1.4.2 Notes on inverters

Over-current protective function

The inverter has an over-current protective function. The current for this protection is adjusted to the maximum current rating of the applicable motors by default. Therefore, when the inverter is used to control a motor with a relatively small capacity, it is necessary to readjust the over-current protection level and the electronic thermal protective function. In such a case, follow the procedure specified in 5.13 to readjust them.

Inverter capacity

An inverter with a small capacity (kVA) must not be used for a motor with a relatively large capacity even if the motor is operated under a small load. If an inverter is used this way, the output peak current rises high because of a current ripple, thus causing the motor to trip easily.

Power factor improving capacitor

No power factor improving capacitor should be connected on the output side of the inverter. When the inverter is used for a motor equipped with a capacitor for power factor improvement, remove the capacitor from the motor. Connecting such a capacitor causes the inverter to break down and the motor to trip, or breaks the capacitor itself.

Inverter

U

V

IM

W

Remove the power factor improving capacitor and the surge absorber, if any.

Power factor improving capacitor

Use of an inverter at a voltage other than the rated one

There is a need to connect it to a power unit supplying a voltage different from the rated voltage, increase or reduce the supply voltage to the inverter's rated voltage, using a transformer, etc.

Use of a set of inverters, which requires circuit-breaking devices

MCCB1

MCCB2 Breaking fuse)

 

MCCB3

 

MCCBn

INV1

INV2

INVn

Circuit-breaking of defective inverter

When two or more inverters are connected to the same power line as shown above, it is necessary to select a circuit-breaking characteristic ensuring that, for example, if a short circuit occurs in INV 1, only MCCB2 trips but not MCCB1. If it is difficult to select a proper characteristic, then insert a breaking fuse between MCCB2 and INV 1 in this case.

■Note on the disposal of inverters

Be sure to dispose of inverters as industrial wastes, when they become unnecessary.

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Toshiba VF-A7 instruction manual Over-current protective function, Inverter capacity, Power factor improving capacitor