Today I came across a magnetic motor starter that has been discontinued by the manufacturer. It’s a Siemens 14CS32A. The customer still wants to use it to control an exhaust fan and I don’t see any reason not to since it still functions properly.
There are two main functions of a magnetic motor starter (MMS): to start and stop a motor and to protect the motor from overheating, which can damage the motor and make it a fire hazard. The MMS has two components: a contactor and an overload relay. A contactor is an electrically controlled switch that turns a load on or off. It’s a type of electromagnetic switch known as a relay and it uses an electromagnetic coil to pull and release an armature that closes or opens contacts.
The overload relay has a dial that I can adjust with a tweaker. I’ll set it to the FLA stamped on the motor’s nameplate.
If current exceeds the amount set:
- The NC contact in the control circuit opens
- Current stops flowing
- The coil’s magnetic field vanishes
- Gravity moves the armature back down
- And the contacts open.
In newer overload relays there aren’t even any electrical connections to the load’s wiring. CT’s measure the current flowing to the load and open the NC contact when the current is too high.
This specific MMS is controlling an exhaust fan motor and I need the fan to shut off when there is smoke detected in the area. I do that by opening the control circuit feeding the coil, and to do that I use a RIB.
Here is a wiring diagram that show how it works.
Buddy,
The San Francisco Electrician
