Fuse Blown But Why Not Tripping on Star

In star connection phase voltage is acting on winding and in delta connection line voltage on same winding. Line voltage = 1.732xPhase voltage. So lower the voltage lower is the current drawn for a given winding.

1. The changeover time may be too short, not allowing the motor to accelerate to near full speed, the surge at changeover may be causing the relay to trip.

2. The relay will usually be set based on full load current in delta. If the motor stalls, it will draw 2x FLA only in star. May take about one minute to trip. Since changeover will take place much before that, a 6x surge will come at the minimum, and trip the relay. Does the motor start at all in star?

Guys, he did mention that a fuse was blown. He just wants to know wht the OL didn't open until it switched to Delta.

Star/Delta starting is designed to reduce start current during the first few seconds when the motor is drawing it's maximum current. With one phase missing the motor is drawing almost maximum current on the other two phases and since it isn't rotating this maximum current was still present when it switched to Delta.

The OL opening only when it switched to Delta shows that the OL current sizing is about right.

_{The relay will usually be set based on full load current in delta. If the motor stalls, it will draw 2x FLA only in star. May take about one minute to trip. Since changeover will take place much before that, a 6x surge will come at the minimum, and trip the relay.}

This is what i said...that the OL will act only in Delta even if motor stalls. Is that any different from what you are saying ? i am not being confrontational, just curious.

Also, the locked rotor current in delta may be enough to blow the fuse, which means usually that the fuse is undersized.

If it's still drawing locked rotor current in Delta then the fuse should blow.

Nope. If SCPD and relay are properly selected, relay will trip. Please check Type 2 co-ordination tests specified in IEC 60947-4.

I stand corrected. I guess still having locked rotor current when the Delta switch occurs comes under the category of "operating overloads".

It would be helpful if plckid could provide the Overload Relay rating and Fuse rating and type - but I have a feeling that he is busy elsewhere by now.

The fuse information would be valuable in checking that the cables are correctly sized, however the Original Poster doesn't suggest that there is a cabling problem.

What is the nameplate current of the motor,what is the setting of OL relay,is it in phase or line,is the load too much,what kind of load is it connected to?.

A star delta motor will have starting power of 1/3 than Normal with star configuration so the flow current in each winding will be 0.58 times of delta but initially the inrush current will be approx 3-5 times of rated current , inrush current accured maight be in mili second .

In your case I think the inrush current no impact to trip the breaker so you have to increase ther setting of current limit of the overload relay .

you have to calculate the rated current , and than you allow to set 1.1 -1.2 x of calculated current value.

Better you check the current flown in delta after normal operation by using suitable current meter , if you find abnormal current flows ( higher than the calculated current) , please check the loads etc.

One more reason for fuse blowing problem could be wrong motor terminal connections in delta. We have to ensure that only the corresponding winding terminals are joined for a proper delta connection. We had experienced this problem once, wasted whole day over it and ultimately found that the motor supplier( a reputed one) had marked the motor terminals wrongly.

In star if the two ends of any winding(phase)is revered what will be the effect?.

A fuse will blow.

As Mr.Joney451 mentioned below I understand that the motor was working ok before. Now suddenly motor working OK in star but fuses blowing when changing to Delta and no one tampered with connections. If this is correct then open delta contactor, one or two contacts must have damaged.

did the fuse "blow" only once? and the tripping occurred more than once?

what is your motor driving? a fan or a blower?

Well Thanks for the reply. Sorry I for the late replies. Here is the diagram

before getting more confused . . . .

just check the DELTA connection . . . your DELTA may just be wrongly connected . . .

This is working for years .No body changed anything. So I dont changing wires is the solution.The question is one of the phases failed there is excessive current on the other two phases which should make the OL trip on the main contactor.

I will try and attach big image in Pdf when i get back to home .

no need - your image is legible - - - check the actual connection of the delta - it may not be as per the drawing . . .

Am i right in thinking that the starter is using two relays and five contactors ?

In Υ, 412C3.6 and ...8 are On i suppose ? Relay 412OL16.5 is in circuit. To properly protect the motor against OL and LR, it must be set to 1/3 of the full load current.

When it changes over to Δ, contactor ...8 drops and contactor ...6 closes. Relay 412OL16.8 is also within the Δ circuit. Two relays in series is not normal. Further, the motor current seen by both relays is now 1/√3*FLA (line) of the motor. i don't know what to set on the second relay....but the first one is most likely to trip even at full load.

As this is working for a long time, i must be misreading the circuit badly. i am looking forward to some explanation of the scheme....

there are no five contactors . . . there is a lock out switch near the motor (presumably for maintenance) - common in motors for cooling tower pumps in chilled water airconditioning plants and other applications where the MCC is located far from the motor to avoid control cabling and also make the system reasonably safer for maintenance personnel . . .

Your description and the drawing do not match

The description given by you do not match the drawing

On behalf of soebfatehi, this is the mechanically coupled switch which is near the motor....136CB16.5

Three contactors, one Time-delay-relay one over-load relay makes a complete fully automatic star-delta motor starter. The reasons for fuse blowing can be:

- due to a fault in the contactor

- due to sudden high current, may be due to jerk load.

