Soft start motor

The advantage of using frequency converters is that the possibility exists to use a ramp function when starting and stopping the motor (soft start). By using this function, it is possible to avoid starting both fans at full speed with closed dampers it also reduces stresses on the fan transmission (belts) at the start. 

Soft starting minimizes the starting mechanical and thermal stresses/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. 

Since the motor s fixed parameters can now be varied to suit a particular load requirement, there is no need to pre-match a motor with the load. Now any motor can be set to achieve the required characteristics to match with the load and its process needs. Full-rated torque (TJ at zero speed (during start) should be able to pick up most of the loads smoothly and softly. Where, however, a higher 7 s, than is necessary, a voltage boost can also be provided during a start to meet this requirement. (See also Section 6.16.1 on soft starting.) The application of phasor (vector) control in the speed control of an a.c. motor is shown in a block diagram in Figure 6.12. 

This field is very large and a detailed study of the subject is beyond the scope of this handbook. We will limit our discussions to the area of this subject that relates to the control of a.c. motors and attempt to identify the different solid-state devices that have been developed and their application in the control of a.c. motors. ()nly the more common circuits and configurations are discussed. The brief discussion of the subject provided here, however, should help the reader to understand this subject in general terms and to use this knowledge in the field of a.c. motor controls to achieve from a soft start to a very precise speed control and, more importantly, to conserve the energy of the machine which would be wasted otherwise. For more details of. statie controllers see the Further reading (Sr. nos. 2, 4., 5, 8 and 12) at the end of the chapter. To... 

It enables the motor to have a soft start. The motor picks up lightly and quickly and reduces the pressure on the supply source. 

Electronic soft-start The voltage and frequency applied to the motor is controlled ... 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

Pumps should be started slowly to prevent water hammer (a surge resulting from a sudden change in liquid velocity). Water hammer can cause cracks in the outer shell of the membrane modules as well as compaction of the membrane itself (compaction results in lower flux through the membrane at constant pressure). Also, water hammer causes the membrane modules to move in the vessel, which can cause wear to the O-rings used on standard interconnectors and lead to leaks of feed water into the permeate (see Chapter 4.3.3). An increase in pressure of no more than 10 psi per second is recommended.3 Some motors may be equipped with a "soft start" that regulates the speed with which they start up. Other considerations to minimize water hammer include ... 

Should a large tank not be an option, the alternative is multiple skids, some of which will sit idle. The key is to rotate skids on and off so that no one skid experiences most of the down time start-ups. It is also recommended that an off-line or shut-down flush be employed, and that the motor be equipped with a soft start. For the example above, two 350-gpm skids would be required. Both skids would be on line during the 500-gpm draw. A product tank with enough capacity to handle the 200-gpm overage would be required. 

The controller is of the variable voltage, variable frequency type, thereby providing a complete speed range for the motor at constant torque. The system inherently provides soft start which is necessary to alleviate high torsional stresses within the motor-pump uniL that may otherwise damage the shaft and couplings. 

The steady-state torque equation just discussed is the base to build simplified models of the IM in order to analyze/simulate systems where (part or all of) the motor dynamics can be neglected because it is considered too fast in face of the evolution rate of the overall system and its excitation. An example is the soft starting of an IM where stator voltage and frequency are softly increased until they reach the operating values. Even a starting with direct connection to the network could... 

SOLID-STATE MOTOR CONTROL FOR AC INDUCTION MOTORS Electromechanical versus Solid-State Soft Starting Selection of Solid-State Motor Controls Starting Methods for Solid-State Motor Controls Environmental Conditions for Solid-State Motor Controls Wiring Considerations for Solid-State Motor Controls Cost Considerations... 

The key features of the voltage-ramp soft-start unit are that it can softly start varying loads, it does not limit motor torque, and it is easily adjusted. In general, it is desirable to bring the load up to speed as quickly as possible given the mechanical and electrical power distribution system constraints. 

The key features of the current-ramp soft-start unit are that it limits the motor inrush current to a specific amount, it can provide physical soft start, and it limits motor torque. These units are used mainly for electrical soft start (reducing inrush current). The shutdown of production is a... 

Current-ramp units can be applied on variable-torqne, some constant-torqne, and high-inertia loads. They are not recommended for loads that vary considerably since they may not be able to start the load sometimes. As with any type of rednced-cnrrent or soft-starting means, it is necessary to evalnate high-starting-torqne appUcations to determine feasibihty. Some loads demand such high torque at locked rotor that any kind of rednced cnrrent will not allow the motor to start. 

The most significant cost to consider when purchasing a reduced-voltage soft-start motor control is the purchase price. But there are several other costs which must also be considered. In both new and existing installations, it is necessary to figure in the cost of instaUmg the device. Depending on the location selected, the cost to install and wire the device could exceed the cost of the device itself. 

The benefits derived by the use of a VFD are closer and more responsive control of motor speed less mechanical wear on systems because of inherent soft start substantial energy savings achieved on variable-torque loads (e.g., fans and pumps) and elimination of noisy mechanical speed-control systems and their associated maintenance problems. Some of the main considerations in applying a drive are the following ... 

In Europe the main motor is usually an AC motor, using a star-delta starter. An inverter for the main motor is becoming more common, particularly with the smaller decanters. The inverter enables a soft start, and allows speed adjustment for different process requirements. Inverter motors can cause unwanted electrical interference, and harmonic wave forms, on the main electrical supply lines. These problems can be minimised by using electrical filters and the latest advanced electrical technology. 

The power required to accelerate the high inertia bowl to speed can be considerably higher than that required to rotate the bowl at speed. Thus, to start the decanter, the main drive motor need special consideration. This could be one of a number of alternatives, a soft start motor, a specially wound motor, a star-delta starter, or a motor with a fluid coupling. A patented alternative (Alfa Laval) to these methods, used on large decanters, is a dual set of motors [19], Both motors are energised for start up and one is switched off when the bowl is at speed. 

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