SSR vs EMR for motor switching

C

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CWH

We are designing a part of a machine that has a motor that has to be switched on/off and forward/backward rather frequently.

Initially we started with eletromechanical relay (EMR) but we had many motors spoilt in the course of running. So now, some of us are suggesting to use solid-state relay (SSR) in place of this EMR.

My question is, can somebody explain theoretically can/how SSR actually helps prolong the lifetime of the motor.

No doubt, the SSR is better in this case because it is not susceptible to arcing and mechanical wear during switching, but this only helps prolong the relay's lifetime.

Hint: Is it something to do with the zero-switching characteristic of all SSR?

Thank you for your help.
 
A basic SSR isn't going to do much to help the life of your motor, but you might be thinking of a soft starter, which is a solid state starter, but it has features above those found in a basic SSR. A soft starter limits the current inrush to the motor, which can be more than six times the locked rotor current of the motor. This huge current inrush is hard on the motor.

There are many soft starters on the market today that have many features and are priced very competitively with conventional starters, even if you use cheap IEC starters. Two of my favorites are the AB SMC-3 family for small motors, and the SMC-flex family for large motors. I just installed an SMC-3 in a small 5hp application that cost me lots less than a NEMA contactor with an electronic overload would have, or had I used a garbage IEC contactor with electronic overload, it would have been close to the same cost.
 
M
SSR vs EMR:

The SSR turns on exactly at the zero crossing mark and then power ramps up along with the sinewave. The EMR turns on anywhere, including the full conduction point.

The SSR turns off at the zero crossing point. The EMR turns off anywhere in the sinewave creating an inductive flyback voltage surge as the mechanical contacts open. The resulting voltage surge can be destructive but it has a tendency to be most harmful to the smaller mass electrical products nearby such as PLC, PCs, etc.

A high quality SSR responds at the zero crossing mark so the system response is < 8.3 mseconds for turning on or turning off, when at 60Hz. An EMR has variable mechanical delays in turning on or off that may impact the motor depending on how your control system is using the EMR.

More information about solid state motor starters is online at: http://www.power-io.com/new/motorstarters.htm
 
Can you tell me what is the difference between the solid-state relay and the solid-state starter, since you mentioned that they have roughly the same characteristics?

I was thinking that with a zero-switching for a SSR, it will limit the starting current of the motor and hence increase the lifetime of the motor.

Thank you for your help.
 
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