star delta motor protection
star delta motor protection
(OP)
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.





RE: star delta motor protection
Reliability-wise, you are limited to a current-balance {negative-sequence overcurrent} relay. There are many grades and prices for such devices.
RE: star delta motor protection
(0.58=1/SQRT 3)
RE: star delta motor protection
Quando Omni Flunkus Moritati
RE: star delta motor protection
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
RE: star delta motor protection
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
RE: star delta motor protection
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.
RE: star delta motor protection
RE: star delta motor protection
RE: star delta motor protection
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.
RE: star delta motor protection
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.
RE: star delta motor protection
Regards,
Afterhrs
RE: star delta motor protection
http://www.ssusa.cc/reference/wiring_diagrams/standard_...
It looks similar to a start-delta starter scheme but the motors are not the same.
http://www.ssusa.cc/reference/wiring_diagrams/standard_...
Quando Omni Flunkus Moritati
RE: star delta motor protection
RE: star delta motor protection
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
RE: star delta motor protection
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).
RE: star delta motor protection
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
RE: star delta motor protection
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
RE: star delta motor protection
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?
RE: star delta motor protection
http://www.ssusa.cc/reference/wiring_diagrams/standard_...
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.
RE: star delta motor protection
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_...
The purpose of the S relay and contacts?
///Normally, S relay is an electromagnetic contactor. Its purpose is in the motor starting sequence to connect the motor winding into y-connection to reduce the motor starting current to one third. After a specific predetermined time for the specific motor expires in the time delay relay, S opens and 2M contactor closes, thus connecting the motor in delta-connection for its ongoing duty. Some manufacturers include this schematic in their catalogs, e.g. Square D in the Digest. More information is available from the manufacturer.\\\
Is it for making the Delta connection for the motor windings?
///Actually, it is for y-connection. See the above explanation.\\\
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?
///Yes, the 2M contactor increases the voltage to line to line across each motor winding set.\\\
RE: star delta motor protection