Soft start

Sometimes the load itself may call for a soft start and a smoother acceleration and a reduced voltage switching may become essential. 

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. 

In a V/f control generally, only the frequency is varied to obtain the required speed control. Based on this frequency, the switching logistics of the inverter control circuit control the inverter s output voltage using the PWM technique to maintain the same ratio of V/f. A W/control is, however, not suitable at lower speeds. Their application is limited to fan, pump and compressor-type loads only, where speed regulation need not be accurate, and their low-spccd performance or transient response is not critical and they are also not required to operate at very low speeds. They arc primarily used for soft starts and to conserve energy... 

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... 

Approximate torque curve during a soft start Normal torque curve Load torque... 

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

This part-also deals w ith static controls and drives, soft starting and process control through solid-state technology (phasor and field-oriented controls) using IGBTs as well as energy conservation,... 

There is special coverage of fluid couplings for soft starting and speed control, A comparison between static drives and variable-speed fluid couplings is inade. 

This is going to be a eurrent-mode eontrolled half-bridge topology. It will have soft-start to minimize the start-up surge of the system. The power supply must meet UL, CSA, and VDE safety regulations. 

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. 

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. 

Similarly if you toggle the enable pin at a certain rate in many ICs you can get to see current overshoots too. This often depends on allowing the input capacitor to discharge below the UVLO threshold, but not to the point where it hits the internal POR (power on reset) threshold. Because in that case, if you suddenly enable the IC, it has no soft-start anymore, and you will hit max duty cycle, and possibly staircase if the current limit circuitry is not well designed. 

Concluding the Mexican adventure, by examining the stamped date codes, we finally traced the problem to some manufacturing tolerances on our switcher IC. We also then created an Apps fix (in this case, a soft-start tweak if I remember right), and we were on our way back, with no liability. That was of utmost importance. 

Figure 12-7 Soft-start May Do Nothing to Control the Peak Currents During Power-up...

So where does soft-start really help Mainly in bringing up the output voltage rail smoothly, maybe to avoid jerking the system connected at the output of our power supply. Yes, perhaps within a certain range of output C and L, it can also help control the stresses on the switch at power-up. But that support is hardly unconditional. Maybe it helps somewhat in lowering the overshoot of the rail at startup. But as mentioned in Question 2, it can itself be a reason for the overshoot too. Therefore, in all the AC-DC Flybacks we... 

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 ... 

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