The saga with my Holbrook C10 continues and I'm beginning to accept that it must be the motor. It's a 3 speed motor rated at 2HP but is a monster and is buried inside the machine. The rating plate says it's a British Thomson-Houston (BTH) KVZ4120. It has an 8 hole flange that could be unbolted with a struggle but there's no visible way to separate the drive - any suggestions on how it might be coupled?
A separate but related question - can anyone explain the difference between a conventional motor and an inverter-rated motor?
In which case it's usually just the keyed shaft going into a female gearbox or could be a splined drive.
Main differences between invertor rated motors are the way they are bound and shellaced. if we are talking home shop use here then don't bother, the difference isn't justified by the price increase.
If you need a new motor give me a shout I have got about 30 2HP brand new motors 1425 rpm 6 wire Italian or German made, bought as a job lot to get dealer pricing. I can sell these at £30.00 each
Inverter rated motors are desined to cope with the loss of cooling ( slow fan ) that under speed running can bring.. some even have seperate cooling fans that have there own motor. Inverter motors do not need to be de rated for slow speed use by the same margin as normal motors.
The fast edges of VFD waveforms result in higher than normal voltage stress on the first few turns of the motor windings. In some extreme cases this may generate low level corona with long term degradation ofthe insulation. Inverter rated motors have uprated insulation with particular attention to the winding starts.
This is a matter of degree of safety factor not a go/no-go matter. Non inverter rated motors may have a somewhat increased failure rate but in home shop use, as John comments, most standard motors are already adequate and the increased risk is acceptable.
Over the commonly used frequency range 0 to 100Hz pretty well any motor is OK. However some VFD drives can operate at frequencies as high as 400Hz and this certainly needs motors designed for operation at this frequency.
Thanks John, that's an attractive offer - foot or flange mount? To get the existing range of spindle speeds I'd need to run a 1425 motor over the range 33 to 100Hz - is it really going to be happy at that frequency? If I decide to go through all the pain a motor change would bring I'm wondering whether a 2-speed motor would be better or whether I should increase to 3HP whilst I'm at it.
The motor runs fine off load at all 3 speeds (2875/1450/950) but under load at high speed (clutch engaged) the currents all get silly and things trip out. The same happens at mid speed selection and the inverter frequency set about 70Hz. Coupling all this with some slight intermittent noises I'm wondering if the motor bearings are shot and allowing a bit of rotor/stator contact under load.
My worry at the moment is how the hell to get this monster of a motor out without having to suspend 1.5 tons of lathe from a skyhook and extracting it from underneath. I'm hoping someone from the Holbrook group will tell me that it's a five minute job if you do it like this...
The reason I was asking about inverter-rated motors is that the inverter handbook seems to suggest that vector control is better in some way than V/f control but that you need to know the motor characteristics and that these are only specified for inverter-rated motors. I guess it probably makes half of very little difference for home use. Have you found that a blower is necessary at low drive frequencies?
These are all foot mount but I can get one changed to foot and flange at a slight extra cost.
Now you have me puzzled. Are you trying to run all three speeds from a 240v 3 phase invertor. If so this sounds like your problem. In the UK we can't run two or three speed motors from 240v 3~ because of the way they swap the wiring around when bringing extra coils in. [ Jim help me out on this one please ]
You have to use just one speed with the windings in delta and then the invertor will manage the speed range.
If you mean Converter instead of invertor then ignore what I have posted.
I first tried running from a 3.7kW Transwave converter but the lathe motor wouldn't start at 2850 (fine at 1450 and 950); a 5HP 2850 pilot motor made no difference. I'm now running a 415V inverter from a 240-415 autotransformer (in fact I'm using the transformer phases from the Transwave) and the inverter is connected directly to the input of the motor speed selector (all lathe contactors are bypassed and I'm only changing the selector switch when the inverter is off - I plan to build a solenoid interlock using the inverter run status output to stop me from doing something dumb). The inverter works fine on single phase 415 but I may need to increase the size of the DC bus cap to help with clutch engagement transients - turning-on the inverter auto-restart option gets over the problem but a few more microfarads should do the job.
I'm an Electronic Engineer by profession - and now I wish I hadn't skipped the machines and fields lectures in favour of the digital electronics lectures. :-)
Thanks. Having a better picture now. The VFD on my lathe (a retrofit by me) can drive up to maybe 400Hz, but the motor simply starts to skip pulses at about 120Hz (and rapidly getting slower*)). At 120 Hz, I have nearly no power. Useful range is up to 90 Hz. I assume a special motor would fix that. Do you know at about what prize factor?
I have had for some time what are ABB servo motor's They are from what I see as three phase motors and have a optic disk in there as weel I believe for sensing either position or rotation.
These are permenant magnet motors and I have been thinking of using them as generators as the plate says
BEMF 257 V Max rpm 3000
Cont s torq 10 Nm 13.8 Amps Peak S torq 30 nM at 42.7 Amps.
These are quite small motors and flange mounted.
I have often considered trying these on a 240 Volt VFD to see if they work. I am guessing they were run like this with the disc wheel to provide feedback anyone can any views?
In the ideal case, correct motor voltage should be directly proportional to frequency. Standard VFDs can only do this up 50Hz. Beyond this it's constant voltage so that at at
100Hz the motor is getting less than half its proper drive voltage. This coupled with rapidly increasing frequency sensitive iron losses severely degrades the motor performance.
Operation at frequencies above 100Hz needs motors with thinner laminations to reduce iron losses and wound for correct drive at this higher frequency. 115/200V 3 phase 400Hz motors are common items in aviation elctronics.
That's interesting Jim. So if I find that I need to replace my 2 HP 3 speed 2875/1450/950 motor it looks like I need to either find a 2 pole (2875) motor that will be happy running at 950 or less, or a 2 speed 2/4 pole motor. Presumably to get a 2 pole running happily at 950 or less I'll need to use a significantly beefier motor than 2HP and add a blower, so maybe a 3 HP 2 speed 2875/1450 would be a better answer. What does the team think?
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