It starts at the design. The accel tensions under load are much higher than running tensions. Thus the design must be adequate for this condition.

Relook at the design and modify the system to accomodate the startup loads under full load condition. ■

I am not sure if you are trying to address a new or existing system issue. If all else fails there is always the soft start or reduced torque start star / delta motor starters. The former can work with across the line motor starters whereas the latter will need a specially wound motor. This is all assuming that the rest of the hardware was designed properly in the first case. ■

Most failures at start-up can be traced to instrumentation faults. There are a number of reasons for the condition, such as vibration. 95% of faults are instrumentation. This is why so many accidents happen when operators bridge the signals. One fault deserves another. ■

Originally Posted by sganesh

During starting both speed sensor ( Zero Speed Switch - ZSS ) and Over Load Relay are out of line for few seconds and many break downs happen during that time.

I am wondering how to plug this serious gap?

Assuming you are dealing with a belt conveyor then employ Mr Nordell to design a suitable control system. ■

Resolution of the speed sensor is critical with both accuracy and timing to output signal at low speed.

Zero speed switch is not the answer. Bolts weld to poor pulley that saps its strength.

Get an appropriate tachometer (signal converter) and encoder with 99% accuracy at less than 5% speed. Usually, this is a 2-step or adaptable speed measuring sensor that has a variable timing window that matches with speed to be measured. At low speed (1% range) you need countable pulses (digital leading and trailing ) that return enough pulses in the time band you wish to work in, such as the scan time of the PLC you are using. Typically, at least 5 pulses are advised. You can work out the math for a 15-20 millisecond scan time. You should also not issue control on one signal that may have noise. We stack and compare for continuity to see if the analog or digital signal is truly valid and then logicall act.

Our tachometers are accurate in the 1% speed range and adapt to higher speed with variable scan window becoming much faster as the belt speed increases.

I believe the bigger problem to be that most engineers do not become involved in specifying the necessary signal properties for the full speed range. Sine wave signals are not appropriate, cheap but, inaccurate.

We do not market our speed equipment. However, if you wish a spec on the tachometers, send an email. I believe we have electronic flyer on the units. ABB & Siemens have been our customers as well as the mining companies, when we supply control systems. The devices are military grade, have opto-isolators for high voltage rejection, yada, yada .... and are very accurate and fast with the right encoders. They have active and passive line drivers..... ■