3 Phase Motor with star delta starter shuts down after around an hour

Thread Starter

niraj_khadka

Joined May 10, 2016
4
Hello all.
In our village we have a local community owned water distribution system where we use a 12 hp motor with a 12 hp motor starter (star delta) to pump the water.
The star delta starter have the following components:
1) Metasol MC-22b contactor
2) Metasol MT-32 thermal protection relay
The picture is attached herewithin.

Previously due to low power issue on our village the motor was run on a DOL mode that is directly from the middle contactor and both the thermal realy and the primary contactor was byapssed. This way the pump was running fine although the submersible pump wires were a little warm.

Now we bought a 60kva voltage stabilizer and conntected it. Now the motor is also connected as it should be in star delta starter. The motor runs okay for an hour and shuts off automatically. The thermal protection relay MT-32 is so hot to touch .
The phase to phase current are as folows as shown by the ammeter display
RY -----> 22A
YB ------> 22A
BR -----> 22A

Before shutting off itself the ammeter reads around 26A.

Since this is a phase to phase current and the motor only starts in star connection but runs on delta connection later so it means that the line current is root(3) times phase current which is over 40A, the limit of MT32.
Please see the picture of how the starter was connected previously..1.jpg 1.jpg 2.jpg 1.jpg 2.jpg 3.jpg 4.jpg 5.jpg 6.jpg 7.jpg
 

MaxHeadRoom

Joined Jul 18, 2013
28,527
What is the FLA on the motor plate?
Is the inlet and outlet of the pump fairly wide open (full load), if so you may need to reduce the outflow side somewhat.
I assume the thermal relay has tripped at the point it stops?
Max.
 
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Thread Starter

niraj_khadka

Joined May 10, 2016
4
My guess is the same but i think its not good to replace a thermal realy with a high ampere one because in the metasol datasheet it is given that mc-22b is supposed to use mt-32, and if i need to use mt-64 i neeed to chage the contactor as well..
 

Thread Starter

niraj_khadka

Joined May 10, 2016
4
Another important thing.. The phase to neutral voltage in nepal is suppose dot be 220v but in my village the volatge is only 170V.
 

MaxHeadRoom

Joined Jul 18, 2013
28,527
170v is definitely aggravating the problem, It is best to go to the cause and remedy that rather than increase the Th. relay, a bit like retouching the X-ray's instead of having the operation!;)
What is it between phases?
Max.
 
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MaxHeadRoom

Joined Jul 18, 2013
28,527
An aside: It seems rather strange that the three phase appears to be wired in Earth wire conductor/colours in some instances?
Maybe this was an excess inventory and on hand or Kathmandu has a different standard ?:(
Max.
 

tcmtech

Joined Nov 4, 2013
2,867
Another important thing.. The phase to neutral voltage in nepal is suppose dot be 220v but in my village the volatge is only 170V.
That's your problem right there. The motor is running way below it's designed minimum working voltage which pushes its current draw up above it and the protection circuitries intended working limits.

If it took 22 amps per phase at its normal 220 VAC input rating to put out 12 HP to do so at 170 volts input would require ~28.5 amps thus leading to your motor and thermal overload devices running way too hot for too long of periods of time.

If you have a voltage stabilizer unit on the system what is its input and output voltages when all of this is happening?

If it's what is putting out 170 volts obviously it's not able to raise the input line voltage high enough for the motor to work properly or it's wired wrong.
 

ballie247

Joined Jul 22, 2011
1
A very often ignored problem in remote areas is that of power factor / power factor correction. Everybody adds induction motors to the utility supply and seldom adds local capacitors for power factor correction for the specific piece of equipment being connected. Cumulatively, this adds to a bad power factor at your connection point to start with. In my experience in a wheat and maize mill, bad overall power factor caused motor overheating and even failures. When a plant-wide assessment was done and primary incoming power factor correction was implemented, these problems disappeared. Then there is also the little matter of harmonics. (different but related can of worms) Lots of information is available on the net. I placed a link that could just help clarify some of the basics.

http://www.plantengineering.com/sin...-factor/fb7e4029c995bb046039cb9d70b1fc6b.html
 
Check your lines if there is possibility you can install step up transformer with automatic compensation of voltage change. But I think this is expensive. The only problem is that you can burn your pump, luckily it is in water. Maybe installing thermo protection inside pump and increasing amperage of thermal protection will solve problem. But if thermal protection, or thermal fuse fails you can burn your pump if it is very expensive this plan is bad.
 

Lomac

Joined Dec 15, 2023
1
My 7.5 HP 'Kiloskar' Submersible Pump installed in my Underground Reservoir, has just been re-wound and I fitted it back in the same Star Delta Motor Starter. It worked for about 3 days satisfactorily. Today when I started the Pump it switched off as soon as it went on to Delta. This happened before and it burnt out. Can anyone please tell me why is this happening and what to do to rectify this problem.
 

MisterBill2

Joined Jan 23, 2018
17,866
If the OP has access to a clamp-on ammeter then I suggest checking the current in each motor lead while it is running in the DELTA configuration. One poor connection will result in excessive current in one leg of the motor and tripping of the overload. And in that assembly there are quite a few places where a slightly loose connection could be located. Tripping after a time of running points toward that being the problem. Voltage checking will probably not sense the problem, it is a current issue.
 

GetDeviceInfo

Joined Jun 7, 2009
2,190
You need to start with your motor FLA and service factor, which dictates the overload setting. You then must confirm current draw (use an amp clamp) to see If the motor is not overloaded. Your overload setting looks funky. Low voltage will be a problem. Auto step up transformers are generally cost effective.
 

MisterBill2

Joined Jan 23, 2018
17,866
Amazingly enough, the voltage difference is much less than the current difference when there is a weak connection with a 3-phase motor. On the line side the difference is almost nothing, on the load side the motor delivers a voltage but the current is the wrong phase. which is why the voltage check scheme is not nearly as accurate. It is much more than just an ohms-law case.
 

GetDeviceInfo

Joined Jun 7, 2009
2,190
Sagging line voltage can be a real problem with motor starts. Long leads can exasperate the problem. Transformer start is a practical method to boost current to a lagging motor. But that’s for starts. For continuous run, the load demand is met by the power formula. Low supply voltages and a lagging motor will reduce power factor, possibly decaying to stall. It’s here where the overload plays it’s roll. Motor heat is largely due to power factor. As apparent power rises over real, the difference is heat.
 

MisterBill2

Joined Jan 23, 2018
17,866
Sagging line voltage would certainly be a serious issue, but the development of overload tripping after running some time points rather much toward a slightly poor connection on one phase. Pointing also in that direction, for this instance, is the reality that the starter is implemented with a large number of independent devices. The screw in one connection being a half-turn loose could be the cause of the problem. A simple check of each phase of motor current would verify or disprove that as a problem cause. And a simple tightening of the connections could be a solution. As a backup for that check, use the same clamp on meter to check the mains input current, which should also be very close to equal.
This check has an added benefit that it does not need to interrupt the operation at all.
 

MisterBill2

Joined Jan 23, 2018
17,866
A single loose connection screw can be tightened in less than a minute, solving the problem. And checking the phase currents with a clamp on meter had quickly located which phase it was.
 
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