Constant torque. Is it worth the cost?

[MrWild]

A Hytachi 2hp VFD can be as cheap as $210, or it can be as high as $290+ for a conkstant torque model. Is the constant torque worth the added cost? I'm leaning toward the SJ200 model, but if the cheaper variable torque model isn't that much different, does it pay to opt for the better one?

[Al_The_Man]

I haven't used Hitachi, but I use the equivalent in Mitsubishi, including the encoder feedback type, which are even better when you want accurate rpm control.
A VFD can be just variable speed or torque/vector control where the torque is attempted to be maintained across the speed range. You get what you pay for.
Al.

[in2steam]

i suppose that depends upon the application...
chris

[Mike Nash]

I just took a quick look at the L200 vs the SJ200. The L200 is actually a V/Hz control type as opposed to the SJ200's open loop vector (sensorless vector.) Both can be run in constant or variable torque modes. With some mfgs a variable torque setting may allow a higher horsepower output because it reduces the overload current allowed. Centrifugal fans and pumps are ideal examples of variable torque loads, requiring very little if any overcurrent at low frequencies. www.driveswarehouse.com is mislabeling the drives with the constant and variable torque labels, although it is true that many (most) variable torque loads work fine (if not better sometimes) with V/Hz control.

On the flip side of all this, the SJ200 will have much better torque than the L200, especially at very low speeds. It will also have tighter speed control. I didn't look for it, but the L200 can possibly have slip comp added so you can manually add in compensation to correct for speed droop when the load increases. You can also add torque boost. The reality for most machine tools is that this is probably not a big issue. Situations requiring vector drives and greater than 100% torque capability tend not to be machine tools. You don't usually start a lathe or mill spindle with the tool already buried in the work.

So depending on your application, it's up to you to decide if the extra $$$ are worth it.

[perrybd]

Originally Posted by Mike Nash I just took a quick look at the L200 vs the SJ200. The L200 is actually a V/Hz control type as opposed to the SJ200's open loop vector (sensorless vector.) Both can be run in constant or variable torque modes. With some mfgs a variable torque setting may allow a higher horsepower output because it reduces the overload current allowed. Centrifugal fans and pumps are ideal examples of variable torque loads, requiring very little if any overcurrent at low frequencies. www.driveswarehouse.com is mislabeling the drives with the constant and variable torque labels, although it is true that many (most) variable torque loads work fine (if not better sometimes) with V/Hz control. On the flip side of all this, the SJ200 will have much better torque than the L200, especially at very low speeds. It will also have tighter speed control. I didn't look for it, but the L200 can possibly have slip comp added so you can manually add in compensation to correct for speed droop when the load increases. You can also add torque boost. The reality for most machine tools is that this is probably not a big issue. Situations requiring vector drives and greater than 100% torque capability tend not to be machine tools. You don't usually start a lathe or mill spindle with the tool already buried in the work. So depending on your application, it's up to you to decide if the extra $$$ are worth it.

Interesting--Variable torque and constant torque is itself a misnomer. I'd appreciate it if they'd just call the drives a heavy duty and a standard because that's essentially what the difference is; not that one only can have a set torque.

Anyway, I noticed you mentioned driveswarehouse. I'd try Variable Frequency Drives | VFD's.com. I work for its parent company Energy Management Corporation - Motors, Drives and Generators, and I'd be willing to bet that they have the drives listed right on the site, considering you can download the manuals straight from it.

[perrybd]

Originally Posted by MrWild A Hytachi 2hp VFD can be as cheap as $210, or it can be as high as $290+ for a conkstant torque model. Is the constant torque worth the added cost? I'm leaning toward the SJ200 model, but if the cheaper variable torque model isn't that much different, does it pay to opt for the better one?

If your looking for some info on the SJ200-015HFU, Variable Frequency Drives | VFD's.com has some in stock. You can also download the manual there.