How to Determine Phase Rotation on a De-energized Motor

megger 560060 motor and phase rotation testerMotor and Phase Rotation Tester – Megger

  • Complete phase-sequence and motor-rotation testing in one instrument
  • Ensures correct phase hookup in one easy test
  • Rugged and portable tester
  • Performs additional polarity and continuity checks

DESCRIPTION

The Megger 560060 Motor and Phase Rotation Tester permits the electrical contractor or industrial maintenance electrician to permanently connect and tape the terminals of the motor being installed, without having to first energize the motor by a temporary hookup from a power source, if available, to determine motor rotation. Therefore, the test set eliminates the need for temporary connections that can be time consuming, costly and quite hazardous, particularly where many large, high-voltage motors are involved.

Also, certain types of drives should never be rotated in the wrong direction. In such cases, the temporary hookup or trial method, having a fifty-fifty chance of being wrong, can do serious harm.  The three motor leads on the left side of the test set are for attachment to the terminals of the motor being tested for rotation determination.

Fuses are inserted in the motor A and C test leads as protection in the event the user accidentally touches these leads to an energized circuit. These standard fuses are easily removed and replaced from their panel-mounted holders.  The three lines leading to the right of the test set are for direct attachment to energized ac power systems up to 600 volts to determine the system phase sequence.

A four-position switch selects the test to be made — system phase sequence, motor rotation and transformer polarity. The selector switch connects a D-size dry cell into the circuit when the rotation of a motor or the polarity of a transformer is being checked. In the OFF position, both the meter and the battery are disconnected from all circuits.

A push switch is connected in series with the battery and opens the circuit during transformer polarity testing.  The dry cell is easily removed and replaced from its panel mounted holder by a coin-slot access cap.  The dc zero-center ammeter indicates correct or incorrect rotation or polarity by deflecting its pointer to the right or left. A zero or null adjuster is provided for the ammeter.

APPLICATIONS

The Motor and Phase Rotation Tester provides a positive way to identify the leads of a disconnected polyphase motor; it also identifies true phase sequence of energized 60-Hertz AC power lines up to 600 volts. Both are necessary to ensure that a motor will rotate in a prescribed direction when energized.

There are three other important uses for this unique testing device:

  1. It can determine the polarity of power and instrument transformers
  2. It can identify phase and polarity of winding sections of multiple-winding (delta- and star- connected) motors
  3. And it can be used as a continuity tester in checking electrical circuits.

FEATURES AND BENEFITS

  • Determines rotation direction of one-, two- or three- phase motors before connection to line
  • Determines phase rotation or sequence of energized power circuits
  • Determines polarity of instrument and power transformers
  • Determines phase/polarity of unmarked motor windings
  • Identifies true phase sequence of energized AC power lines up to 600 volts (Higher voltages can be tested by interposing a step-down transformer.)

This Tester is used to identify the leads of a disconnected polyphase motor so that when connected in phase rotation ABC (or with procedure modification CBA) it will run in the desired direction. The Tester is also used to identify phase rotation ABC (or with procedure modification CBA) of energized AC power lines up to and including 600 volts. Other uses include the determination of transformer polarity and testing of circuit continuity.

The above features also provide in a single instrument, facilities for identifying phase and polarity of winding sections of a multiple winding motor.  Where connection diagrams are lost or terminal markings are obliterated, this identification process is necessary before a motor can be reconnected.

Theory of Operation

When direct current is applied to the windings of a polyphase induction motor, a field is set up and the rotor iron becomes magnetized. If the magnetized rotor is turned, the field will rotate with it for a short time because of hysteresis in the iron. The motion of this field induces voltage in the windings. The direction of the induced voltage depends on the direction of rotation. The same factors that determine the direction of a rotating field in a connected motor determine the direction of voltage induced when the motor is manually turned while connected to the motor rotation circuit. The motor rotation circuit makes use of the above principles in order to determine motor rotation.

megger 560060 motor and phase rotation testerThe circuit is a bridge in which two adjacent phase sections of the motor winding are balanced against a potentiometer. The simplest case where each phase section is a single coil side is shown in figure 13a. With the rotor at rest, the ZERO ADJ. potentiometer, R1, is adjusted to give zero current in the meter M1. At. this point there is equal voltage across each of the two phase sections.

When direct current enters one phase (at terminal C) and leaves the adjacent phase (at terminal A) a field will be set up as shown by the air gap arrows in figure 13a. Now when the rotor is turned so that it moves from the one phase toward the adjacent phase, a voltage will be induced in the one phase that is opposite in direction to the direct current. A voltage will also be induced in the adjacent phase but this will be in the same direction as the direct current. Where the induced voltage is opposite to the direct current, it reduces the total voltage across the phase. Where induced voltage is in the same direction as the direct current, it adds to the phase voltage. Since the phase voltages were balanced before rotation, the induced voltages adding to one phase and subtracting from the other, cause an unbalance of the circuit. The unbalance voltage drives current through the meter in the positive direction and therefore causes a CORRECT reading.

If the motor were connected to a polyphase power system so that A phase follows C phase (sequence A, B, C) the rotor would also move in the same direction as just described. Thus the motor tagging, when CORRECT deflection is obtained, indicates the proper phase connections. To show how this simple theory is applied to more complex windings, consider a two pole, three phase, star connected motor reduced to its simplest form in which all the coils of one phase group are represented by a single coil located at the center of the phase group which it represents.

A developed view of the winding is shown in figure 13b. Also shown is the schematic coil arrangement. In all diagrams in figure 13, the direction of applied d-c is indicated by arrow heads on the wiring. Direction of induced voltages is indicated by arrows parallel to the wiring. In figure 13b the rotor surface is represented by the rectangle. Flux is shown as being distributed over the entire rotor surface in order to show the effect of a distributed winding. The shaded portion indicates flux entering the rotor. Unshaded area shows flux leaving.

No indication of flux magnitude is necessary, but it may be noted that the magnitude is zero at the point where reversal takes place. This null point in the field is found at the mid-point of any group of conductors carrying current in the same direction. The arrow at the side of the rectangle indicates the direction of motion of rotor and flux.  Figure 13c shows an open delta connection of coils on a three-phase motor. Figure 13d shows a two-phase motor. Figure 13e shows a three-phase delta connected motor.  Download the continuation of this discussion on Theory of Operation

Download the Megger 560060 Data Sheet

JM Test Systems is a Stocking Distributor of Megger products

JM Test Systems has the Megger 560060 Motor and Phase Rotation Tester for purchase.
The Megger 560060 Motor and Phase Rotation Tester available to rent  from JM Test Systems.

Call us today for a quote at 800-353-3411 or check us out on the web at www.jmtestsystems.com

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