Induction Motor DC test for Stator Resistance and Locked Rotor Test

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.

Table of Contents

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 (X_r) 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=V_DC/I_DC

R₁= V_DC/2I_DC

By finding the value of R₁ the stator losses at the stator can be measured and the P_rot losses can be calculated by eliminating stator losses from the input power (P_IN).

What is the Locked Rotor Test of Induction Motor

There is another test that performs on the motor to know different constraints of motors locked rotor test is also known as the block rotor test.
In this motor the rotor is static and input supply given to the motor and corresponding current, the voltage measured according to supply.
The lock rotor test also likes the short circuitry test of induction motor.
You can see the circuit diagram of the Locked Rotor test. During this test, we will apply the AC voltage at the stator of the motor and set current at full load value.
After setting current at the full load value the motor parameters like current-voltage and power are computed.

The corresponding circuit for this test is drawn in given a diagram.
You can see that as the rotor is not moving, hence the value of slip will be one.
So, the rotor resistance whose formula is R₂/s will become R₂(has very less value) as the slip value is one.
As the values of R₂ and X₂are very less so the total input current will pass by these two parameters, in its place to move by higher magnetize reactance (X_M).
So the circuitry of the motor in these situations behaves like a series circuit that has X1, R1, X2, and R2.

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.

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 (P_in), voltage (V), and current (I) are calculated before the rotor becomes hot (heat up).
The input power of the motor is given as.

P_in=√3V_TI_Lcosø

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

PF= cosø= Pin/√3V_TI_L

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

Z_LR=V_ø/I₁ =V_T/√3I_L

The angle of total impedance is given as.

Z_LR=R_LR+jX’_LR

= Z_LRcosø +j(Z_LR)sinø

Hence the resistance of the locked rotor is

R_LR =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’₂are the rotor and stator reactance at the testing frequency.
The resistance of the rotor can also write as.

R₂=R_LR-R₁

In this equation, R₁ is the value of stator resistance which we calculate during the DC test.
The entire rotor reactance (X_r) 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.

X_LR=f_rated/f_testX’_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.