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1. SQUIRREL CAGE AC MOTOR. NO LOAD TEST

1.1 INTRODUCTION. DESCRIPTION OF THE EXPERIMENT

The three-phase induction motor carries a three-phase winding on its stator. The rotor is either a wound type or consists of copper bars short-circuited at each end, in which case it is known as squirrel-cage rotor. The three-phase current drawn by the stator from a three-phase supply produces a magnetic field rotating at synchronous speed in the air-gap. The magnetic field cuts the rotor conductors inducing electromotive forces which circulate currents in them.

The no-load test on a squirrel cage AC motor shows the operating conditions in the magnetic circuit of the motor and gives information on the absorbed current and losses in no load con-dition. It is normally performed at rated frequency by applying balanced phase voltages to the stator terminals. The three-phase induction motor behaves as a transformer whose secondary winding can rotate. The basic difference is that the load is mechanical. Besides, the reluctance to the magnetic field is greater on account of the presence of the air-gap across which the sta-tor power is transferred to the rotor. The no-load current of the motor is sometimes as high as 30 % to 40 % of the full-load value.

ObjectivesBy performing this experiment, the students will learn how to determine the losses of a squir-rel cage AC motor in no load operation while reaching the following main objectives:

¾ To understand the schematic diagram corresponding to the no load test of the squirrel cage motor.

¾ To perform the squirrel cage motor wiring connections, in order to run the no load test. ¾ To obtain the characteristic curve related to the no load test (V0 - no load voltage I0 - no load current):

I0 = f(V0) and cosφ0 = f(V0)

1.2 COMPONENTS LIST

The modules required for this experiment are: ¾ DL 1021 Three-phase squirrel cage asynchronous motor ¾ DL 10065N Electrical Power Digital Measuring Unit ¾ DL 1013M2 Power Supply Module

1.3 PROCEDURE OUTLINE

Schematic diagramThe no-load test is useful not only to observe the working conditions of the motor magnetic circuit, but also to obtain data both for drawing the characteristic curves of the machine (I0 and cosϕ0) and for calculating the conventional efficiency (Pm and Piron). It consists in supplying the asynchronous motor with its rated voltage, leaving the rotor free to rotate without any braking torque.

Under these conditions, the absorbed current is given by the vectorial sum of the magnetizing current and the small active component produced by the losses in the iron (stator) and mecha-nical (friction and ventilation).

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In order to obtain the no load circuit characteristic curve, follow the figure 1. The no-load elec-trical parameters can be measured using the ammeter A and the voltmeter V. In this schematic diagram, the AC stator configuration is presented. The electrical power has been measured with a single wattmeter as the asynchronous motor is, due to its construction and operating conditions, a symmetrical machine under every load condition.

Figure 1. Circuit diagram for the squirrel cage AC motor no load test.

Characteristics curvesFrom the diagrams, in correspondence of the current rated value, we obtain respectively: I0, iron losses PFE, mechanical losses PM and power factor cosϕ.

Figure 2. Characteristic curves of the squirrel cage AC motor no load test

The no-load rotational losses (winding, friction, and core losses) will be seen in the no-load measurement. Given that the rotor current is negligible under no-load conditions, the rotor copper losses are also negligible. Thus, the input power measured in the no-load test is equal to the stator copper losses plus the rotational losses.

The Pm + Piron = f(V0) curve is practically a parabola, with an offset from the V axis equal to Pm (figure 3). In fact, when V0 varies, the mechanical losses do not change since they are related to the speed that remains fairly constant.

On the other hand, the iron losses do change (the voltage variations cause a proportional va-riation on the generated magnetic flux) and, since a quadratic proportion exists between the iron losses and the induction, their graph will have a parabolic shape.

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Figure 3. The characteristic curve for extrapolation of the Pm + Piron

The separation of Pm and Piron is possible through a graphical way, when the cross point betwe-en the curve and the Y-axis has been determined. That point cannot be experimentally mea-sured because when the supply voltage is too low, the asynchronous motor is inclined to stop. The cross-point, therefore, has to be determined by graphical extrapolation using the amount of the curve that has been measured: to reduce the difficulty of this operation it can be consi-dered the fact that, in the cross-point, the curve is tangent to the X-axis.

Setup and connection diagram

Figure 4 shows the schematic diagram of the no load test, where the squirrel cage AC motor is supplied with AC three-phase voltage from the variable power supply section (0÷240V/8A) of the DL 1013M2.

The schematic diagram from figure 4 is close to the student’s theore-tical knowledge (it contains the classical electrical symbols). We invi-te you to use this diagram while performing the experiment, having the wiring diagram from figure 5 as a reference.

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Figure 4. Schematic diagram for the squirrel cage AC motor no load test

The squirrel cage motor stator can be connected in star or delta, but for this experiment we have selected the delta configuration. The electrical parameters of the motor DL1021 will be measured using the DL10065N module.

Follow the diagram below to connect the power cables:

Figure 5. Wiring diagram for the squirrel cage AC motor no load test

Before starting any wiring activity, check all the power connections: all switches must be OFF.

Do not forget to connect the ground terminal! As shown in the dia-gram with specific symbols, all the equipment is connected to the protective network with a dedicated connector and cable.

Experimental procedure and learning plan

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Before starting the experiment, connect all the modules to the main power supply using the supply cables.

Perform the circuit configuration shown in the wiring diagram in figure 5. Power ON the DL 10065N measuring device.

Follow the next steps to enable and prepare the DL 1013M2 power supply for use:

¾ Raise up all the switches on the power supply.

