Sabtu, 14 Agustus 2010
How to select the correct motor starting method - Overview
Full voltage starting of motors can produce objectionable voltage flicker, mechanical stress to gear boxes or belt drive systems and create pressure surges or water hammer in pumping applications. Starting a motor at reduced voltage can help reduce or overcome these problems. If the load cannot be accelerated to full speed using full voltage and current, it cannot be accelerated to full speed using reduced voltage and current.
There are several factors to be considered when selecting the starting equipment for any electric motor driven load. These include, but are not limited to:
1. The source of power and the effects the motor starting currents will have on the source and the stability of the system voltage
2. The starting and breakdown torque characteristics of the motor (motor speed torque characteristics)
3. The motor starting characteristics (torque) that correspond to the motor best suited to the load
characteristics at full load and speed
4. The starting characteristics of: motor torque load torque, accelerating load torque (load inertia) the motor starting, accelerating and running torque on the driven load
5. The available short circuit capacity of the distribution system
6. The operating speed range of the connected load.
7. Process considerations: shock, vibration, mechanical hammer, the control and maintenance of different starting methods.
The initial inrush currents, locked rotor currents and the resulting torque values produced, are the factors that determine whether the motor can be applied directly across the line, or whether the current has to be reduced to get the required performance to match the load requirements and/or utility line voltage flicker or voltage dip specifications.
Full voltage starting can be used whenever the driven load can withstand the shock of instantaneously applying full voltage to the motor and where line disturbances can be tolerated. Full voltage starting uses a main contactor to apply the motor stator windings directly across the main system voltage. This type of starting method provides the lowest cost, a basic and simple design of controller, resulting in low maintenance and the highest starting torque.
Reduced voltage starting may be required if full voltage starting creates objectionable line disturbances on the distribution system or where reduction of mechanical stress to gear boxes or belt drive systems is required. It must be noted that when the starting torque will decrease proportional to the percent squared of voltage applied (i.e. 50% voltage produces 25% torque =0.50 squared). This phenomenon also occurs in the opposite manner when the voltage is increased.
There are three main reasons to apply reduced voltage to medium voltage motors:
1. To reduce the mechanical effect of across the starting and stopping
2. To limit the inrush current inherent with full voltage starting
3. To reduce the effects of pressure surges and water hammer in pumping systems.
This reason for applying reduced voltage has various different names; it can be called mechanical shock, mechanical stress, or various other names. The effect is the same. When a medium voltage motor is started at full voltage the torque being applied from the motor to the driven load rises to a very high value almost immediately. This can cause damage to the bearings in the motor or the load, the rotor of the motor or to the mechanical coupling method which connects the motor to the load. The load itself can sustain damage depending on what the application may be. In the case of conveyor application if the load is started to quickly the belts of the conveyor can be stretched or broken. If the motor is connected to the load via chains or belts these coupling means can be damaged as well by sudden starting techniques.
As utility power systems continue to be run at maximum capacity the effect of starting medium voltage motors across the line can put stress on the factory’s power distribution system. The lights go dim, process control systems can fail or trip out or you may be restricted as to when and how often you are allowed to start the motor.
Load Related Reasons
By soft starting the load you may see improvements in the way the equipment performs. For example, when a soft starter is applied to an agitator by slowing ramping up the speed of the motor the material being agitated tends to splash less and causes fewer problems than when started across the line. In the case of mill applications the material will start to move slower than when started across the line and cause less wear on the driven load. The ways in which a soft start can improve system performance are only limited to the number of applications that the end user can think of. New uses are being thought of and applied all the time.
It is important to reiterate that when the voltage is reduced when starting a motor, so are the current and torque values. It should be apparent that a motor that will not start a load at full voltage, it will not start that same load under reduced voltage conditions. This conflict between torque and current requirements of induction motors is one typical dilemma facing the user of reduced voltage starting equipment. It may be only one of several problems but is the most common and most important.
Reduced voltage starting can be accomplished in several different ways.
This method also reduces the voltage, current and torque to the motor according to the reactor tap setting. It is possible to reduce the motor terminal voltage as required by placing a primary reactor in series with the motor windings, for a period during starting. The use of a reactor during starting results in an exceptionally low starting power factor. Reactors must be carefully designed and applied since any saturation in the reactor will produce in-rush currents close to those seen during full voltage starting. Reactor starting has one major advantage; the voltage to the motor is a function of the current taken from the line. It can therefore be assumed that during acceleration the motor voltage will rise as the line current drops. This relationship results in greater accelerating energy at higher speeds and less severe disturbances during the transition to full voltage.
Autotransformer starting automatically switching between taps of an autotransformer reduces the voltage, current and torque to the motor according to the tap setting used on the autotransformer. There are two very distinctive characteristics of an autotransformer starter.
1. The motor terminal voltage is not a function of load current and remains constant during the
2. Due to the turn ratio advantages, the primary line current is less than the secondary motor currents. A three-coil autotransformer is connected in a wye configuration and connected to the motor in such a way as to supply reduced voltage to the motor when the line voltage is applied to the Autotransformer. Several sets of taps are usually available to the user to provide different values of reduced voltage (NEMA standards are 80%, 65% and 50% of the full line voltage).
Solid State Reduced Voltage Starting
The use of solid state Reduced Voltage Starting can provide a smooth stepless method of accelerating and smoothly decelerating a squirrel cage induction motor. This type of starting method, when properly applied can provide an efficient and reliable means of smoothly starting and stopping a motor and load. The use of solid-state reduced voltage starting will perform, in most cases, more efficiently than field coupling, eddy current drives and clutches. The stepless ramped acceleration and deceleration capabilities of these types of starter reduces the inrush currents to the motor, eliminating transitional shocks to the load and reducing voltage flicker on the distribution system.
Selection of Appropriate Starting System
The selection of an appropriate starting system requires the reviewer to compare or weigh the importance of several factors.
Cost and Economics:
When determining the starting method, the economics of the decision can also provide important tips to the selection of an appropriate controlling means.
The capabilities of the mechanical and electrical support facilities and personnel can have an important impact on the starting method determination.
Remote Control Requirements:
As businesses become increasingly more competitive, the reductions in the area of personnel related to the control and operation of industrial processes are becoming dramatic. This required reduction of personnel has subsequently hastened the development of the remote control capabilities of modern motor control equipment. In many cases, production flow and efficiency rate adjustments can be critical to the quality of the end product. For example in the case of remote pumping stations, the ability to control the speed of a booster pump, based on the product in the line at the time, can allow a remote location to monitor and adjust flow rates to maximize the capacity of the pipeline.
Process Control Requirements:
In today’s competitive environment, industry is endeavoring to continually improve processes to produce high quality products, at accelerated periods, at the least possible cost. If the process requires variation of speeds, a controller that will vary the motor speed would be appropriate. If depressed distribution voltage, during motor start cycles, is an issue, a solid state or other reduced voltage starting method may suffice.
Physical size restraints:
The limitations of available physical floor space could be a major concern when retrofitting new equipment into an existing control area. The physical construction of equipment housing the newer technology may not be suitable incorporated within existing facilities. Careful consideration may be required for the removal of heat generated by semi-conducting devices, printed circuit cards, transformers and other electronic control devices. Cooling systems may be needed to aid in the maintenance of a suitable environment for these types of equipment.
Ease of use:
One consideration that is sometimes overlooked is the capabilities of the personnel required to service and maintain the equipment. Simpler more traditional starting means may be suitable where basic personnel training levels are maintained. The simpler relay control logic systems may be adequate for the system configuration required for the specific process.