If buying a new MCC it would seem smart to buy only autotransformer type starters even if you had existing wye-delta motors.The only negative I found was that the transformers take up more space. However I wanted to be sure I wasn't overlooking some important consideration.
I think you have to choose one of them depend on application and design requirement.I hope this info give you some clearness of purpose
All what I know about:
A: The most common types of reduced voltage starters are Autotransformer, Part Winding, Wye-delta (open and closed transition) and Solid State.
AC adjustable speed drives (inverters), though they can be considered as a type of reduced voltage starter, will not be discussed here due to their cost.
Reduced Voltage Autotransformer Starters: The reduced voltage autotransformer starter provides a reduced voltage to the motor terminals during starting using taps on a three phase (coil)autotransformer -motor's coil connected in star or delta depend on nominal voltage of power supply and mnf. requirement.
The autotransformer voltage taps allow adjustment for a range of current and torque requirements. The standard taps are 50%, 65%, and 80% of the full line voltage. Starting torque are therefore 25%, 42% and 64%. While the current to the motor is 50%, 65% and 80% respectively, the line current will be 25%, 42% and 64% of the full voltage rating.
Application - Autotransformer starters can be used with any standard induction type motor. Motor connections are the same as for full voltage starting. Designed for reduced voltage starting of standard squirrel cage motors, the autotransformer starter provides the highest starting torque per ampere of line current, and is an effective means of motor starting for applications where the inrush current must be reduced with a minimum sacrifice of starting torque.
Autotransformer type starters are used where torque is required, but current must be reduced. Applications include conveyors, crushers, extruders, mixers, fans, compressors and chippers. These starters are closed transition, an arrangement that maintains a continuous power connection to the motor in the transition from reduced to full voltage. This avoids the high transient switching current characteristic of starters using open-circuit transition, and provides smoother acceleration.
Part Winding Starter: This starter is not a true reduced voltage starter. The Part Winding starter is designed for motors which have two separate sets of windings, and the starter energizes half of the motor's windings with full voltage during starting and then the other half of the windings for a run condition.
Typical starting current is 50-65% of the motor's full voltage inrush current and starting torque is approximately 50% of what would be developed at full voltage.
This starter is the least expensive of the reduced voltage starters, however, there are several limitations which restrict its use. As stated, the motor must have two separate windings, which is standard on dual voltage (230/460V) motors, but these motors can only be started part winding on the lower voltage (230V). Some single voltage motor designs can be started by part-winding method, but usually a special motor must be built (therefore not a common distributor stock motor). Having a special wound motor for part-wind starting would cause the end-user or distributor to stock a special motor, increasing inventory cost, or there would be a possibility of the equipment being down while a new motor is made or the existing motor is rebuilt.
Another restriction to the application of a Part Winding starter is that standard motors have an allowable acceleration time (only half the windings energized) of one to two seconds (maximum three seconds) based on the motor design.
Applications - Only loads which have low starting torque requirement such as low inertia fans and blowers, and some compressors designs can be started using part-winding starter method. The applications are limited by the simple fact that the load must be able to be accelerated from zero to full rated speed in only two seconds, with only half the motor capacity. If the load cannot reach full speed before the second winding energizes, the motor's torque and inrush current will jump to the full voltage values and defeat the purpose of the starter. Essentially the Part Winding starter is used for only two things; it is the least expensive of the reduced voltage starters and it meets the Utility Company's requirement of ``Motors over `X' horsepower must be started by reduced voltage.
Wye-delta Starters: This starter is also not a true reduced voltage starter in that the motor terminals will see full line voltage. The motor windings though are connected in a wye configuration during starting, which means the individual winding is designed to have line voltage across it (i.e. 460V) but the voltage across each winding is line voltage divided by the square root of three (i.e. 265V) or 58%. This gives the affect of reduced voltage on the windings and the motor draws only 33% current from the line. Since the voltage on each winding is at 58% and the torque is the square of the voltage, the motor output torque is 33%.
