Difference between Reversing and regular starter

anks

Member
Join Date
Dec 2004
Location
California
Posts
66
hi,

what is the difference between reversing starters and regular starters for motor protection and also what is the difference between solid state relay & eutectic alloy relay.

thanks
 
The difference, as far as motor protection is concerned is nothing. This is because a reversing starter is nothing more than two contactors instead of one for the non-reversing.

A reversing starter uses one contactor for forward (pass through) and the second to swap phases for reversal when energized.

In both cases, the motor protection is "after" the contactors.

Not sure EXACTLY how a solid state overload functions. A eutectic alloy overload relay heats up from motor current and releases a mechanism to drop out the motor.

To visualize this, imagine that a shaft is held in a pot of solidified solder. The motor overcurrent heats the solder around the shaft and frees it. When the shaft is free to move, it opens a switch, dropping out the starter coil. This is how eutectic alloy overloads work.
 
There's no difference in motor protection techniques for reversing and non-reversing starters, outside of the use of aux. contact interlocks for the fwd and rev parts of the starter.

A eutectic alloy relay is used in combination with a ratchet wheel to activate a tripping device when an overload occurs. A eutectic alloy is a metal that has a fixed temperature at which it changes directly from a solid to a liquid. When an overload occurs, the heater coil heats the eutectic alloy tube. The heat melts the alloy, freeing the ratchet wheel and allowing it to turn. This action opens the normally closed contacts in the overload relay.

A solid stateoverload relay does not actually generate heat to facilitate a trip. Instead, it measures current or a change in resistance. Current can be measured with current transformers, then converted into a voltage which is referenced by the overload relay. If the relay notices that the current is higher than it should be for too long a period of time, it trips.

$

Aha i'm 2 seconds too slow. :p
 
One other factor is that a reversing starter usually employs a mechanical interlock between the forward and reverse contactors to prevent both from being operated at the same time, plus an electrical interlock for the same purpose.
 

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