Most often the electrical problems affecting industrial plants including voltage sag and temporary rise, harmonic, transient, and the unbalanced voltage and current.
In a balanced three-phase system, each phase voltage should be the same or very close to the same.Phase voltage imbalance is not a measure equal degree.Voltage unbalanced three-phase system is the voltage difference between the phase in the index.Imbalance will affect the performance of the three-phase motor and shorten its life.
The transient effect on the motor can be very serious.Motor winding insulation may breakdown, leading to premature failure and expensive unscheduled shutdown.
The transient voltage test motor
Transient voltage, it is to point to the harmful short circuit voltage spike, there are many types of the root cause, including within or outside the factory.
Adjacent load cut or cut out, the power factor correction capacitor Banks and even the distant weather factors will create a transient voltage on the power distribution system.These transient voltage on the amplitude and frequency difference is very big, will cause the loss of motor winding insulation or breakdown.
However, due to the low frequency, and happens differently, so is very difficult to find these transient source.For example, the transient may occur on the control cable, it does not necessarily directly result in equipment failure, but may affect the normal work
A good way of recognition and measu雷竞技apprement of the transient is the use of which has the function of the transient three-phase power quality analyzer, such as the Fluke 438 - II and motor power quality analyzer.The transient feature can be set on the instrument is higher than normal voltage 50 v.Then the instrument can be displayed on the screen above 50 v voltage, transient potential problem.
If found that the transient when the initial measurement雷竞技apps, a good approach is to measure and record the power quality changes over time, you can use the advanced industrial power quality recorder, such as the Fluke 1750 three-phase power quality recorder.
What are the cause of the unbalanced voltage?
Unbalanced three-phase system will cause the three-phase motor, and other three phase load performance degradation or premature failure occurs, causes include:
- Due to the torque output below normal, produce mechanical stress on the motor.
- The current in the motor and three-phase rectifier is higher than normal.
- Unbalance current through the neutral line of Y type three-phase system.
On the motor terminal voltage imbalance caused by high current imbalance, the latter is probably six to ten times of the former.Unbalance current will cause the torque pulsation and mechanical stress, increase loss, as well as the motor overheating.Voltage and current unbalance also heralds the maintenance problems, such as loose connection and contact wear.
Can occur at any point on the power distribution system of the imbalance.Should be to average distribution box each phase load on the distribution.If a particular phase of the load on the, far more than other phase voltage will be lower than other phase voltage.Distribution box connected to the transformer and the three-phase motor work may be at higher temperature, increased noise, serious vibration, even cause permanent fault.
How to calculate voltage imbalance
The calculation method of voltage unbalance is simple.As a result, the percentage is not balanced, can be used to determine the next step in the diagnosis of measures.Calculation is divided into three steps:
- Determine the average voltage and current
- Calculate the maximum voltage or current deviation
- Maximum deviation divided by the average voltage or current, and then multiplied by 100% : unbalanced = (according to the average voltage or current to calculate the maximum deviation/average voltage or current) x 100
Manual calculation imbalance can only determine unbalanced voltage or current of a point in time.雷竞技appFluke 438 - II motor driven analyzer displays real-time unbalanced voltage or current, including any kind of imbalance.
