I was looking at:
And now I wonder what the difference and ramifications are of constant
torque and variable.
I'm assuming you're comparing the Hitachi SJ200 and SJ300. Both good drives
and this is a good vendor. The SJ300 use only three phase input - it has a
phase loss sensor that will trip out the drive if you try 1 phase input.
DAMHIKT. I've heard you can jumper one of the hot input wires to two of the
input leads but I'm not sure it will work.
The SJ200 will have less torque at lower RPM. I don't know if the amount is
all that significant. I'm using this drive on my Monarch 10EE, my CNC knee
mill, and my Hardinge CHNC lathe.
Torque is proportional to amperage. Since the motor has a
current rating, that limits the available torque. So, from zero
to the rated speed, the torque rating is constant. If you slow
the motor to 50% of rated speed, you only get rated torque. So,
a 1 HP motor at 50% speed only puts out 1/2 Hp. That's why
belt-type varispeed drives are sometimes better than a VFD, as
they multiply torque at lower speed.
At higher than rated speed, the drive generally cannot supply
more voltage to the motor, so the voltage becomes constant.
This under-excites the rotor magnetism, and the motor gets
weaker (less torque). So, you feed rated current at higher
frequency, but steady voltage, to the motor, and it runs faster,
but you get less torque. This works much the same way as a
belt-type varispeed drive would work. This is variable-torque
mode, above the nominal 60 Hz.
The constant torque mode is below 60 Hz.
In other words, below the motor's base frequency you get constant torque
and HP is proportional to speed (1/2 speed = 1/2 HP) , above base
frequency you get constant HP and torque is inversely proportional to
speed (2x speed = 1/2 torque).
Regards, Gary Wooding
(To reply by email, change feet to foot in my address)
Good explanations above on how VFD's work in general.
If you cruise this web site, you'll see that the two more expensive VFD
lines add the claim "Constant Torque" over the less expensive (SJ200) lines.
These are the flux vector(SJ300) and sensorless vector(SJ100) drives. I'm
not sure what you get for the higher dollar units. I do know the less
expensive line works fine for RCM machinists.
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