Datasheet
K6CM
17
Effect of voltage imbalance
The ZCT (IRT) calculates the Ior from the phase difference between the measured Io and the voltage. If there is a voltage imbalance, the phase
of each phase voltage will change, and an error will occur in the calculation of Ior. Use the measurement results of the case when the voltage
imbalance ratio is within 3%.
Range indicated by the measured value during insulation deterioration
The accuracy of the insulation resistance value is decided under the influence of several parameters. (The parameters are as described on
page 16)
An example of the range shown by the measured value as a result of deterioration of the insulation resistance is shown in the figure below.
Ioc may further increase depending on the manufacturer and structure of the motor, which may result in a decline in accuracy.
Inverter special measurement
A case is known where a motor is driven by setting the system voltage and inverter to the same frequency.
(Ex. System frequency: 60 Hz, inverter frequency: 60 Hz)
In the past, since it was not possible to remove the frequency component of the inverter in such a configuration, it was difficult to measure the
insulation resistance.
In the inverter special measurement, the measurement of the insulation resistance is made possible by detecting the minute deviation in frequency.
The 60 seconds required as the measurement time is the time for accumulation of the data necessary for this measurement.
Comprehensive Current Diagnosis Type Technical Data (Reference)
Use the following expression when calculating the current value from the motor capacitance.
(Example) When a 5.5-kW motor is used at 200 V
Therefore, the CT K6CM-CICB025 in which 22 A is within the measurement range is selected.
The measurement range is within 20 and 100 A even in the case of K6CM-CICB100, which means that 22 A is within the range and this model
can also be used. However, during selection, priority must be given to a CT having a small rated current value in order to realize more accurate
measurement.
Note:
In the expression shown above, general values must be used for the power factor and efficiency, and the load factor must be 100%. However,
depending on the actual operating environment, the actual current value and measured value may be different. If the CT is used at a current value
that is below the lower-limit value of the measurement range of the CT, the measurement error of the degradation level will increase. Therefore, if
possible, measure the current during a steady-state operation with a clamp meter, etc., and select a CT corresponding to the current value.
Range shown by the measured value
0.2 MΩ ±35%
Insulation resistance of motor
Measurement value when Ir has
the opposite phase of Ior
Measurement value when Ir has
the same phase as Ior
0.2 MΩ
1 MΩ
0.1 MΩ
Elapsed time
Conditions
• Top runner of Company A
(200-V type, 7.5 kW, 4 poles)
• Wiring length of ZCT and motor: 40 m
• The through-wire is fixed in the center of ZCT
• Voltage imbalance ratio: 3%
Current value of motor (A) =
Motor capacitance (kW) × 1000
Motor voltage (V) × √3 × Power factor (0.9) × Efficiency (0.8)
Current value of motor =
5.5 × 1000
200 × √3 × 0.9 × 0.8
= 22 A