Question about VFD (variable frequency drive)

[[email protected]]

Hi,

I read an article intruducing the principle of VFD at:
http://www05.abb.com/global/scot/sc...52573070053da9e/$file/lvd_eotn01u_en_reva.pdf

When I read it, I do not understand the line:

"As long as this ratio stays in proportion, the motor will develop rated torque."

It means that we only change the frequency to drive the motor to change thetorque?

It seems there is also a method to change the AC voltage to drive the AC motor (I am not sure that there is the precise statement, but I think theoretically it can change the on/off time to change RMS voltage at the motor). That VFD introduction does not include voltage variation?

Thanks,



...........
Fig. 5 shows
the torque-developing characteristic of every motor:
the Volts per Hertz ratio (V/Hz). We change this
ratio to change motor torque. An induction motor
connected to a 460V, 60 Hz source has a ratio of 7.67.
As long as this ratio stays in proportion, the motor will
develop rated torque. A drive provides many different
frequency outputs. At any given frequency output of
the drive, you get a new torque curve.

 

[Jamie]

Hi,

I read an article intruducing the principle of VFD at:
http://www05.abb.com/global/scot/sc...52573070053da9e/$file/lvd_eotn01u_en_reva.pdf

When I read it, I do not understand the line:

"As long as this ratio stays in proportion, the motor will develop rated torque."

It means that we only change the frequency to drive the motor to change the torque?

It seems there is also a method to change the AC voltage to drive the AC motor (I am not sure that there is the precise statement, but I think theoretically it can change the on/off time to change RMS voltage at the motor). That VFD introduction does not include voltage variation?

Thanks,

Normal mode for a basic VFD is volts to hz... Which means by the time
the drive reaches max output, it is generating max voltage and frequency
the motor is designated for.

At minimum output (zero speed), it will generate no voltage and no hz..

At 50% output, it is generating half the motor rated voltage and half
the motor base frequency..

It is a proportional operation and since this is true, the torque
capabilities for the motor equal.. In other words, we can generate max
current for that motor at any speed except 0 of course.

Lets say you have a motor that handles 10 amps max, if you were to
lower the frequency to 10 hz for example, you also need to lower the
voltage to the same proportion. Reason for this is induction of the
motor coils. Lower frequencies will generate higher currents and currents
are what drives the machine (magnetically). If you don't lower the
voltage to match the ratio of frequency, you'll exceed the motor's current
rating. Of course, higher Hz requires higher voltages to maintain that
max current due induction.

If you wanted to flux the motor with constant current then look at
operations like vector control. Vector systems attempt to maintain a
near constant current in the motor, even when it's not spinning (0 hz).

Basically, it would appear like having pure DC current being applied
to the coils, which basically it is. This will give you a stiff holding
drive shaft. THe control electronics will modulate this constant current
(Mag current) to form the AC frequency and voltage required to move the
shaft. It's almost like having a servo motor as a results in effective
output.

When things are operating smoothly in vector mode the motor current
should be reading the same through out the power band. If the current
starts to increase in vector mode it means that you don't have the
mag current up to the 50% mark or, you have exceeded the motor and
drive's ability and most likely will respond to things like speed and
over voltage errors.

Jamie.