high torque A.C. synchronous motor having instant start , stop , reverse characteristics.
For further information and guidance on the applications of D.C. Motor Control Panels contact our sales office.
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