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Overcurrent relay circuit position and variable settings in star and delta connected induction motor.


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Good evening, 

I have a few questions requiring your expertise:

1)

For a motor with a nameplate FLC of 44A:

The overload relay trip setting should be set to 44A when connected in Delta and set to 44/(root 3)=25.4A when connected in Star. 

Is this statement correct?

2) 

Does the position of where the overcurrent relay is connected in the power circuit have any effect what the trip setting is set to?

For example, if the OCR is connected in series AFTER the main contactor, would it have the same trip setting is it was connected before the contactor? In this case, my understanding is the OCR is monitoring the line current as it is connected to the supply line in series with the contactor and the motor which is why the OCR is different as per my 1st statement. 

There are cases where there is no individua  OCR in some motor starter circuits  but instead a motor circuit breaker GV-2 with both thermal overload and short circuit protection, so in essence, it acts as a combined circuit breaker and OCR. 

Would the GV2 have a different thermal overload setting than an individual OCR because it is connected before the contactor?

Thanks very much,

Ewan 

 

 

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Hello Evan

1) That is correct.

2) Provided that the circuit is only carrying the motor current, then the position is not important, however if there is power factor correction installed, ensure that the over current protection is wired on the load side of the power factor correction connection point.
The combination of motor current and power factor current is a reduced current and is not an accurate representation of the current flowing into the motor.

Best regards,

Mark.

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For a motor with a nameplate FLC of 44A:

Sorry I got my original statement in reverse.

The overload relay trip setting should be set to 44A when connected in star and set to 44/(root 3)=25.4A when connected in Delta. 

Is this statement now correct? 

 

thanks, ewan 

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Hi Ewan

If you take a given motor that is designed for operation on 400V in delta, and that motor has a rating of 44Amps, then in delta connection, the voltage across the winding, is 400V and the maximum current through the winding is equal to the line current (44A) divided by Root 3 = 25.4 Amps

Connecting the motor in star, will reduce the voltage across the windings will be 230V and so the rating will reduce to one third of the KW rating.

It is usually argued that the current through the winding that will not damage the winding insulation, is the same in both delta connection and in star connection and from a I2R perspective, that is correct, but the flux in the iron is much lower, so the motor will run at increased slip and increased slip means higher iron loss and higher iron loss will increase the heating and so we now have additional heat and potential long life shortening.

We find similar factors when a motor operates at rated load at 75 Hz rather than 50Hz. In theory expect that below 50Hz, we reduce the KW rating with the speed rating and above 50Hz, we limit the maximum load to the 50Hz KW rating, but the reduction in flux does reduce the torque capacity, Hence the constant KW rating (KW = T x w), but in addition to the reducing torque, we have increasing slip and consequently increasing slip losses and excess rotor heating.

Best regards,

Mark.

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Hi marke, 

If a star connected motor runs with less flux in the iron which increases the slip, that would in imply that the rotor speed is less in star? 

I thought it was just torque and current that  is effected when a motor is connected in star at a reduced voltage of root 3? 
 

thanks, Ewan 

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Absolutely not!!

When I was a boy, the most common form of starter, was the star / delta starter, so all motors sold in New Zealand wire six wire motors that could be connected in star to give a "reduced voltage start, or i delta for run.

The name plate gave a 400 volt delta connection and the motor characteristics in this mode, plus a 660 volt star connection with appropriate characteristics.

These motors can be connected to the 400volt supply in delta with full ratings, or in star with effectively one third of the ratings.

If you take a motor that is designed to be connected in star at 400 volts and delta at 230 volts, then you must never connect that in Delta at 400 volts, you will over flux the iron and let the smoke out and it is very hard to put the smoke back in again.

We usually find that motors under 4 KW are designed to be star connected at 400 volt and delta at 230 volts. These motors are ideal for use with small VFDs running of a single phase 230 V0lt supply.

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Thanks Marke, 

If a star connected motor runs with less flux in the iron which increases the slip, that would in imply that the rotor speed is less in star? 

I thought it was just torque and current that  is effected when a motor is connected in star at a reduced voltage of root 3? 

