Tuesday, December 7, 2010

Motor Starting Schemes

1. Direct-on-line Starting (DOL)

This is an ideal type of starting, and is simple and economical. In this type the full voltage is applied across the stator windings. The torque developed by the motor is maximum. The acceleration is fast and the heat of starting is low (See figure 1). For heavy rotating masses, with large moments of inertia, this is an ideal switching method. The only limitation is the initial heavy inrush current, which may cause severe voltage disturbances to nearby feeders (due to a large voltage drop in the supply line). With this in mind, even local electricity authorities sometimes restrict the use of DOL starting beyond a certain rating, say, 10 hp, for small installations. For large installations, this condition may be of little significance as, in most cases, the power from the electricity authorities may be available on HT 11-33-110 kV. When it is so, a transformer is provided at the receiving end for the distribution of power on the LT side thus eliminating the cause of a line disturbance due to a voltage dip. On the HT side, the effect of a voltage dip, caused by small motors, is of little significance. However, the method of DOL switching for larger motors is recommended only where the supply source has enough capacity to feed the starting kVA of the motor, with a voltage dip of not more than 5% on the LT side. That's why in large industries, nearly all of the motors are started in DOL.

For a large LT motor, say, 300 hp and above, there is no economical alternative other than DOL starting. One can, however, employ delayed action coupling with DOL starting to start the motor lightly and quickly. Soft starting through a solid-state device or a liquid electrolyte are costly propositions. Autotransformer starting is also expensive due to the cost of its incoming and outgoing control gears and the cost of the autotransformer itself. This is also the case with Star/Delta  starting. Moreover, Auto transformer and Y/Delta startings are reduced voltage startings and influence the starting torque of the motor. It is possible that the reduced starting torque may not be adequate to drive the load successfully and within the thermal withstand time of the motor. Unlike in smaller ratings, where it is easy and economical to obtain a high starting torque characteristic, in large motors, achieving a high torque say, 200% and above, may be difficult and uneconomical.
 
With the availability of large contactors up to 1000 A and breakers up to 6400 A, DOL starting can be used for LT motors of any size, say, even up to 1000 hp. The use of such large LT motors is, however, very rare, and is generally not recommended. Since large electrical installations are normally fed from an HT network, whether it is an industry, residential housing, an office or a commercial complex, DOL switching. even when large LT motors are used, may not pose a problem or cause any significant disturbance in the HT distribution network. The power and control-circuit diagram of a DOL started machine is given.

It is, however, advisable to employ only HT motors, in large ratings. HT motors have little alternative to be switched other than a DOL or a soft starting. Y/D switching in an HT motor is neither advisable nor possible for its windings are normally wound in a star formation to reduce the winding's design voltage and economize on the cost of insulation. Autotransformer switching is possible, but not used, to avoid a condition of open transient during a changeover from one step to another and for economic reasons as noted above. Moreover, on a DOL in HT, the starting inrush current is not very high as a result of low full-load current. For example, a 300 hp LT motor having a rated full-load current (FLC) of 415 A on a DOL will have a starting inrush of approximately 2500 A, whereas a 3.3 kV motor will have an FLC of only 45 A and starting inrush on a DOL of only 275 A or so. An HT motor of 3.3, 6.6 or 11 kV will thus create no disturbance to the HT distribution network on DOL starting.

2. Reduced Voltage Starting

When the power distribution network is available on LT, and the motors are connected on such a system, it becomes desirable to reduce the starting inrush currents, for motors beyond a certain rating, say, 10 hp, to avoid a large dip in the system voltage and an adverse influence on other loads connected on the same system. Sometimes it may also be a statutory requirement of the local electricity authorities for a consumer to limit the starting inrush of motor currents beyond a certain hp to protect the system from disturbances. This requirement can be fulfilled by adopting a reduced voltage starting. Sometimes the load itself may call for a soft start and a smoother acceleration and a reduced voltage switching may become essential. A few common methods to achieve a reduced voltage start are described below.

2.1 Star/Delta (Y/Δ) starting

The use of this starting aims at limiting the starting inrush current, which is now only one third that of DOL starting. This type of starting is, however, suitable for only light loads, in view of a lower torque, now developed by the motor, which is also one third that of a DOL. If load conditions are severe, it is likely that at certain points on the speed-torque curve of the motor the torque available may fall short of the load torque and the motor may stall. See the curves in Figure 3,  showing the variation in current and torque in the star and delta positions and the severe reduction in the accelerating torque. When employing such a switching method precautions should be taken or provision made in the starter to ensure that the windings are switched ON to delta only when the motor has run to almost its full speed. Otherwise it may again give almost a full kick as on a DOL and defeat the purpose of employing a star-delta switching. Figure 3 explains this. The power and control circuit diagrams for a typical fully automatic starter is given in Fig. 4

Limitations
 
1. Due to the greatly reduced starting torque of the motor, the accelerating torque also reduces even more sharply and severely (Figure 3). Even if this reduced accelerating torque is adequate to accelerate the load, it may take far too long to attain the rated speed. It may even exceed the thermal withstand capacity of the motor and be detrimental to the life of the motor. One considers this aspect when selecting this type of switching. To achieve the required performance, it is essential that at every point on the motor speed-torque curve the minimum available accelerating torque is 15-20% of its rated torque. In addition, the starting time must also be less than the thermal withstand time of the motor.

2. This type of switching is limited to only LT system for HT motors are normally wound in star. However, in special cases, HT motors can also be designed for delta at a higher cost to the insulation system which may also call for a larger frame size. Also a provision can be made in the switching device to avoid the condition of an open transient during the changeover from Y to Δ.

3. This type of switching requires six cable leads to the motor as against three for other types of switching to accomplish the changeover of motor windings from Y to Δ.
 
2.2 Auto Transformer Starting

For smoother acceleration and to achieve a still lower starting current than above, this type of switching, although more expensive, may be employed. In this case also the starting current and the torque are reduced in a square proportion of the tapping of the autotransformer. The normal tapping of an autotransformer are 40%, 60% and 80%. At 40% tapping the starting current and the starting torque will be only 16% that of DOL values. At 40% tapping, therefore, the switching becomes highly vulnerable as a result of greatly reduced torque and necessitates a proper selection of motor. The control circuit of a fully automatic autotransformer starter is given in Fig. 5

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. The manufacturer of the auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor.

2.3 Soft Starting

Soft starting minimizes the starting mechanical and thermal stressed shocks on the machine and the motor. It results in reduced maintenance cost, fewer breakdowns and hence longer operating life for both. Reduced starting current is an added advantage. In a solid-state static switching device the voltage can be varied smoothly to any required value from high to low or low to high without creating a condition of open transient. For HT motors particularly and large LT motors generally, it provides a more recommended alternative over an autotransformer or a Y/Δ starting.







1 comment:

  1. What about the rating of transformer depending on the staring KVA of motor?The voltgae dip at transformer should not exceed 10% during starting of motor hence the KVA of transformer
    should meet this criteria

    ReplyDelete