Motors on VFD's, do they need Temp. Switches

[Designer69]

I've seen a lot of designs from an engineering firm where any motor they have on a VFD they have a temp. sensor on the motor that's wired back to the VFD.

I can't tell if it's for alarm only or provides a trip function.


Anyone familiar with this and whether it's a requirement or supplemental?


Thanks

 

[Besoeker3]

Most that I have dealt with two per phase, one for alarm and the other for trip. Usually customer specified in our case.

 

[drktmplr12]

I've seen a lot of designs from an engineering firm where any motor they have on a VFD they have a temp. sensor on the motor that's wired back to the VFD.

I can't tell if it's for alarm only or provides a trip function.


Anyone familiar with this and whether it's a requirement or supplemental?


Thanks

the motor mfr might require it as a condition of warranty. that said... you will find one way design engineers deal with this unknown is to just require them to avoid the headache at start up when the mfr's rep asks to see where the temp switch is wired in and you find the control circuit has to be modified.

we don't require them for small motors

 

[LMAO]

as others have suggested it is mostly subjective and application depended. Generally speaking, temperature switches or thermocouples are used on larger motors and wired back to the SACADA or VFD itself.

 

[Jraef]

It used to be that VFDs did not come with Motor Thermal Overload Protection built-in, and standard bi-metal thermal OL relays have a tendency to nuisance trip on the output of VFDs, so the embedded thermistors in the motor were used as the OL protection system. Most modern VFDs sold here now have Motor OL protection built-in, it was required for UL listing starting in 2002, but that is irrelevant outside of North America. So for us, connecting the thermisors is not compulsory, at least from a code standpoint, unless the motor or equipment mfr requires it in their instructions (in which case it falls under NEC 110.3.B).

In many parts of the world it is mandatory, so the VFD mfrs are covering their bases. In addition, I'm pretty sure that all motors used in classified areas and run by VFDs are required to have them (outside of North America).

 

[iceworm]

Motors on VFD's, do they need Temp. Switches
I've seen a lot of designs from an engineering firm where any motor they have on a VFD they have a temp. sensor on the motor that's wired back to the VFD.

I can't tell if it's for alarm only or provides a trip function.


Anyone familiar with this and whether it's a requirement or supplemental?

Are you differentiating between "temperature Switch" and "temperature sensor"

 

[paulengr]

Certain motor applications such as submersible pumps must use thermal switches as overload protection because they are cooled in a way other than air. And the cooling is pretty variable unlike say a NEMA motor where cooling rates are well defined and can be modeled mathematically. So this is where one is required. Second the thermal model approach is generally conservative...the motor thermal capacity is larger. So higher performance is possible by adding thermal sensors or switches. A thermal switch input is just a generic input that is used as an interlock by the VFD. Thermistors aren’t terribly linear but are a step up and many VFDs support them. The ultimate sensor is the RTD. More accurate and faster response than thermocouples but there is a problem, They are easily damaged so most motor shops install a minimum of 6. Standard practice is to use majority voting logic and ignore any that are obviously shorted/open. This fixes the issue so you can get maximum performance. This is usually more common with motor protection relays or with an option card on VFDs. You CAN get thermocouples but these are not nearly as common.

So look at how you protecting the motor from thermal overload first as to whether it is mandatory or not, Then optionally especially in high performance cases temperature sensors may make sense.