wmacdaid
New User
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- VelcroUSA Manchester, NH
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- Electrical Control Tech
Does a 3 phase motor starter protect against single phasing or do I need a 3 phase monitor?
Single Phasing
- Thread starter wmacdaid
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oldsparky52
Senior Member
- Location
- Wilmington, NC USA
Typically you would use a properly sized motor starter (with overload sensors) to protect the motor from single phasing.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The simple answer is that some three phase motor starters contain electronic overload protection which also detects single phasing even on a lightly loaded motor.
Others contain thermal overload protection which by its mechanical design trips at a lower current when single phasing is present.
And some contain overload protection which is not sensitive to single phasing and can lead to a burned out motor.
Note that it is relatively easy to detect single phasing when you try to start the motor. It is harder to detect single phasing when the motor is running but lightly loaded, since it will continue to turn at near synchronous speed and without overcurrent on the remaining phase. (Think about a rotary phase converter.)
Others contain thermal overload protection which by its mechanical design trips at a lower current when single phasing is present.
And some contain overload protection which is not sensitive to single phasing and can lead to a burned out motor.
Note that it is relatively easy to detect single phasing when you try to start the motor. It is harder to detect single phasing when the motor is running but lightly loaded, since it will continue to turn at near synchronous speed and without overcurrent on the remaining phase. (Think about a rotary phase converter.)
oldsparky52
Senior Member
- Location
- Wilmington, NC USA
It's been a long time since I installed a motor starter, but I thought the whole purpose for a motor starter was to protect against single-phasing.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The one absolute requirement for a motor starter is to provide overload protection which the short circuit and ground fault protection for the circuit can not generally do. If single phasing results in an overload it will then protect the motor.
If the phase is lost when the motor is not running, a eutectic alloy thermal overload will not sufficiently protect the motor if a startup is attempted.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
How expensive is it to replace the motor?
ptonsparky
Tom
- Occupation
- EC - retired
Exactly. The motor itself plus the collateral damage.
The newer overloads are enough better at detecting phase loss that I no longer include additional phase loss protection as standard procedure. Smaller motors were a toss up. Depended on the process.
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