I am new to the website so I hope this thread is posted right. Currently I have a PM1236T lathe 3-phase machine. It was purchased two years ago along with a Hitachi WJ200 VFD. All wired up per the instructions provided by Precision Mathews and has been working great. My experience operating the lathe is two years. The speed control feature has never been wired up because I had concerns about the motor operating at different frequencies other than 60Hz. Some reading I did through the internet cautioned about overdriving the motor when operating it below or above 60Hz. The article did go on to say that 4 pole motors are less effected than others. Can anyone share if this is harmful to the motor? My next project is to install the speed control using a 1k ohm potentiometer (2W) for this feature. This was the size recommended by the Hitachi technical support group.
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Using VFD for speed control on a 1236T Precision Matthews Lathe
- Thread starter sewaldrep
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You should be able to drive the factory motor with the Hitachi VFD from at 40 to 90 Hz. without an issue - perhaps a bit wider.
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I would say a bit wider than david said. Most VFD manufactures don't recommend below 20 hz if its a non inverter rated motor. It will poke holes in the insulation due to the lower class of insulation. I wouldn't stay a 20 hz either for heavy work, but something light like threading would be ok. Typically motors that are not vfd rated shouldn't be ran over 3600 rpm in the smaller 56 and 145 frame sizes. They are limited by rotor design/centripetal force.
Thanks David, I plan to install this in the coming weeks. Another concern is using the low voltage inputs for the VFD. The speed circuit cable will be about 5ft in length. The VFD manual mentions using shielded wire and grounding the shield at the VFD only. So my assumption is that these circuits can be impacted by noise. Does any additional shielding need to be done? This is also recommended on the relay logic inputs. So far there have been no issues for me operating the lathe as they recommend. However, I have a reconditioned Bridgeport Mill Series I that was built in the 70s. There is no logic provided for operating the motor like the PM1236. I designed a relay logic controller using 120VAC relays for Forward, Reverse, and Emergency Stop to reduce the possibility of noise on the logic inputs. The 24VDC from the FVD only operates from the relay contacts as short distance from the VFD hopefully reducing voltage drop and interference. Did I waste my time? It may have worked fine running 24VDC through the mill?
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Don't overlook the relationships between the Hz and torque/HP output. That's what drove me to the conservative estimate of 40 Hz.
I'm certainly no expert on this, but I do know that if gearing down the lathe will give more low end performance than dialing down the VFD. This is the primary reason I swapped out the factory motor on my 1340 for a vector-rated Baldor alternative. As for the shield requirements on the low voltage wiring, maybe @mksj will chime in here - he's the guru on all this.
I'm certainly no expert on this, but I do know that if gearing down the lathe will give more low end performance than dialing down the VFD. This is the primary reason I swapped out the factory motor on my 1340 for a vector-rated Baldor alternative. As for the shield requirements on the low voltage wiring, maybe @mksj will chime in here - he's the guru on all this.
Thanks, I don't know that I would ever try to run at 3600RPM for any projects that I do but I am a beginner to machining and have lots to learn. I guess running time at the different frequencies may heat the motor?
Thanks for the graph. I did not consider losing torque at higher RPMs. I need to learn more about the vector-rated motor.
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The directions for the VFD conversion were probably based on the 1340GT basic VFD install instructions that I have posted, you can look up the thread if you want more details. The thing to understand is that you loose the mechanical advantage of the gearbox below the motor base speed of 60Hz and gain the mechanical advantage above 60 Hz. Motor Torque stays pretty flat down to around 15-20Hz with normal (non-inverter motors) and falls off in a non-linear fashion above the base speed. Hp is a bit of the reverse. So what does this mean in the real world, well for heavy turning you probably do not want to go below 30 Hz, threading and light turning is OK down to around 20 Hz. Below this point you loose too much power, and a TEFC motor may encounter some cooling issues with "prolonged" heavy use below 20Hz. The upper end, standard 4P motors are mechanically sound to at least 2X and higher of their base speed, the limitation is standard motors have some degree of performance drop off above 100Hz. There is no detrimental effects of running a smaller (under 10 hp) 4P motor to 100Hz. Their is the mechanical advantage and gives you a wider speed range. Cooling is also not an issue withing the speed ranges mentioned.
On the 1236T and 1340GT, when using a VFD what is usually done is to flip the motor pulley so you are running the larger motor pulley to the larger headstock pulley (requires a BX27 belt). Then use the VFD with a speed range for general turning from 30-100Hz, and down to 20Hz for light turning. This gives you a single speed belt range from around 80-2000 RPM with no belt changes.
