What VFD and motor to get for mill

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Jan 10, 2016
Hey all, I got a new to me knee mill and am wanting to put a VFD on it. How ever I have a few questions first. The slowest speed on the mill now is 270 rpm, and I would like to get it down to around 100 rpm or so for using it to drill large holes. I have done a little bit of research and have come to find that the slower the speed of the motor the less torque it has. The mill has a 1.5hp motor on it now, would it be a good idea to install a larger motor to not have to worry about stalling it out? What kind of motor and VFD should I get for it?


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Jun 12, 2014
I would say the most commonly used and simplest to install VFD is the Teco L510 which is reasonably priced (arouns $170 US). I would go with a 2 Hp VFD. The major limitation is it has no provision for an external braking resistor, so fast braking is not possible with the L510, figure 3-5 seconds stopping times. Hitachi WJ200-015S is more versatile with provisions for an external braking resistor, a better VFD, but a little more complicated. I have posted all the parameters and wiring diagrams for the mill in previous threads (otherwise PM me and I will send them to you). Automation Direct VFDs are favored by many, but they are older designs and the GS2 series does not have sensorless vector control, which greatly enhances the low speed power (torque). The GS3 is more suitable, but similarly priced to the WJ200. Yaskawa, ABB, and a number of other higher end VFDs are better, but much more pricey and you will not see any performance improvement over the WJ200. I would avoid the generic offshore eBay VFDs.

VFDs can provide flat torque below their base speed (60Hz) to 0 RPM, but standard motors usually have their torque and performance fall off below 15-30 Hz and the Hp declines in a linear fashion below 60Hz. So at 30 Hz you only have 3/4 Hp. You also do not have the mechanical advantage of the pulley/gear ratio, so although you can go much slower you can not cut heavier loads. In your case going from 270 to 100 RPM, with a VFD you will have about a 1/2 Hp motor. There are a number of other alternatives, typically a larger motor is used if you want a wider motor speed range, you can also change the motor pulley ratio so the you keep the motor spinning faster relative to the spindle to maintain Hp. So my mill VFD (vector motor) operates from 20-200Hz. Standard motors I would not push past 90Hz, older motors 75Hz with a lower carrier frequency.

Putting in a larger motor is not so simple a task on a mill, motor shaft size is larger and there are mounting issues with a larger motor. So there can be a lot of work and cost. Motor wise you want to look for an inverter motor with a minimum constant torque ratio of 10:1, the inverter/vector motors are more like 1000:1, this is the range that the constant torque is maintained below the motor's base speed. This give better low speed control, and typically they can also be operated up to 2X with constant Hp above their base speed (60Hz). I would use a 2 Hp and change the drive ratios so the motor is at or greater than ~ 30 Hz at 100 RPM. So it all comes down to how deep you want to get into it and spend.
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