star delta motor protection 1

[DanThomas]

We just had a 50HP compressor motor burn up. It has a star / delta starter. One of the delta contacts failed, causing that part of the winding to single phase. The overload relay never tripped. Motor FLA is 60 Amps. Overloads rated at 63 Amps. Overload relay is connected to T1, T2, and T3. The contact that failed was one that goes to T10, T11, and T12. Should there be another overload relay for leads T10, T11, and T12? The existing overload relay may work for an overload condition, but it certainly provided NO protection when one of the contacts failed. Any recommendations would be appreciated. Thank you.

 

[busbar]

Reliability-wise, you are limited to a current-balance {negative-sequence overcurrent} relay. There are many grades and prices for such devices.

 

[rbulsara]

OL relay for a star-delta starter is normally connected in the delta circuit, that is it will sense the phase currents not the line currents. Therefore, it should be set at 0.58 times the desired line current trip (or 0.58*63=36.5A).

(0.58=1/SQRT 3)

 

[jraef]

There you go. That was your problem. And it is a common one too. That is the reason I try to discourage people from using star-delta. Service techs who are unfamiliar with how the motors and starters work make this mistake (and others) quite often. Better to use solid state and a standard motor with standard connections.

Quando Omni Flunkus Moritati

 

[GGOSS]

Hello DanThomas,

It would appear rbulsara has identified the reason the overload did not trip, however it should be noted that many overload relays do in fact have a dual scale thus eliminating the need to do the 0.58 calculation.

Assuming your overload relay does have a dual scale and that it was correctly set-up, it could well be that it does not incorparate a 'differential bar' and is therefore unable to sense current imbalances and/or phase loss conditions.

If you would like to post make & model number, I would be happy to look into this further for you.

Regards,
GGOSS

 

[DanThomas]

Thanks to all for their replies. If I understand correctly, I need to add 2 overload relays to monitor the current in each of the delta windings. The FLA of the motor is 63 amps, so I want each OL relay to be set at approximately 37 amps. I would wire the N.C overload contacts in series with the exististing OL realy. Am I correct?
The existing contactor is from Ingersoll-Rand and just has their part number on it. It's the type that uses the bolt on heater elements. According to the chart in the manual, the heater elements should trip at 65 amps.
Thank you all!
Dan Thomas

 

[jbartos]

Suggestion: The overload relay trips are relatively slow when it comes to larger currents the single-phasing may have caused. Please, provide more info about the motor protection upstream. Was there a circuit breaker or fuses, what size, in what condition was the overload relay, how old, etc.
Please, notice that this protection scheme has been established for many decades, and it is found satisfactory within the industry. It is supported by the industry standards and practices.

 

[rbulsara]

I dont understand two delta windings! You need only one relay for a star/delta starter.

 

[edison123]

Two delta windings have two sets of delta windings, of which one is used at start to reduce starting current. The second winding is switched in near full speed. It is common in HVAC compressor motors.

 

[rbulsara]

edison123:

then how does that make a 'star-delta' starter?..perhaps delta-delta starter? I am not saying you are wrong but I am more confused now.

 

[edison123]

rb,

You are right. Dan does mention in his first post it is star/delta starter. In his last post, he says "I need to add 2 overload relays to monitor the current in each of the delta windings" which would indicate a two delta winding. Since both the star/delta and two delta windings come with six leads, it may cause confusion about the motor and starter configuration. May be Dan clarify this.

However, I have handled many two winding compressor motors where one winding is used for starting to reduce staring current. This type is called partial winding start.

 

[afterhrs]

I have two IR compressors that operate as Edison 123 suggested, so I am also interested in this thread.

Regards,
Afterhrs

 

[jraef]

I think the confusion here may be that some applications use a star-delta like configuration as a 2 speed 1 winding motor. In that configuration there is an overload relay in each set of windings. look at this drawing.

http://www.ssusa.cc/reference/wiring_diagrams/standard_series_drawings/series_110/110_01.pdf

It looks similar to a start-delta starter scheme but the motors are not the same.

http://www.ssusa.cc/reference/wiring_diagrams/standard_series_drawings/series_107/107_21.pdf

Quando Omni Flunkus Moritati

 

[Walden]

The Partial Start motor windingsdescribed by “afterhrs” is also known as Part Wind start and is very commonly used in refrigeration compressors. In the UK this method of starting is restricted to refrigeration compressors (because of the influence of American refrigeration compressor manufacturers after WW2) and there will be many UK electrical engineers who will have never heard of this method of starting a motor. Basically there are two electrically independent, usually star connected, windings each with its own terminals. The motor is started by energizing the first winding and then a 1 second (or less) later energizing the second winding. It is classed (in the USA) as a reduced voltage starter although technically it is not, as the windings are connected to the full mains voltage at start. The windings are designed to give a reduced starting current and the windings are designed to carry a percentage of the overall FLC of the compressor motor. Typical values are 67:33% or 50:50%. Both winding individual thermal overload relays and contactors the size being dependant on the current split.