Normal over-load relay takes time to trip. (bi-metal strip inside the relay to heat up and then disconnect the contactor coil power supply)

If fuse blowing off problem is frequent; it seems fuse rating may not be correct. One mistake most engineers do is, not considering "de-rating factor" of the HRC/ HBC fuse. Most fuses are rated at 25'C and if panel inside temperature is high then we need to de-rate the fuse; that is use higher capacity fuse with out sacrificing motor safety.

These are both overload relays.

1.The symbol is clearly that of a heater

2.The terminal numbering 95/96/97/98 is that of an overload relay as per IEC.

3. The designation of the devices is 412 OL 16.5 and 8

i don't see any timer, though it must be there for an auto-star-delta starter,

Thanks soebfatehi for the clarification about the switch. Makes sense.

kvsridhar -

1. i suppose the two relays will be differently rated.

2. presume that the timer (if present) will be shown in a separate control wiring drawing.

3. original post mentions change in time for star.

4. and i know about local switches because i have seen them commonly in use. however this one has one aux contact. i wonder what role it plays in the control scheme. may be providing an indication that the motor has been locally switched out?

1. Why two relays at all ? Only required if stall protection is Star is required. It will be set at 1/3 the rated current and will be cut out of the circuit at changeover. If it is not cut out at changeover, it will trip. If set high enough not to trip, it will not do any protection in star....so .... why at all???

2. In such a well-made schematic, rather surprising that the timer is not shown.

3. Yes, there must be a timer.

4. You are most likely to be right... an indication at the MCC that motor is under maintenance....

Well, the OP is a fresh electronics engineer, so may not be able to answer any of the doubts we have... let us see..

The timer is in the control circuit, with the relays.

Where, please? You can copy the image from the post#14, save it to your computer, edit that image to show the relevent part, save it again, and post it here. It will be much appreciated.

Here is the schematic of Star-Delta "Fully Automatic c/o":

Thanks, but obviously we are at cross puroses. i have been referring to the circuit diagram provided by pickid, the original poster, in post # 14. Please study that circuit again and let me know what you think.

This is the first time I have seen two OL relays rated for Star-current being used in a Star-Delta starter. Normally we use one in delta or one rated for FLA in the maim supply.

Can you send the control circuit for the starter in post 14?.

Apart from all causes stated, please also check that during change over from star to delta connection, all three contacts sets are closing simultaneuosly, other wise single phasing may occur during changeover and causing fuse to blow.

Regards,

Manindra

This should be a fairly simple problem to fault find, If all checks good a common problem with open transition Star Delta starting is during changeover the motor acts has a generator which will cause the fuse to blow. I dont see why you need a overload on the Star CCT, because if its a class 10 you need about 6 times FLA to trip less than 10 seconds by then it should switch to Delta and if the is dead short it should trip the breaker. Note, not telling you to remove the overload.

Well to clarify there are no 5 contactors. There are 3 contactors as you expect in star delta circuit the other 2 are overloads. The thing at the bottom 136CB16.5 is Isolator Just shuts down power to the motor during Maintenace.

While you were busy elsewhere, we diagnosed this already .. if you read the whole thread, you will see this.

There are many doubts...let me ask one thing....

Why two relays at all ? This is very unconventional, and not recommended also....

Two relays? YOu mean the overloads or the auxilary contacts?

KVSridhar means the Overload Relays . .

Thanks soebfatehi, i do mean two overload relays.

Now coming to fuse blowing, rather than repeating a lot of the discussions, please take a look at the thread : http://cr4.globalspec.com/thread/64502/Motor-Starting-Problem#comment677664

There exists a condition called "frozen rotor" in star-delta starting. This usually causes fuses to blow (only one fuse blowing is strange, though) Please google it and study.

Two overload relays is definitely an unusual thing. It is possible that on very low loads the motor is run only on star connection and is not changed over to delta. Under these conditions, one overload relay could be set for low load current and when the load is larger necessitating delta connection, star overload contact could be bypassed in the control circuit and the delta overload relay would remain in the circuit.However, in the absence of the details of the control circuit, all this becomes only a quesswork. The original problem of fuse blowing in delta whether it still persists is not known. Whether the person who has raised this problem has checked the timer setting, timer operation, the actual motor currents in star and delta, the overload settings, the presence of all the three supply phases etc.is still not clear. These are the only things that might lead to fuse blowing at delta change over.

Even after all these posts, it is unclear whether an overload relay or fuse tripped. We also don't yet know the voltage, full load current, etc. of the motor. But then most electrical posts on CR4 routinely fail to include basic data, let alone full data.

Mr.Tornado is absolutely correct. Not only they don't give essential data but also if a member asks for required data they don't provide. In this case I asked for the control circuit for the said starter which uses 2 OL but still not given..

After so much of brain storming, I would request Mr.Plckid to coduct further trials on the motor with fuse blowing problem and let us have the feed back so that either the problem is solved or some new observation comes forth for further discussions. It would be worth his while to declare the present status.

