What is a slip and frequency of rotor current in stand still and running condition?

When we connect three phase applied to stator winding then rotating flux will set up due to this flux rotor start rotating but it will never catch up the synchronous speed of flux Ns=120 f/p. if it did so then no relative speed between flux and rotor then no EMF will induces, no current and so no torque to maintain rotation. So motor will always rotate less than synchronous speed of magnetic flux Ns = 120 f/p.

The difference between synchronous speed and actual speed Nr is known as slip.

     Slip = Ns - Nr. 

Unit revolution per second.

Express percentage of synchronous speed 

% slip S = ( (Ns -  Nr) /NS) * 100 %

One more term we call it as slip speed

Slip speed = sNs = Ns- Nr 

when rotor is stationary then frequency of rotor current is same as supply frequency. But when rotor start rotating then frequency depend upon relative speed between the two rotating magnetic flux and rotor. Let's say at any sleep Speed the frequency of rotor current is f' .

Ns - Nr = 120f'/ p   -------  (1)

Ns = 120f / p -------------------(2)

Divide equation 1 by question 2 we get

Ns-Nr/ Ns = f'/ f

 Sf = f'

So frequency of rotor current under running condition is

 f' = sf.

Why does the rotor rotate of induction motor ?

 Induction motor has a stationary part known as stator containing status winding and a rotor having bar in squirrel gauge induction motor and three phase winding in slipping induction motor.

When three phase winding of stator fed by three phase supply then a rotating magnetic field  of constant magnitude 1.5 times  of maximum flux due to any phases is generated. Rotating at synchronous speed NS = 120 f/p. 

This flux passage through the air gap and cut the rotor bar as that they are still stationery.

Due to the relative velocity between the rotating flux and the stationary rotar conductor, EMF is induced in the rotor conductor according to faraday's law of electromagnetic induction.

The the frequency of the induced EMF is same as supply frequency, magnitude is proportional to the relative velocity between the two rotating flux and rotor conductor. And direction is given by Fleming's right hand rule.

since the rotor conductors are short circuit by end ring so current start flowing whose direction is given by lenz's law. It opposes the cause producing it. Here the cause is relative speed so to reduce the relative speed rotor starts rotating in the same direction of rotating flux. so it starts rotating.

What is stall fault and unbalance current in EOCR for 3-phase Ac induction motor?

 In AC induction motor when we start it, at starting it will take 5-7 times of motor rated current and it should come down in 4-5 second and will take no load current 40% - 50% of motor rated current.

Then we can now give it on load.

Now suppose if there is jam in mechanical coupling side, motor shaft then what would happen.

In this case if we start the motor then it take it's starting current 5-6 times of rated current and because it is jam either from mechanical coupling,gearbox or motor shaft problem 

Motor can't start rotating and motor will take  current 5-6 times and  will not come down to it no load current as the delay time of EOCR end it will trip in stall fault.

So if it trip in stall fault then it is mechanically stuck up, it happens at start.

If motor trip in unbalance current:- 

If 3 phase Induction motor was in running condition and trip during running in unbalance current. 

Probable cause may be :-

​1. Winding resistance unbalance.

​2. Winding inductance unbalance.

​3. Motor shaft play.

​4. Motor vibration may increase.

​5. Motor power cable burn somewhere/ carbonized.

 May be other cause too.

1. First this is check the motor resistance and inductance by motor checker.

 Procedure to check the motor Resistance:- 

Disconnect all three motor phase connection from motor terminal , and take out the three lead from stud.

Then take reading of resistance of R- phase,Y- phase, B -phase by connecting checker lead to motor phase lead one by one.

Resistance should be balance.

Procedure to check the motor inductance:- 

Disconnect all three motor phase connection from motor terminal , and take out the three lead from stud.

Then take reading of inductance of R- phase,Y- phase, B -phase by connecting checker lead to motor phase lead one by one.

Inductance should be balance.

2. In some cases i found if there is play in shaft then also motor trip in current unbalance. So if motor winding resistance and inductance is balance then check there also.

3. Check motor vibration also as per standard.

4. If motor power burn near socket, somewhere then also I found motor trip in unbalance.

So if motor trip in unbalance then above steps will help to find out the reason of motor trip.

If any queries then do comment.

Regards.

If AC Induction motor trip in unbalance current, what may be the cause and then what to do ?

 If 3 phase Induction motor was in running condition and trip during running in unbalance current. 

Probable cause may be :-

​1. Winding resistance unbalance.

​2. Winding inductance unbalance.

​3. Motor shaft play.

​4. Motor vibration may increase.

​5. Motor power cable burn somewhere/ carbonized.

 May be other cause too.

1. First this is check the motor resistance and inductance by motor checker.

 Procedure to check the motor Resistance:- 

Disconnect all three motor phase connection from motor terminal , and take out the three lead from stud.

Then take reading of resistance of R- phase,Y- phase, B -phase by connecting checker lead to motor phase lead one by one.

Resistance should be balance.

Procedure to check the motor inductance:- 

Disconnect all three motor phase connection from motor terminal , and take out the three lead from stud.

Then take reading of inductance of R- phase,Y- phase, B -phase by connecting checker lead to motor phase lead one by one.

Inductance should be balance.

2. In some cases i found if there is play in shaft then also motor trip in current unbalance. So if motor winding resistance and inductance is balance then check there also.

3. Check motor vibration also as per standard.

4. If motor power burn near socket, somewhere then also I found motor trip in unbalance.

So if motor trip in unbalance then above steps will help to find out the reason of motor trip

If any questions please let me know.

Regards.

What is the minimum value of insulation resistance of an electrical equipment so we can give it in service? How to improve IR of motor ?

What is the minimum value of insulation resistance of an Low tension electrical equipment so we can give it in service?

Well " insulation resistance"  this is very basic term in electrical world.

If some motor, switchgear, transformer trip in Earth fault then we need to check the insulation resistance of the equipment with meger. 

For LT equipment we use 500volt meger. Our equipment is generally of 415 volt ac. So the minimum IR value formula is 

IR min. = KV +1 MEGA OHM.

SO if the voltage is 415 vac then min. Value of IR  will be

IR  = 0.415 +1 Mega ohm

      = 1.415 Mega ohm.

 

If voltage level is 11kv then min. Value of insulation resistance will be

IR min. = 11kv + 1 Mega ohm

             = 12 Mega ohm.

If motor IR of motor at lower side let's say below 2Mega Ohm then what should we do to improve the IR of motor.

Well if motor Insulation is really damage and grounded then it will give you 0 Mega ohm but if it is non zero value then you have a chance to get healthy motor, low IR  may be the because of moisture.

First put blower near the motor terminal let it run for 24 hours so moisture get vaporize. Then put off the blower and wait for 30 minutes and then check IR .

If not improve then send it to the motor repairing center there they will open the motor and dry the stator section part by heating chamber.

By doing this they heat the stator part of motor and dry the stator part. It will definitely work.

By using above step you can improve insulation resistance of motor.

If any queries feel free to comment.

Thanks.