Motor Thermistors

[dvcraven0522!]

I don't much about motor thermistors. Do all motors have them? If not are they required by code or manufacturers.

Thank You
Dan Craven

 

[retirede]

In my experience, you have to special order a motor to get it equipped with thermistors. As such, you’ll find very few in the field.

There is no general requirement to have them, but some special applications (such as some hazardous locations) require some provision to ensure motor windings don’t exceed certain temperature limits and the use of thermistors is one way to accomplish that.

 

[dvcraven0522!]

Ok that would make sense.

In essence I have situation in a class 2 Division 1 ( Grain Handling) environment where all motors except one have the thermistors. Can these be ordered and installed in the field?

 

[ptonsparky]

Ok that would make sense.

In essence I have situation in a class 2 Division 1 ( Grain Handling) environment where all motors except one have the thermistors. Can these be ordered and installed in the field?

Even if they could, are you confident in your abilities to install them such that your workmanship would not be called upon for the entire lifetime of the motor?

 

[retirede]

Ok that would make sense.

In essence I have situation in a class 2 Division 1 ( Grain Handling) environment where all motors except one have the thermistors. Can these be ordered and installed in the field?

The only way I would consider that would be if the motor manufacturer offered a field kit specifically for that motor. And I’ve never seen such a thing.

 

[Jraef]

Can these be ordered and installed in the field?

No, they are embedded inside of the windings of the motor. Any field modifications to an explosion proof motor voids the ratings.

You must use motors that are identified for the environment. Motors are not REQUIRED to have thermistors, but most motor manufacturers ATTAIN the class and division ratings they need by using them. If your motor that DOESN'T have thermistors still says on the nameplate that it is good for Cl. II Div. 1, then it is and you don't need to concern yourself further, because the motor mfr apparently got that rating without needing to use thermistor protection to prevent it from exceeding the temperature limits of 500.8 that apply to your installation.

But if not identified for Cl. II Div. 1 use, , then you can't install that motor.

 

[don_resqcapt19]

The Class II motors that I have installed had "P" leads. Those are connected to a normally closed temperature switch and wired into the control circuit so the starter drops out when the thermal switch opens.

Don't think I ever did one that had a thermistors. Are those only for larger motors that would be connected to a controller such as GE Multilin?

 

[Jraef]

The Class II motors that I have installed had "P" leads. Those are connected to a normally closed temperature switch and wired into the control circuit so the starter drops out when the thermal switch opens.

Don't think I ever did one that had a thermistors. Are those only for larger motors that would be connected to a controller such as GE Multilin?

The IEC folks use thermistors more often that we do here. They just need a Thermistor Trip Relay, relatively inexpensive compared to an MPR like the Multilin. Some VFDs have the ability to program one of their inputs as a thermistor input as well.

 

[don_resqcapt19]

The IEC folks use thermistors more often that we do here. They just need a Thermistor Trip Relay, relatively inexpensive compared to an MPR like the Multilin. Some VFDs have the ability to program one of their inputs as a thermistor input as well.

Can the leads from the thermistor run in the same raceway as the motor leads? Most of our across the line starters use 120 volt control circuits with the control conductors in the same raceway as the power conductors.
This changes with motors on a VFD where there is always a power and a control conduit.

 

[Jraef]

Can the leads from the thermistor run in the same raceway as the motor leads? Most of our across the line starters use 120 volt control circuits with the control conductors in the same raceway as the power conductors.
This changes with motors on a VFD where there is always a power and a control conduit.

So long as the rules are followed, i.e. 600V insulation if it's 480V.
All a PTC motor thermistor does is change resistance, really sharply, at a specific temperature. It's not really a good analog signal like an RTD (although people try to use them that way), there is a steep knee point, almost a "Go / No-Go" aspect to it. So the thermistor trip relay is basically just sending a voltage down that line and if it comes back, it's good, but when the resistance goes super high, the feedback voltage drops and the relay trips.

 

[GoldDigger]

So long as the rules are followed, i.e. 600V insulation if it's 480V.
All a PTC motor thermistor does is change resistance, really sharply, at a specific temperature. It's not really a good analog signal like an RTD (although people try to use them that way), there is a steep knee point, almost a "Go / No-Go" aspect to it. So the thermistor trip relay is basically just sending a voltage down that line and if it comes back, it's good, but when the resistance goes super high, the feedback voltage drops and the relay trips.

I thought that a more appropriate description is that when the resistance goes high the current goes down. If that current is used to generate a voltage inside the sensor input, then that voltage will in fact decrease.
But if the current itself is the input value (like being the current in a relay coil) then the voltage decrease description is strained.
If the sensor input approximates a constant current, then the voltage increases.
(One of the almost infinitessimally rare occasions where I dare to disagree with jraef.)

 

[retirede]

Can the leads from the thermistor run in the same raceway as the motor leads? Most of our across the line starters use 120 volt control circuits with the control conductors in the same raceway as the power conductors.
This changes with motors on a VFD where there is always a power and a control conduit.

I’ve seen many a class 1 motor with a separate condulet where the thermistor leads exit.

 

[dvcraven0522!]

Thank you for all the responses and discussions.

I essence what we are doing is demolishing all of the existing starters and disconnects and providing a new MCC in an existing doghouse on the roof to get out of C2D1 environment. All of the motors will have soft starts. My question is are the thermistors typically wired in such a way that it can trip the soft start?

 

[Jraef]

Thank you for all the responses and discussions.

I essence what we are doing is demolishing all of the existing starters and disconnects and providing a new MCC in an existing doghouse on the roof to get out of C2D1 environment. All of the motors will have soft starts. My question is are the thermistors typically wired in such a way that it can trip the soft start?

Depends on the soft starter. Some have a thermistor input, some do not. For those that do not, you will need to add a thermistor trip relay and wire it into the Start / Stop circuit.