Toshiba VF-A7 Influences of leakage currents and measures against it, Measures to be taken

1.4.3 Influences of leakage currents and measures against it

Warning

An electric current can leak through an input or output cable of the inverter because of its insufficient capacitance and, sometimes, affects the peripheral systems. The amount of a leakage current depends on the carrier frequency, the length of the input/output cable, etc. It is advisable to take the following measures to prevent leakage currents.

(1) Influences of a current leaking into other systems via the ground

An electric current can leak not only into other circuits of an inverter but also into other inverters through grounding wires. Such a leakage current can exerts influences on various electronic devices, for example, malfunction of ground leakage breakers or relays, ground relays, fire alarms, sensors, etc., noise on CRTs and display of incorrect current values on a

CRT screen.

Power supply

Inverter

IM

Inverter

IM

Measures to be taken:

1.Lower the PWM carrier frequency.

Use parameter  to lower the PWM carrier frequency.

2.Use high frequency-ready ground leakage breakers (e.g., Esper Mighty series(manufactured by Toshiba Schneider Electric Ltd.)). When these ground leakage breakers are installed, there is no need to lower the PWM carrier frequency.

3.If sensors and CRTs are affected, they can be restored by lowering the PWM carrier frequency as described in 1 above. However, if lowering the PWM carrier frequency results in an increase in magnetic noise, contact your Toshiba dealer.

※Cautions for applying models with a built-in noise filter.

For the models with a built-in noise filer, the leakage current value at power supply of Δ(delta) connecting wire (1 phase earth) can be larger than normal inverter, so be careful.

<Standard leakage current value> VFA7-2004PL～2037PL : about 4mA VFA7-2055PL, 2075PL : about 13mA

(2)Influences of a current leaking from a cable into other cables

①Thermal relay

If a current leaks from an output cable of an inverter to other cables because of its insufficient capacitance, the high-frequency elements of the leakage current sometimes increase the effective current value, and thus cause external relays to malfunction. For a model with relatively long cables (longer than 50 m) or a model designed for motors with small current ratings (several amperes), especially 400V model with a small capacity (3.7kW or less), the external thermal relays can malfunction more easily because a leakage current can be too large as compared with the current rating of the motor.

A-12