# VFD's, Please explain what is ment by constant torque vs variable

I was looking at:
https://www.driveswarehouse.com/Drives/AC+Drives/Sensorless+Vector+VFD/SJ200-007NFU.html
And now I wonder what the difference and ramifications are of constant
torque and variable.
Thanks,
Wes
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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.
Karl
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Wes wrote:

https://www.driveswarehouse.com/Drives/AC+Drives/Sensorless+Vector+VFD/SJ200-007NFU.html
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.
Jon
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Jon Elson wrote:

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
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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.
Karl
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Karl Townsend wrote:

I suspect the primary advantage would be the ability to soft start hard to start loads without having to oversize the motor and drive.
Pete C.
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