Terminals in the back but lateral side of the motor

[appunni]

Sir,
We have three old 180kw induction motors in our pump house. On the back but lateral side of the motors I saw terminals with ferrules s1 and s2. I could not understand the use of those terminals. I measured the resistance between the terminals and found that the value was in the order of kilohms. When the motor was energized I measured the voltage to neutral in both terminals as about 110 volts. The supply of the motor is three phase 415 volts.
What is the purpose of these terminals? I request for your valuable comments. Thanks in advance.
With regards,
Appunni

 

[ScottyUK]

Is the voltage a 'real' voltage, or capacitive pick-up from the windings? When the motor is off, can you measure an resistance to any of the main terminals? If there is no resistive connection, I wonder if it is a temperature sensor? From your resistance measurements, and only two wires, I guess it is probably a thermistor rather than an RTD or thermocouple.

 

[rbulsara]

Asking the mfr would shed some light too even without full details.

If not RTD it can also be strip heater terminals. Usually there are more than one RTD ina motor, one per winding at least. If there are only two terminals, then it is more likely to be a heater.

Measure the voltage between the s1 ans S2 when motor is running. RTD resistnaces could be easily relarted to a standard chart of RTD. Most common is PT-100 type RTD. You may need to do some trial and error.

RTD temp will change measurably with temp rise. A strip heater's R will not change significantly with temp.

 

[jbartos]

Suggestion: It might be a sensor that monitors the motor running by generating 110V

 

[edison123]

How big (areawise) are these two terminals and the leads connected to it? If they are thin, probably they are thermister leads (as scottyuk says, they cannot be RTD's due to the high resistance). If they are thick, they could be heater terminals (but then why they should have kilo-ohms resistance beats me).

 

[stardelta]

Another possibility could be a seperate high induction winding to allow for DC injection braking, however in pump drive applications there generally is no need for this facility. The motor could however been designed for, or used in another application.

 

[edison123]

stardelta,

DC injection braking (dynamic braking) is applied to stator terminals. Moreover, appunni has mentioned high resistance (kilo-ohms) between these terminlas so that rules anything connected with stator winding.

 

[stardelta]

Edison.....your half right. Modern DC injection braking systems do use the stator winding but before this technology was available it was common practise to include a seperate winding for the purpose. The winding was highly inductive, ie lots of turns of a fine wire. Hence the high resistance. These windings were often used in older motors for machine tool applications. Any other experienced Stator/Armature winders out there going to back me up on this or is it going to turn into another long running debate between the theorists and the hands-on guys??

 

[edison123]

As a "theorist", I would like to know the necessity to use a separate, higly inductive & resistive winding for dynamic braking when a stator winding is available for this purpose.

And yes, I am also keen to learn from "other experienced stator/armature rewinders"

 

[stardelta]

I cant explain the theory behind what we term as braking windings or for that matter how they function, the comments of a designer are needed. We just put back into a rewound stator what comes out and from the description in the original posting it sounds to me like it could well be in this case a braking winding. True to say they are not very common these days but they do exist.

 

[edison123]

stardelta,

As a "hands-on guy" with 20 years experience in rewinding all types motors, I can tell you that I have never come across such a braking winding. So, naturally I am curiuos to learn if such windings do exist.

Given the high resistance (kilo-ohms), I am thinking about the size of the conductor (really, really thin) and what current it can carry (really, really small)and how it will help in dynamic braking in this case (none, at all). So, if you can post some more details, I would be glad to change my view.

Btw, dynamic braking (using stator windings) has been in use at least since 1930's (yes, as a "theorist", I do have some really, really old books that describe the dynamic braking) and so, it is not so "modern" after all.

To cap it, I would say s1&s2 being thermistor leads is the most plausible explanation (as stated by scottyuk).

 

[jbartos]

Suggestion: It appears to be some sort of tacho, generating voltage when the motor runs

 

[stardelta]

I have never seen a Giant Panda......but I know they exist! The point I am making is that I would not dismiss a fact purely because I have no experience or understanding of the matter. I am not prepared to get into a argument with a complete stranger over the existance of braking windings nor am I prepared to waste time looking for references to back up the fact. They way I see it you have simply yet to come across one. I made a suggestion to the original posting to offer advice, not to prove my knowledge or experience in Motor repairs. However I admit to being at a loss to understand how a Thermistor circuit is capable of generating 110 volts across it.

 

[ScottyUK]

stardelta,

I agree that 110V across a thermistor would be interesting, although possibly shortening the life of the thermistor to milliseconds.

The original post noted that the 110V was measured relative to neutral, not across the thermistor. If the thermistor is embedded within the winding, there will be capacitive coupling between the sensor and its associated wiring, and the conductors of the winding. The coupling capacitance will be small, probably a few pF.

The instrument used to make the measurement will display a voltage dependant upon the voltage divider formed by the capacitance and the instrument input impedance. In the case of a modern DVM, the input impedance is very high, so it is quite possible to measure 'ghost' voltages which have no galvanic connection and are present solely due to parasitic capacitances between conductors close to each other.

On the other subject, I'm in agreement with Edison - I've never seen the winding arrangement you describe for DC braking, although that may merely prove my ignorance of motors from the dim and distant past. Are you sure this wasn't the starting winding for a 1-ph induction machine, which has similar characteristics (lots of turns, high resistance)?