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Thread: 5 wires for 3 phase?
07-25-2007, 05:28 PM #1
Hi, I have a 3-phase motor that requires 5 wires. In addition to L1, L2, and L3, it requests that P1 and P2 (on the motor) be hooked up to terminals 3 and V on the motor starter. What is the purpose of those 2 extra wires?
07-25-2007, 05:45 PM #2
Many 3-phase motors have thermistor connections that are connected to a thermal sensing unit. If they are true Thermistors they would either be PTC (Positive Thermal Coefficient) or NTC (Negatiave Thermal Coefficient) . . . Or more likely they are thermal CLICK switches in which case they are typically Normally Closed when the motor is below the temperature threshold. If the latter situation is the case (you read 0 Ohms between P1 and P2) then these should be in series with the motor coil so that if the motor goes over temp, the motor starter automatically drops out and shuts the motor off.
07-25-2007, 05:52 PM #3
Ok, that makes sense because it's in series with the starter coil, and the diagram shows a switch inside the motor also in series with P1 and P2. It seems odd because the starter already has overload coils inline with the power output terminals.
07-25-2007, 06:19 PM #4
"It seems odd because the starter already has overload coils inline with the power output terminals."
The overloads in a magnetic motor starter are an indirect measure of motor overload.
The thermal protector which is buried within the motor windings is a direct measure of motor overload.
07-25-2007, 06:56 PM #5
"Incipient smoke detector."
07-26-2007, 12:16 AM #6
They more directly measure motor TEMPERATURE, not just overload. You can have too high of an ambient temperature or blocked air flow on a fan cooled motor and the thermistors or klixons will take it off line before an OL relay will see the effects.
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