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Troubleshooting Three-Phase Motors With A Clamp-On Meter

Polypase motors are easy to troubleshoot with a clamp-on meter. whether the problem lies with the motor or with the power distribution system can be determines in a matter of minutes.

Three-Phase power is commonly found in commercial and industrial properties. Some small businesses also have a three-phase service because it is a much more efficient power distribution system then single-phase distribution. The three-phase motors are less complicated than single-phase motors. There are no centrifugal switches, separate start windings, start capacitors, or run capacitors. Basically, a three-phase motor consists of a rotor, two bearings, and a stator. The stator contains three sets of run windings 120 degrees out of phase with one another. The three sets of windings displaced by 120 degrees set up an apparent rotating magnetic field which is why a three-phase motor does not need a separate start winding to start the rotor rotating at start-up.

Three-Phase motor lead connections.

Most three-phase motors are either 3, 6, 9, or 12-lead motors. Three lead, three-phase motors are single-voltage motors, while the 6, 9, and 12-lead three-phase motors are dual voltage motors. The motor leads are labeled T-1 through T-12. The three phase lines coming into the motor's connection box are labeled L1, L2, and L3. All three-phase motors are reversible and are reversed by switching any two phase lines. The connections patterns for various voltages can be complicated for those who do not work with three-phase motors everyday and for those just starting out so I have included the most common connections here.

Three-Phase motor failure

A recent study conducted by leading manufacturers of polyphase motors showed that forty-four percent of motor failures were caused by the heat resulting from overloads or single-phasing. Besides single-phasing an unbalance in voltage between phases will lead to overheating. Overheating reduces the useful life of the motor winding's insulation by fifty percent for every 10 degrees Centigrade (18 degrees Fahrenheit) about the windings rated operating temperature.

What is single-phasing and how does it happen.

In its simplest terms, single-phasing occurs when one of the phase legs loses voltage. This can happen under a number of different circumstances. An open phase can occur on the primary side of the three-phase transformer bank. A distribution system fuse may have blown. The three-phase transformer bank may be running open delta. One of the motors' safety disconnect fuses may have blown, or one of the across the line motor starter’s over-loads (O.L.s) may have opened, to name the most common causes.

Some common symptoms of phase voltage unbalance.

  • Unexplained (Nuisance) tripping of the motor's protective devices.
  • An unusually high number of motor failures.
  • A motor's failure to start dependably or a motor starts with considerable difficulty.

Using a clamp-on meter to detect an unbalance voltage condition.

  1.  With the motor running, measure and record the current flowing in each of the phase lines-L1, L2, and L3, with L1 connected to T1, L2 connected to T2, and L3 connected to T3. Shut the motor down and pull the safety disconnect.
  2. Reconnect L1 to T3, L3 to T1, and L2 to T2. Close the safety disconnect switch, restart the motor and record the current readings. Shut the motor down and pull the safety disconnect switch.
  3. Reconnect L2 to T1, L3 to T2, and L1 to T3. Close the safety disconnect switch and restart the motor. Record the current flowing in the three phase wires.
  4. Calculate the average current and maximum deviation for the currents recorded in steps 1, 2, and 3. If the currents recorded for L1, L2, and L3 in steps 1 was 34A, 32A, and 24A respectively, the average current is 30A. The maximum deviation = the average current – the lowest phase line current = I average – IL3 = 30 – 24 = 6 Maximum Deviation = 6A.
  5. Compare the maximum current deviations computed for steps 1, 2, and 3. If the maximum deviation current always involves the same motor T lead, the problem probably lies with the motor. If the maximum deviation current always involves the same phase lead, L-Lead, the problem probably lies with the power supply.

Using a clamp-on meter to detect single phasing.

  1. Measure the current flowing in each of the phase lines-L1, L2, and L3.
  2. In a properly operating system, all three phase currents will be equal.
  3. If the motor is single-phasing, one of the phase-lines will be drawing 0 Amperes.

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Comments (2)

Interesting sir. Can be very useful in the future, thanks.

Tony Vowell

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