Proa

multimeter

 One of the most useful equipment for an electrician is the multimeter.
The name "Multimeter" shows that are many instruments together in the same case.

In this photo we have:

1. Voltmeter to measure voltage in volts ac and dc to dial numbers in black-choice and switch ac - dc in proper position. Measured by the voltage present in the test circuit in parallel connection.
2. Ohmmeter and diode controller to measure resistance in ohms and voltage threshold in volts for the diode in green numbers. Measuring without voltage in the concerned circuit.
3. Frequency meter to measure frequency in Hz in white numbers. In a parallel connection.
4. Ammeter to measure current in amperes in orange numbers. Measuring under voltage connected in series in the cable that we want to measure the amps.
5. Capacitance meter to measure capacity in farad at the blue numbers. No voltage. The ends of the capacitor goes into the slots with the symbol of the capacitor. If the capacitor is electrolytic correct in the write polarity.

The black connector is permanently attached to the connector COM (From the word common). The red probe goes into the slot with the same color you choose on the rotary knob.

In position control knob channels there is also the function of "pager", in which between terminals if the resistance is less than 20 ohms the buzzer sounds

In all options the rotary switch knob shows the number of the maximum value that can measure the multimeter. E.g. position 2 volt for up to two volt, if the voltage is greater displayed is displaying 1 with a dot (1.) to notify us that the voltage is greater than 2 volts and choose the next larger scale. It is best to start for an unknown expected value measure from the widest possible choice (p, e.g. for voltage 600 volts) and descending scale if needed, so we achieve the greatest possible accuracy.

The three-phase motors rotate clockwise or counterclockwise depending on the sequence of the three phases are provided.

The end of the three phases, which typically are named with English letters R, S, T, is clockwise and the motor to be connected in series with the U, V, W, will turn clockwise. But this is not always correct, because it depends on the manufacturer of the motor,  how has wrapped its coils and the "visual" of each. Besides, there is no right rotation for the motor; depends on the work that we want to achieve.

The change of direction is made in mutual permutation of two of the three phases. The plan change effect is accomplished by swapping the first to the third phase. The same effect can be rotating the second to the third or the first to the second.





A simple plan to control the rotation shown in the second drawing.

Design automation control direction three phase electric motor rotation

To avoid case to arm simultaneously two relays we put in a series on automation so-called latching contacts. Thus if one is armed relay can arm the other, even if pressing the START button, must necessarily be deactivated first reinforced by pressing the stop and then to activate the other. One relay that "latches" the other with a closed contact (normal close NC) to prevent the arm, hence the name of the contact.

This pattern of changing direction is not ideal because it requires the operator's attention to the actual stopping of the motor, wait some time before starting to reverse. Could someone, while the motor turns at once, press the stop and immediately after, without waiting at all, press the START of the other direction, bringing the motor and load abruptly and perhaps dangerous. This can be prevented by addition of another insurance provision.