Internal thermal.

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If the motor says on the nameplate that it is”Thermally Protected” or in some other way indicates that, you don’t need an external OL relay. If it has embedded thermal sensors that use resistance (RTDs or PTC thermistors), you must have some sort of relay that interfaces with them. Those are usually way more expensive than an OL relay.
 

paulengr

Senior Member
No there is no limitation. In fact submersible pumps are a big challenge because they are water cooled. The solution is to use a “Klixxon” or equivalent. There is usually 3 or 6. They are just epoxied to the coils. There are both NC and NO versions. Flygt likes to put them in series with a moisture probe so you can detect both over temperature and moisture past the outer seal using just two wires.

A more fancy version as mentioned uses an RTD. Generally you want 6 or more. In combination with a protection relay that uses majority voting gives you the best thermal protection money can buy. The reason for multiple sensors is that RTD reliability is not as good as the motor itself. There are no “better” models on the market…I tried, same result no matter the brand. Often you see this on motors over 250 HP.

There are also PTCs. More hardy but not as accurate. Thermocouples are used but rare. Slightly more reliable but again not as accurate.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
If the motor says on the nameplate that it is”Thermally Protected” or in some other way indicates that, you don’t need an external OL relay. If it has embedded thermal sensors that use resistance (RTDs or PTC thermistors), you must have some sort of relay that interfaces with them. Those are usually way more expensive than an OL relay.
The motor has a thermal switch in it wired to a starter. Trying to think how well these will work on a phase loss.
Past experiences have shown they fail open years before the motor has actual issue. Also they have been used in conjunction with an overload block. Not this time.
 

retirede

Senior Member
Location
Illinois
The motor has a thermal switch in it wired to a starter. Trying to think how well these will work on a phase loss.
Past experiences have shown they fail open years before the motor has actual issue. Also they have been used in conjunction with an overload block. Not this time.

If it has a single thermal switch I don’t see how it can provide adequate protection.
 

garbo

Senior Member
Is an internal thermal allowed for overcurrent protection of a motor? 25 HP 480. Mfg supplied starter.
Had a lot of them on 2 to 10 HP Explosion proof motors in a starch molding room at a candy company years ago. Never had touble with them but had to watch when you had motors rewound. Some shops would remove them and not say anything. We always used overloads on these motors.
 
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