Which motors require stator RTDs?

Guest,

Mandatory? I don't know of any. Remember that the NEC is not a design or specification document. It is a standard giving the minimum requirements that must be met. Look at 110.3, both (A) and (B). These are very general sections that require you to consider the application, heating effects of use, and any other factors that may apply; and also require following all instructions from the manufacturer or the listing agency (such as U/L). Many times, electricians will ignore the importance of the general requirements of article 110.

The section you reference, 430.32(A)(4) is for motors of 1 Hp or greater. Your basic requirement is to protect the motor against overheating and to protect the personnel and plant against fire or injury.

1. Subsection (1) works by approximating the heating within a motor with a device that heats a sensing element within the starter. If you have any condition that makes this approximation unreliable, then this method is not a good choice. These can include motors run at variable or slow speeds, motors in harsh environments where cooling is difficult, motors subject to voltage variations or imbalances, and others.
2. Subsection (2) requires thermal protectors integral with the motor. For larger motors, they are unavailable.
3. Subsection (3) is permissible if the motor is normally not subject to overloads and part of an "approved assembly". That means, an assembly that has been approved together for a particular use. The general nature of your question probably rules this out.
4. Subsection (4) is really the best choice. The problem is to locate these thermal detectors (whether RTD's, thermistors, or TC's) in the places where cooling is most difficult, the places where the motor is most likely to overheat. This relies then on experience and judgment. Motor repair shops and manufacturers can help. Motors much smaller than 1500 Hp can be obtained with embedded thermal detectors. The problem is increased cost and increased lead time on an order.

Ultimately, you have to balance the degree of protection and safety that is given by the use of embedded temperature detection (the most accurate method for monitoring motor temperature) against the problems of decreased familiarity maintenance personnel will have with this type of protection, additional wiring needed, and availability. It only takes destruction of one motor to justify the additional cost, particularly if reliability or continuous process needs are important. Embedded RTD's can also be used in predictive maintenance and process monitoring, if desired.

In addition to this, depending on the process and motor design, you may need to be monitoring bearing temperature, vibration, cooling flow or cooling temperature, and other parameters for the motor(s).

Be safe--John M.

it all depends on whether how important is that motor for u.Generally all HT motors having voltages of 3.3 KV or6.6KVor11KV need RTD for windings and bearings.Depending upon the type of cooling IC61 ORIC51 RTD's for Air temp is also used.In India we use 6Nos RTD's 2 per phase for stator 2 for bearing and one or 2 as ATD.In addition a mercury in steel dial type thermometer is also used for bearings with settings for alarm and trip

Gentlemen,

Thanks very much for your comments, I guess that the choice is a matter of cost vs benefits, criticality of the application and other considerations that might be based on the company's philosophy.