Sorry for late replies was away from home. Here is the thing. I spoke the electrecian the other day he said he said he reset the overload (2x) times. When I came the site the fuse was blown. Here is the control circuit

Which OL is tripping(main or delta)?. Why main contactor is 37kW while delta and star contactors are 45kW?.What is the rated voltage,kw of motor(how many wires),range of OL relays?. Is this the first time it is tripping or was it working for some time?. Has the connected load increased recently?.What is the voltage drop before tripping problem started?.

As i said this was working for years so dont tell me wrong wiring or check your delta connection.

The delta overload was tripp 2x times a

Load was the same

No idea about rated voltage of the motor.

i have asked some questions, please check them, ask your electrician or some higher technical person, and let me know.

i have never seen a motor protected by TWO thermal overload relays. It is counterintuitive. But as you say, this system is working for years, so i must be missing something. i also can't see any timers.

If this is too much trouble, don't bother.

Is this a quiz?. In professional forums facts and figures are important. To your doctor or lawyer if you don't tell the truth your problem will never be solved. In this case rated voltage,kw and range of both OL relays are important.

NO timers because it is controlled by plc.

OL ratings is on the diagram which i posted.

We still don't have very good info for diagnosing this problem, such as:

Motor KW, FLC, voltage, code letter indicating starting current as multiple of FLC.
Time delay before switching to Δ.
Fuse type and nominal rating.
Ambient temperatures at motor and at location of overload relays.
Nature of driven equipment--fan, pump, conveyor, other.
Time vs overcurrent curve of overload relays.
Etc.?

ok guys . . . now get this . . .

1. motor drives a fan as i had suspected . . .

2. motor does not have timer because the control is through a plc (some kinda controller) output command sequence . . .

3. the fuse blew only once after the time setting had been changed . . .

4. motor was tripping on change to delta . . .

5. two overload relays are present . . . a "fresh electronics engineer" does not know why as also experienced electrical engineers who keep badgering him to tell why . . . so maybe stooping asking that question again and again may shorten this discussion and take it forward (unless we are around to play shmik-mty (pls ask what that means)) . . .

6. the circuit has been in place and working fine for some reasonable period of time before the problem came up and it now persists (maybe) . . .

7. two overload relays are present . . . and presumably odd and un-explainable . . . so is any hailstorm in the Sahara desert . . . that is why it is worth writing about . . . ok now that it is there can anyone explain why the fuse blew? without asking a "fresh electronics engineer" to feel responsible or guilty about a circuit he never designed?

8. a local isolator is used to isolate the motor during maintenance and it has an aux contact to trip the starter when the isolator is turned off . . . what is not clear (in the absence of the plc logic chart is whether the motor is allowed to come ON when the isolator is closed afterward? or does it require switching at the starter / plc panel? this however is a side issue in the tripping / fuse question so i will not press this question . . .

9. so based on the information available, any guesses as to why the fuse blew? and relay tripped? no penalties for bad guesses . . . but no repeat questions please . . .

This diagram for Y-Δ starting was provided by kvsridhar in another thread. It may be useful here.
______

DOL and Star-delta are both available. But the choice depends on

1. whether your utility from which you get power, will permit such a large inrush current when you start DOL.

2. whether your mechanical load can withstand the high starting torque that DOL starting gives.

3. whether your load is ok to be star-delta started ... (ΥΔ gives only 1/3 torque as compared to DOL)

If you can afford it, use a Softstarter. ΥΔ is not really a good choice.

Hello colleagues.

I am also fresh electric engineer and have a bit same problem as stated above.

I have three pumps with motors of 22 kW, and rated current 44 A.

All of them are conected to frequency converter system.

System works by rule that converter drives one pump until 100% speed, and than if load exceed, it starts new pump in delta-star connection, and this first one continue to drive below 100%.

The pump number 2, when is driven by frequency converter works normal, but when converter drive some other pump (1 or 3) and then starts pump nr. 2 in delta-star conection, star connection is ok, but by changeover fuses blow.

Fuses are 63 A (C type), and overload relays are set on 26 A.

Ampermeter shows that star connection starts with cca 70 A, and then goes to cca 40 A. When changeover occures, delta sometimes goes to 110 and fuses don't blow, but sometimes it dont reach even 70 A and fuses blow. First case is usually when I start manualy pump 2 in delta star connection, and second case is during automatic operation when system start pump 2 in star delta operation because pump 1 or 3 were not enough to satisfy hidraulic requests.

Thanks for your help.

sorry i saw this thread and your query after a very long time - btw your question is not similar to the original thread so the solutions offered there should not confuse you . . .

i am sure you have solved your problem already by now . . . just in case you have not -

as a first step - check your STAR DELTA timer setting - check the open interval between star and delta

and also check if you are changing over from star to delta too early . . .

personally i opine - a good hydraulic electric system designer would not bring in P2 in star delta when P1 on ac drive is overloaded without worrying . . . check the system engineering if not already done please