Applications - Wye-delta starters are used with delta-wound squirrel cage motors that have all leads (6 or 12) brought out to facilitate a wye connection. This starting method is particularly suitable for applications involving long accelerating times or frequent starts. Wye-delta starters are typically used for high inertia loads, although they are applicable in cases where low starting current is necessary and low starting torque is permissible.
Reduced Voltage Solid State Starters:
The solid state starter provides a smooth soft start of standard three phase AC induction motors using stepless reduced voltage. The solid state starter uses SCRs (silicon controlled rectifier) to apply voltage motor. Two SCRs are connected back to back per phase, and their firing angle is controlled to increase or decrease the voltage to the motor. This method allows a stepless voltage ramp and therefore there is no transition (no current surge during transition).
Start/Stop Methods - The solid state starter offers various starting and stopping methods. Starting control methods include soft start (voltage control), current control (current limit), and special load (pump) control. Stopping methods include soft stop, emergency (coast) stop, DC injection braking, and special load (pump) control.
Application - Solid State starters can be used with any standard induction type motor. Motor connections are the same as for full voltage starting. Using the voltage control method, the voltage is set to a low value, generally between 0 and 50%, and slowly increased to provide a slowly increasing torque to the load. The time from the initial voltage to full line voltage is programmable usually between 1-60 seconds.
The other primary method of starting is current control. Using this method, the maximum limit is set for starting current, and using current feedback system, the voltage is output to the motor terminals based on this current value. As the motor approaches full speed, the voltage increases to full line value.
My Best Regards, Stele Alex.
Many additional features are available either as standard or options on solid state starters. These include soft-stop for slowly stopping a load, emergency (coast) stop, and DC injection braking to quickly stop the load. With most solid state starters, there are also many standard or optional protection features such as single phase, overtemperature, overload, SCR failure, phase reversal, and ground fault.
Solid state starters are used where torque is required, but current must be reduced, and where starting torque has to be limited. Solid state starters are also used on high inertia loads, where low torque must be applied
saws and centrifuges. Applications also include conveyors, crushers, extruders, mixers, compressors and chippers where torque is required but current must be limited. Many solid state starters are used on pumps in order to control the increase in pressure during starting and control the water hammer during stopping. Along with Toshiba, some manufacturers offer a special pump control circuit which can help with slowly opening and closing check valves, and with the pressure oscillations sometimes caused by the long acceleration and decelerations times.
What is the difference between a Wye-Delta open transition starter and a Wye-Delta closed transition starter ?
A. The Wye-Delta open transition starter develops current and torque surges in the transition from starting Wye to Delta run. The Wye-Delta closed transition avoids current and torque surges by adding a fourth contactor that inserts a resistor bank when the starter does the transition from starting Wye to Delta run.
MAMOOAMH, why would you recommend an AT starter for the upgraded MCC! I believe the first step would be to find out why the Wye-Delta was originally chosen (low inrush current, low torque, other)!
If reduced starting-current is the primary reason, and the motor is adequate, other more economic starting methods are available than the
AT. Alternatively, if increased starting-torque is required, then in all probablity the original motor will be inadequate, and the AT can't compensate.
By now this problem is well solved. I am wondering if you have considered a company (newer to the US market) called Motorboss from Somar www.somar.co.uk? Motorboss is a motor controller that has the benefits of the electronic start however it stays on-line an continues to monitor torque. It will deliver the correct amount of voltage to the motor to keep it operating efficiently trought the start and running cycle. By correspond directly, I would appreciate feedback from those who are "in-the-know.
Since the transition of star-to-delta connection of the motor windings are being performed outside the motor case (starter/contactors are inside the MCC remote to the motor), would the distance of the MCC from the load (600m) be a major factor to consider?
1) Economics.
There are six leads, so the an additional de-ratng factor must be used, which will increase the conductor size (mmq) as well as the conduit size (if used)!
2) Voltage-Drop.
During Start & Run, the two sets of conductors are in series, which exacerbate the VD limits!
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