¾ Turn the key clockwise from position 0 to 1. ¾ Switch the selector "a0b" to position "b". We will use the AC part (0÷240V/8A) of the power supply. This action is necessary in order to start the power supply.

¾ Press the green “start” button from the power supply module.

Before using the power supply, make sure that the safety connector (dongle) K1 is installed on the DL 1013M2 module (see figure 5).

Supply voltage to the squirrel cage AC motor (DL1021) using the DL 1013M2 power supply.

Make sure that the knob of the power supply is turned counterclockwise at position “0” and that the main selector is switched to “b”. Switch the selector of DL 1013M2, corre-sponding to the variable AC voltage "L1L2L3/ 0÷240V•8A", from off (O) to on (I) position.

Gradually increase the voltage and, while adjusting the knob, read the voltage on the 10065N module, until the motor is supplied with its rated voltage; let the motor in free rotation for some minutes, for the frictions in the supports to stabilize.

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Fill the following table with the input current I0, power P0 and Cosϕ0 data measured with the DL 10065N module (use the arrows to switch between voltage, current and power), correspon-ding to each voltage value.

Table 1. Measured values of the squirrel cage motor (delta)

Calculate the power factor using the following formula and compare it with the power factor measured using the DL 10065N module:

Calculate the losses using the formula below, using the value of the phase windings resistance (Rph) measured in the previous exercise, and the no load phase current (I0ph):

Pm + Piron = P0 - 3Rph ⋅ I20ph

Trace the Pm + Piron vs. V0 curve shown in Figure 3 and determine the value of Pm using the graphical extrapolation method.

When the experiment is completed, turn off the power supply and switch all the selectors to “off”, the “a0b” to position zero and turn the knobs fully-counterclockwise to the zero position.

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1.4 QUESTIONS

Answer the following questions related to the experiment.

1. How many types of induction motors rotors are there?2. The no load test of a 3-phase induction motor gives which of the following? (a) magnetic loss(b) variable loss(c) both magnetic and variable losses(d) none of the above3. During no load condition the induction motor will have a power factor?(a) Negative (b) High (c) Moderate(d) Low4. Which of the following supply is given to the rotor winding of a squirrel cage AC motor?(a) No Supply(b) DC Supply(c) AC Supply5. Is it true that the squirrel cage induction motor requires an external resistor circuit in the rotor during starting?

1.5 PROPOSED EXERCISE 1

Repeat the example using the same procedure, but this time follow the diagram from figure 6 and connect the power cables accordingly:

Figure 6. Wiring diagram for no load test of the squirrel cage AC motor (star connection)

Repeat the first 2 steps off the previous procedure.

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The third step from the procedure must be adapted for the corresponding level of supply volta-ge. Supply voltage to the squirrel cage AC motor (DL 1021) using the DL 1013M2 power supply.

Make sure that the knob of the power sup-ply is turned counterclockwise at “0” posi-tion and the main switch to “b”. Switch the selector of DL 1013M2, corresponding to the variable AC voltage "L1L2L3/ 0÷430V•5A", from off (O) to on (I) position.

Gradually increase the voltage and while adjusting the knob, read the voltage on the 10065N module, until the motor is supplied with its rated voltage; let the motor in free rotation for some minutes, for the frictions in the supports to stabilize.

Fill the following table with the input current I0, power P0 and Cosϕ0 data measured with the DL 10065N module (use the arrows to switch between voltage, current a power), corresponding to each voltage value.

Table 2. Measured values of the squirrel cage motor (star)

Calculate Pm + Piron, and trace the Pm + Piron vs. V0 curve in Figure 3 to graphically determine the value of Pm using the extrapolation method.

1.6 PROPOSED EXERCISE 2

Star/Delta starting of the squirrel cage motor

The star-delta starting method consists of applying nominal voltages to the three-phase sta-

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tor winding initially connected in star. When getting to a speed of 90-95% of the synchronism speed, the stator connection is switched to the delta connection. This switching can be done manual or automatically.

Figure 7. Star/Delta configurations of the squirrel cage AC motor

The value of line currents decreases three times in the star connection, thus reducing AC motor starting current.

Direct connection to the network involves increases in the startup current, like it is shown in figure 8a. Direct start is abrupt and fast with dynamic shocks in the kinematic elements of the AC motor and with important Joule effects in motor windings.

Figure 8. Characteristic of the star-delta starting: a) direct starting of AC motor; b) mechani-cal characteristic of star-delta starting

Using the star/delta starting method, the starting torque decreases three times (in star confi-guration) compared to the starting torque with the stator winding connected in delta.

Experimental procedure

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For this experiment we will use the DL 2035 Star/Delta starter. Follow the diagram from next figure and connect the power cables accordingly.

Figure 9. Wiring diagram for star/delta starting method of the squirrel cage AC motor

Repeat the first 2 steps off the previous procedure.

Supply voltage to the squirrel cage AC motor (DL1021) using the DL 1013M2 power supply. With the DL 2035 in the “0” position, set the voltage to the nominal value for Delta connection (230V). Switch the DL 2035 to the “Y” position and measure the current. When the motor will reach approximately the rated speed turn the switch to “Δ” position and measure the current.

Calculate the starting current in star (IsΥ) and delta (IsΔ) if, according to figure 8a, its value is 5*In.

IsΥ = .......... ..........

IsΔ = .......... ..........

1.7 CONCLUSIONS

Balanced voltages are applied to the stator terminals at the rated frequency with the rotor uncoupled from any mechanical load. Current, voltage and power are measured at the motor input. The losses in the no-load test are those due to core losses, winding losses and friction.