 

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Reducing the flux (due to reduced voltage across the winding) will reduce the magnetic forces and thereby the torque. To produce the same torque output, the rotor current must increase so that the rotor field is increased. This is achieved by an increase in slip within the motor, so yes, for a given shaft power (or torque) the slip will be higher in star than in delta.
NB : with a standard low impedance cage, the increase in slip still amounts to a very small change in shaft speed.

As the slip increases, the slip losses increase and so the power dissipated in the rotor increases.

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  • 2 weeks later...

Hi Marke, 

Is my understanding of why it is bad for a star/delta motor to be stuck in star correct?

1) Star delta starting is used as a method of starting larger motors at a reduced voltage of root 3 , which reduces the large starting current drawn by the motor during start up, therefore protecting the stator windings from burnout due to overheating. As the voltage across the motor windings are reduced by root 3 during starting in Star, this causes the line current drawn to be reduced to one third of the delta current.

As the torque will be reduced according to the square of the applied voltage; if the voltage is reduced by a factor of root 3, then the starting torque in star will be reduced by the square of the factor,therefore by a factor of 3, which reduces the starting torque to one third of the delta torque.

The power output of the rotor is also reduced to one third of a delta connected motor.

The design requirements of the motor is allow it to be started in star BUT run in delta, to match the required torque and power requirement of the driven load. 

Therefore by running the motor in star means running the motor at one third of the torque and power output, meaning that the rotor will not match the required torque and power demand by the load, therefore the motor will stall and overheat. 

Can you please pick out any technical inaccuracies in my answer. 

 

Thanks, 

Ewan 

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Hi Ewan

That sounds pretty good to me.

Essentially, in star, the rating of the motor will drop to one third, so you can not get full capacity out of the motor.
The reduced voltage causes reduced flux, so for a given amount of torque, the slip increases and as the slip increases, the slip losses increase and rotor heating becomes an issue.

The losses in the motor during start, are not the reason for using a reduced voltage starter, the reduced voltage starter is used to reduce the supply current and thereby reduce supply voltage disturbances, plus to reduce the start torque and thereby reduce mechanical damage, but in the case of the open transition star delta starter, the open transition can result in a massive torque transient that can do far more damage than and DOL start.

NB: the total energy dissipated in the rotor during start, is equal to the full speed kinetic energy of the load, irrespective of full voltage or reduced voltage starting. There can be additional energy loss in the rotor if the start torque is too low causing the start torque to be close to the load torque of the driven load (low acceleration torque)

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If the current protection relay is connected in series with the winding, then it is sensitive to the current flowing through the winding. If you have a motor connected in star, then at on third of the motor rating (in delta) the current flowing though the winding will be within the trip setting, but at full shaft load, the current will exceed the rated winding current and the motor will trip. If the over current protection is connected in series with the supply to the motor, then you could run excess current through the motor winding and it would not trip, but the windings would fail.

The problem with current sensing protection with star and delta connections, is that it is sensitive to the primary (stator) current that it is measuring and does not indicate what the rotor is doing. If the motor is well loaded in star connection, the slip will be higher than in delta and so the rotor dissipation will be higher and there will be an elevated rotor temperature rise that will not be detected.

True thermal protection is always more effective in protecting the motor, but temperature rise can occur due to :

  • Over current due to over load
  • Excess iron loss due to elevated flux levels caused by high supply voltages
  • Excess rotor loss caused by star connection of a delta motor, or low supply volts
  • Excess rotor loss because of harmonic voltages
  • Excess rotor loss due to imbalanced phase voltages
  • Excess stator loss due to harmonic voltages
  • Mechanical losses due to bearing problems - EDM, lubrication, alignment etc
  • Reduced cooling due to low speed of fan
  • Reduced cooling due to clogged cooling system - cooling fins, fan air intake etc
  • Reduced cooling due to damaged fan
  • Poor alignment of the motor and driven load, or incorrect bearings for the type of load.

Most of these problems will not show up on pure current measurement only, but some will be determined by appropriate temperature measurements.

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