 Hello friends, I hope all of you are doing great. In today’s tutorial, we are gonna have a look at the Induction Motor DC test for Stator Resistance and Locked Rotor Test. The rotor resistor (R2) plays a very important character in the working of the induction motor. It also assists to find the torque-speed curve design and calculate the speed at which the maximum (pullout) torque will exits. To measure the total resistance that exists in the motor circuit locked rotor test is done on the motor. As this test computes the entire resistance value of circuitry.

To calculate the resistance (R2) of the rotor we should have the idea of R1 (stator resistance) so that it can subtract from the total resistance. The test is done on the motor to find the resistance (R1) of the stator is named as dc test of induction motor. In today’s we post will have look at working, equation, circuit and other parameters of these two tests. So, let’s get started with an Induction Motor DC test for Stator Resistance and Locked Rotor Test.

#### Induction Motor DC test for Stator Resistance

• In the DC test of Induction Motor, dc supply is given at the stator of induction motor. As input is DC so there will be no induced voltage in the rotor and no current will move in the DC. The reactance (Xr) of the motor will also ‘0’ at DC.
• So, the only element which is restring the current in the motor is the resistance of the stator and it can be calculated by the DC test.
• The circuit diagram for the DC test is shown in the given diagram.
• You can see that in this circuit the motor has a Y connection circuit and two terminals of this connection are linked with the DC supply.
• To do the test, the current at stator is set to the rated value, then the voltage is measured by the voltage meter which is connected.
• The current at stator is set to the rated value to get the same heating condition which the motor has during its normal working.
• As we know that the two terminal of stator windings are connected with the DC input so the current move through 2 windings, due to this the total resistance in the path of current will be (2R1). Which is given here.

2R1=VDC/IDC

R1= VDC/2IDC

#### The problem of the Locked Rotor test of induction Motor

• There is a one-issue which generally we must face during this test.
• During normal working, the frequency at stator will small as the input supply frequency which is fifty of sixty hertz.
• At initial situations, the frequency of the rotor will also have the same value as input supply frequency has.
• Nevertheless, at usual working situations, the motor has slip (s) almost two to four percent, and the resultant rotor frequency is in the range of one to three hertz.
• It makes a problem in that the input supply frequency does not signify the normal situations of the rotor.
• Meanwhile resistive of the rotor is the main function of frequency for class B and C motor the improper rotor frequency can cause disturbing findings of this test.
• Characteristic cooperation is to use a frequency twenty-five % or less than the rated frequency.
• Whereas this tactic is suitable for fundamentally constant resistance rotor structure like class A and D, it left a lot to be anticipated (desired) when one is working to find the normal rotor resistor value of a variable-resistance rotor.
• Since these and alike difficulties, a great deal of care should be taken in during these tests.
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#### Equation of Locked Rotor test of induction Motor

• After setting the test frequency and voltage, the current moving in the motor will rapidly reach the rated value, and the input power (Pin), voltage (V), and current (I) are calculated before the rotor becomes hot (heat up).
• The input power of the motor is given as.

Pin=√3VTILcosø

• The blocked rotor P.F can be calculated by this equation.

PF= cosø= Pin/√3VTIL

• The magnitude of the total impedance in the motor circuitry is given as.

ZLR=Vø/I1 =VT/√3IL

• The angle of total impedance is given as.

ZLR=RLR+jX’LR

= ZLRcosø +j(ZLR)sinø

• Hence the resistance of the locked rotor is

RLR =R1+ R2

• And the reactance of a locked rotor will be.

X’LR=X’1 + X’2

• In this equation, the X’1, and the X’2 are the rotor and stator reactance at the testing frequency.
• The resistance of the rotor can also write as.

R2=RLR-R1

• In this equation, R1 is the value of stator resistance which we calculate during the DC test.
• The entire rotor reactance (Xr) mentioned to the stator can also be calculated. As the reactance (X) is directly proportionate to the frequency (f), the total corresponding reactance at the nominal working frequency can be calculated by this formula.

XLR=frated/ftestX’LR= X1+X2

• Inappropriately, there is no simplest method to isolate the dependency of the stator and rotor reactances from each other.
• Over the ages, understanding has revealed that motors of specific design categories have specific relation amid the rotor and stator reactances.
• The given diagram explains the relationship between these two reactances. So that all about the Induction Motor DC test for Stator Resistance and Locked Rotor Test, if you want to know something else about these tests or edit something you can comments below. See you in the next tutorial, single-phase induction motor.

You can also read some related articles to the induction motor. That is described here.

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### 1 Comment

1. Haseeb Alam says:

Could you maybe specify what will be the effect of AC on the DC resistance measured in the stator resistance test? And also how to take it into account when modelling it.