What Frequency VFD increases , Voltage frequency or Current frequency ?

[munzir]

What Frequency VFD increases , Voltage frequency or Current frequency ?

By increasing the frequency does the current or voltage increased ?

 

[hareeshravi]

Hi,
VFD's are used to change the speed of Induction motors, which can be modelled as an inductor.
Depending on the inverter topology (Voltage Source / Current Source) corresponding variable is changed.
However, both Voltage and current frequencies will be same.

The current/voltage/Torque profile does change for a simple VFD, which is why V/F control is used.

 

[chuckey]

VFDs usually produce a PWM wave form, so the carrier frequency can be altered which is normally just done to stop noise or mechanical resonances, or the actual modulation frequency, which actually drives the motor. In all cases the voltage frequency changes and the current HAS to follow (I = V/Z).
Frank

 

[munzir]

What is a carrier frequency ?

 

[chuckey]

With Pulse Width Modulation, pulses are sent out at a defined rate, this is the carrier frequency and is very much greater then the frequency of the waveform that is to be recreated, i.e. for a 50 HZ sinewave the carrier would be 8- 40 KHZ. If the pulse that is sent is very narrow, the 50 HZ component would be very low, so the output voltage (at 50 HZ) would be low. If the pulse is wide then the 50 HZ component will be greater, so the re-constituted 50 HZ will be large.
Frank

 

[munzir]

means IGBT change the normal frequency to carrier frequency by switching on and off ?

 

[chuckey]

No the IGBT just amplifies the pulse waveform.
Frank

 

[munzir]

Does a VFD increase current and voltage also if its changing the frequency from 0 to 400 Hz ?

 

[chuckey]

A VFD keeps the current constant which means that its voltage is proportional to frequency. i.e. motor .1A @230V @ 50HZ. at 100 HZ, V = 460V, at 10 HZ V = 46V. This is for a constant torque, but wildly differing HPs.
Frank