motor thermal overload protection

[mull982]

I have a 4160V 2300hp 3 pole motor that has an solid state overload relay. This motor has 6 stator rtd's monitoring the temperature on different parts of the 3 stator windings. Recently one of the rtd's on one of the stators has been ginving a high reading which has been tripping the overload device? What whould cause the temperature in only one of the stators to increase? I would think that if the motor saw an overload condition, then the temperature rise would be seen in all three stators? Can anyone offer any ideas as to why only one stator temp is increased?

On a seperate note I had a question regarding the bi-metal motor overloads that are typically used on starters for LV motors. When looking at 430.32 A(1) and A(2) there are maximum overload ratings for an overload device and thermal protector. Which of these two sections would the bi-metal overloads fall under, and what is the difference between the two sections? Does on corrospond to a solid state relay, while the other corrosponds to a thermal strip?

 

[zog]

I have a 4160V 2300hp 3 pole motor that has an solid state overload relay. This motor has 6 stator rtd's monitoring the temperature on different parts of the 3 stator windings. Recently one of the rtd's on one of the stators has been ginving a high reading which has been tripping the overload device? What whould cause the temperature in only one of the stators to increase? I would think that if the motor saw an overload condition, then the temperature rise would be seen in all three stators? Can anyone offer any ideas as to why only one stator temp is increased?

QUOTE]

My guess is a failed RTD

 

[rcwilson]

Motor Protection

Motor Protection

One RTD reading high temperature could be a localized cooling problem in the stator, or a failed RTD as Zog suggests. Experience says there?s 90% probability of a bad RTD if the reading is way out of line.

That's one reason why we specify 6 RTD's. (The other is to improve the probability of detecting a localized hot spot.)

430.32 A(2) refers to temperature switches or other detectors embedded in the motor windings or integral with the motor housing that trip the motor on over temperature. (The reset button on the garbage disposal motor is one example, but that typically would fall under Section B " One Horsepower or Less..".)

Bi-metal overloads in the motor starter fall under 430.32 A(1). Most protective relays for 4.16kV motors fall under 430.32 A (1) & (4).

I've never seen a medium voltage motor protection scheme that relied only on the RTD temperatures to protect the motor. During starting, the motor winding heats up inside the insulation faster than the RTD mounted outside the insulation can detect it. A combination of winding temperature and over current protection is normally used.

 

[mull982]

Bi-metal overloads in the motor starter fall under 430.32 A(1). Most protective relays for 4.16kV motors fall under 430.32 A (1) & (4).

If I understand correctly then if I'm using a bi-metal relay rated for 125% of motor FLA (based upon 430.32) and I'm still having problmes with the overloads tripping on startup or normal operation, then per section 430.32(C) I can use overloads rated up to 140% motor FLA?

 

[Jraef]

One RTD reading high temperature could be a localized cooling problem in the stator, or a failed RTD as Zog suggests. Experience says there?s 90% probability of a bad RTD if the reading is way out of line.

That's one reason why we specify 6 RTD's. (The other is to improve the probability of detecting a localized hot spot.)snip...

That is why I always enable the "RTD Voting" feature if available; it requires more than 1 (or 2) RTDs to show high temperature before a trip is initiated. Otherwise, it is just sent to the warn relay.

Re: 430.32 A (2), it reads;

"A thermal protector integral with
the motor, approved for use with the motor it protects on
the basis that it will prevent dangerous overheating of the
motor due to overload and failure to start..."

Notice the words "integral to the motor" in there. That means, the motor has built-in thermal protection devices. Usually they are referring to either "Kilxon" style thermal cutout switches in the motor housing, or devices called "thermistors" (different from RTDs) which operate on a fixed temperature value and are monitored by an external "thermistor trip unit". Either are set to open up the control circuit under thermal conditions equaling what would be resulting from motor current at the stated values.

I don't think that typically RTDs qualify for this because they are not pre-set to operate at any particular value; that is up to the person programming it and a little too vague as far as the code is concerned. So RTDs are technically considered supplementary to the Overload Relay, but most people consider them to be more accurate where real temperature is concerned and just use the OL relay to satisfy the NEC if they have them in place.