 

[DanThomas]

Thanks again for all the replies. One of the links that Jraef posted (

http://www.ssusa.cc/reference/wiring_diagrams/standard_series_drawings/series_107/107_21.pdf)

is exactly the circuit used in the compressor we have. If you look at the schematic, you'll see that there is no overload protecion for leads T4, T5, or T6. In my case, one of the contacts failed (T5), which put the motor in a "single phase" condition.
The overload relay trips out at around 64 amps (line current). However, in a delta circuit, the phase current is .58 times the line current. This means that at full load the "phase current" is approximately 37 amps per phase.
Although this configuration has been employed for many years, I believe that it is only effective for an overload condition, and nothing else. As in my case, the existing overloads did nothing to protect the motor when one of the delta contacts failed.
That is why I asked if putting one overload relay in each delta winding (rated at 38 amps) would offer more protection against single phase conditions. The main overload (rated at 64 amps) would protect the motor against overloads.
I called Ingersoll-Rand and they said that they see this happen all the time, and that he overload usually only works if there is an overload (compressor starts to bind).
Does this make things any clearer? Thanks again for all the input. Dan Thomas

 

[TheDOG]

DanThomas;

Adding an extra overload would not prevent your motor from failing. However, your Overload relay was not set correctly to begin with! It must be set to 37 amps. As GGOSS stated previously, the O/L may not have differential protection - which would not protect against single-phasing situations.

Suggestion: Replace the O/L with one that has differential protection (electronic) and set the O/L to 0.58 x the motor FLC (37 Amps).

 

[GGOSS]

Hello DanThomas,

I have reviewed the circuit/schematic in the link provided by jraef and can confirm that it is a standard Star/Delta starter.

A single overload relay (with differential bar) will protect the motor against motor overload conditions, phase assymetry conditions and loss of phase conditions. To eliminate confusion, you should know that there are motor windings betwen T1 and T5, T2 and T4, & T3 and L6. Current flowing into the motor via T1 will exit via T5. As an open circuit on T1 or T5 will act to prevent current flow, a single overload relay (with differential bar) connected as shown does in fact provide phase loss protection.

I hope the above clarifies the situation for you.

Regards,
GGOSS

 

[jbartos]

Suggestion to DanThomas (Electrical) Jan 19, 2004 marked ///\\Thanks again for all the replies. One of the links that Jraef posted (

http://www.ssusa.cc/reference/wiring_diagrams/standard_...)

is exactly the circuit used in the compressor we have. If you look at the schematic, you'll see that there is no overload protecion for leads T4, T5, or T6. In my case, one of the contacts failed (T5), which put the motor in a "single phase" condition.
///Please, what was the upstream motor starter protection?
There are many applications where a motor is sensitively protected by an inverse time circuit breaker (NEC permits up to 150% of the three phase wound rotor induction motor full load amps. In very old times, there was nothing better.\\The overload relay trips out at around 64 amps (line current). However, in a delta circuit, the phase current is .58 times the line current. This means that at full load the "phase current" is approximately 37 amps per phase.
Although this configuration has been employed for many years, I believe that it is only effective for an overload condition, and nothing else. As in my case, the existing overloads did nothing to protect the motor when one of the delta contacts failed.
That is why I asked if putting one overload relay in each delta winding (rated at 38 amps) would offer more protection against single phase conditions. The main overload (rated at 64 amps) would protect the motor against overloads.
I called Ingersoll-Rand and they said that they see this happen all the time, and that he overload usually only works if there is an overload (compressor starts to bind).
///The compressor starts in the most adverse motor shaft load condition.\\Does this make things any clearer?
///Definitely it does. Especially, if the motor line protection is not by a sensitive three pole inverse time circuit breaker. Incidentally, this has not been addressed yet.\\ Thanks again for all the input. Dan Thomas

 

[rbulsara]

I agree with GGOSS.

edison, thanks for the clarification.

Not to pollute this thread, but I have a little problem with the control circuit of the 'Other' drawing posted by jraef showing 1 winding, 2 speed wiring. It seems that if you press the low push button, only low contactor will close. How exactly the motor will run in low, with high contactor open?

 

[Kelley8]

Not to deviate from Dan's question, but I have a general question about the starter being used.

http://www.ssusa.cc/reference/wiring_diagrams/standard_series_drawings/series_107/107_21.pdf

The purpose of the S relay and contacts? Is it for making the Delta connection for the motor windings?

When the S relay drops out and the 2M contacts close, does that somehow increase the voltage if indeed T1 and T5, T2 and T4....(etc.) are connected?

Thanks.