In the commissioning of the frequency converters, it is recomendable to take a dv/dt measurement to determine that the voltage delivered at the motor terminals is accepted by the standards.
The voltage measurements to be taken between motor phases, and also between phase and ground as described below. It is important that the measurements are taken with high resolution because they are measurements with high frequency components. They are measurements that, in principle, will be for internal use.The standards that apply are the following:
- IEC TS 60034-25:2014(ed 3.0) (AC electrical machines used in power drive systems -Application guide) - Section 7 Electrical machine insulation electrical stresses.
- IEC 6000-34-18-41:2014, Annex C.
- Dv/Dt phase-phase
The dv/dt phase-phase measurement is taken with a voltage probe between two phases at the motor terminals while the inverter is tripping. To ensure sufficient pulse width, it is best that the motor is at high RPM (no load is required). As with the phase/ground voltages, two records must be taken:
- Peak voltage
Opening the oscilloscope window, the record to be taken is approximately as follows. With the oscilloscope trigger you have to take the maximum peak of the voltage:
- Peak-Peak voltage
To take the Peak-Peak value, open the time window and take at least one period (no more than 3 periods in order not to lose resolution). You have to see the maximum and minimum peaks in the same record.
An example of voltage between phases of the peak-peak motor:
This Peak-Peak value is the one that has to comply with the standard, as will be explained in the last section.
- Peak voltage
- Dv/Dt phase-phase
The dv/dt phase-to-ground measurement is taken with a voltage probe at the motor terminals, measuring the voltage between one of the motor phases (e.g. phase R) and the motor ground.
Important: in the case of phase-to-ground measurements, the motor must be running at rated speed. As in the previous measurements, two records will be taken:
-
- Peak voltage
Opening the oscilloscope window, the record to be taken is approximately as follows. With the oscilloscope trigger you have to take the maximum peak of the voltage.
- Peak-Peak voltage
To take the Peak-Peak value, open the time window and take at least one period (no more than 3 periods in order not to lose resolution). You have to see the maximum and minimum peaks in the same record.
A couple of examples of phase-to-ground voltage of the peak-peak motor:
Al igual que en las medidas fase/fase, la tensión pico-pico es la que tiene que ser comparada con la norma. Viene explicado en el siguiente punto.
- Peak voltage
Interpretation of measurements. IEC 60034-25:2014 Standard.
The IEC 60034-25:2014 standard analyzes both phase-to-phase and phase-to-ground peak-peak values. The current version (2014-10, edition 3.0) does not mention either 'rise time' or DvDt as the previous version did.
As per standard, the peak-to-peak values cannot exceed the values listed in the following table: -60034-25(ed3.0), 7.4 limits and Responsibility, table 4:
The value in the table has to be multiplied by the rated voltage of the motor. That is to say:
