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What are the causes of premature motor failure in single-phase motors?

Published date: 18 July 2020

What are the causes of premature motor failure in single-phase motors?

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All Product models, all serial numbers.

Install, Repair, Cause.

One of the most common causes of premature motor failure in single-phase motors is through incorrect wiring. The root cause of this has been identified as DIY installations and confusion between the old American colour coding (red; yellow & black) and the new European colour coding (black; brown & blue).

In 2003 Franklin Electric SA lead four training courses throughout South Africa in an effort to increase the level of expertise in the borehole industry. These were received with great enthusiasm and all in all 2003 was a huge success for everyone concerned.

As money becomes tighter there is a growing concern for the DIY market where the end user thinks he can save a little money by "Doing It Yourself", experience tells us this is not the case.
Franklin Electric's single-phase motor is essentially a "two phase motor" with a Start and a Run winding. The different sizes of the copper wire can be seen.The thinner wire is the start winding, and the thicker wire is the run winding.
NB! This start winding is not designed for continual use and must be removed from the circuit via the relay supplied in the Franklin Electric control box
Should there be any uncertainty of the colour coding from the Lead out cable to the drop cable then there is a simple procedure to follow in order to get the wires in the correct place.
Failure to correctly connect the motor to the control box could result in premature failure of the control box and / or motor.
In some cases motor protection will not detect the fault fast enough or even at all. The most common symptoms of incorrect wiring are blown start capacitors, and in a lot of cases burnt motors where only the start winding is damaged.
In all cases the failure is relatively fast; therefore care must be taken to ensure that the wires are correctly connected before starting the motor.
In order to determine the correct placing of the wires an ohm (W) meter is required.
There are three possible sequences to measure the three wires.
Step 1:    Number the wires 1; 2 & 3

Step 2:    Measure & record the winding resistance (W) between the wires (1 & 2) (1 & 3) (2 & 3) Note: You should get a low (1 & 2) reading a middle (2 & 3) reading and a high (1 & 3) reading.

Step 3:    The wire NOT used to get the highest reading would be the common. Y (UW) The connection in the motor is as per (Fig 1)Understanding this it is easy to determine the wires.
    The highest reading is obtained through measuring through the run wire and start wire (1 & 3)(both windings are measured)
    The middle reading is the thinner wire (start) and the common wire (2 & 3)
    The low reading is the thicker wire (run) and the common wire (1 & 2)
NOTE 1: A thinner wires resistance would be higher than a thicker wire.
NOTE 2: Since the common wire is not a winding, the two smaller readings should add up to the highest reading (as long as the measurement is taken on the motor, as the drop cable would create a slight discrepancy in this addition.)

Step 4:    The wires used in the middle reading are the Start and Common, (common being known) the remaining wire is the Start. R (Z)

Step 5:    The wires used in the lowest reading are the Run and Common (common being known) the remaining wire is the Run. B (V)

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