Insulation issues with VFD's is less of an issue with modern motors, as the insulation has gotten better. In addition dual voltage (non-inverter) motors say a 230/460V run at 230V is very unlikely to have any issue after years of use with a VFD. Issues of VFD induced insulation degradation and bearing erosions occurs with the higher voltages, longer cables and larger motors in 24/7 use environments. I would not worry about it with newer motors, older motors can have more issues so settings are more conservative. There are inverter/vector motors like what David runs on his 1340GT which can only be run with a VFD, they have full torque down to almost 0 RPM, and full Hp up to 6000 RPM with no issues. This requires using a smaller motor pulley and keeping the spindle speed to under 2000 RPM.
As far as speed pots anything from 1, 2 or 5K in a linear better quality pot, 1-5W will work just fine. They do sell some that fit into the 22mm hole, but quality is just OK. I prefer to use a mil-spec. type pot that are typically rated for millions of turns. It is one area where I have seen failure issues with cheaper/factory speed pots, the RPM will not stay stable either due to poor contact or contact bounce from vibration.
On the 1236T and 1340GT, when using a VFD what is usually done is to flip the motor pulley so you are running the larger motor pulley to the larger headstock pulley (requires a BX27 belt). Then use the VFD with a speed range for general turning from 30-100Hz, and down to 20Hz for light turning. This gives you a single speed belt range from around 80-2000 RPM with no belt changes.
Insulation issues with VFD's is less of an issue with modern motors, as the insulation has gotten better. In addition dual voltage (non-inverter) motors say a 230/460V run at 230V is very unlikely to have any issue after years of use with a VFD. Issues of VFD induced insulation degradation and bearing erosions occurs with the higher voltages, longer cables and larger motors in 24/7 use environments. I would not worry about it with newer motors, older motors can have more issues so settings are more conservative. There are inverter/vector motors like what David runs on his 1340GT which can only be run with a VFD, they have full torque down to almost 0 RPM, and full Hp up to 6000 RPM with no issues. This requires using a smaller motor pulley and keeping the spindle speed to under 2000 RPM.
As far as speed pots anything from 1, 2 or 5K in a linear better quality pot, 1-5W will work just fine. They do sell some that fit into the 22mm hole, but quality is just OK. I prefer to use a mil-spec. type pot that are typically rated for millions of turns. It is one area where I have seen failure issues with cheaper/factory speed pots, the RPM will not stay stable either due to poor contact or contact bounce from vibration.
Thanks, MKSJ. This is very helpful. All of you folks have lots of knowledge on this thank you so much for the info. Thoughts on relay logic controller vs. running 24VDC all over the Mill? I plan to look at some of the other categories for information on why I would need a brake and a tachometer(Speeds and Feeds Calculation?). Still learning about machining. I think a brake might help me with cutting Metric Threads? Sorry if I need to ask that in another part of the Forum.
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The VFD should have electronic braking, requires a braking resistor to get quick stops which helps to stop the lathe in thread relief groove. Metric threading is a bit more tricky, as it requires you not to disengage the half-nut (threading dial only works for imperial threading). So it takes some practice to learn threading and there are different approaches to threading, compound vs. cross slide. Depending on the VFD, often I recommend programming two different braking rates controlled y a small switch, that way you can use faster stopping for threading and slightly longer braking times for higher speeds/routine work.
One thing to understand about VFD braking times, the time you set say 2 seconds is for the maximum Hz. So if 2 seconds at 100 Hz, it will be 1 second at 50 Hz and 0.5 seconds at 25 Hz. So this goes back to the speed range, if you thread at 20-25 Hz, you will have quicker braking times, there is also less centrifugal force in the spinning gears using a higher speed ratio vs. low speed. Threading speed/SFM is usually a function the type of cutter, material type, dimensions, etc. and typically more limited by your reflexes w/o making a mistake. Last hing you want to do is crash your lathe, so always take your time, do not get distracted, and think about what your doing.
One thing to understand about VFD braking times, the time you set say 2 seconds is for the maximum Hz. So if 2 seconds at 100 Hz, it will be 1 second at 50 Hz and 0.5 seconds at 25 Hz. So this goes back to the speed range, if you thread at 20-25 Hz, you will have quicker braking times, there is also less centrifugal force in the spinning gears using a higher speed ratio vs. low speed. Threading speed/SFM is usually a function the type of cutter, material type, dimensions, etc. and typically more limited by your reflexes w/o making a mistake. Last hing you want to do is crash your lathe, so always take your time, do not get distracted, and think about what your doing.