What is preferable, running a motor faster or slower than nominal 50hz rpm?

[Mark Davison 1Participant @markdavison1]

[Mark Davison 1Participant @markdavison1]

I’m considering converting a small horizontal mill from single phase to VFD controlled 3 phase in order to provide a broad range of speed control.

It is currently fitted with a 4 pole/1500rpm motor.

I would like to be able to drive it slower that the 1500rpm allows.

Would i be better getting a 6 pole/1000rpm motor and running it above 50hz to achieve the current speed. Or, stick with 4 pole and run it below 50hz when i want to reduce the speed. I am leaning towards 6 pole/1000rpm and running above 50hz when require, but have no idea what the optimal configuration is?

[Michael GilliganParticipant @michaelgilligan61133]

Probably not much in it … But the 6 pole motor should be inherently smoother; so that would tip the balance for me [unless the price differential is too large]

MichaelG.

[Ian ParkinParticipant @ianparkin39383]

I have 18 machines all with inverters and on a machine that needs variable speed i will happily run at 10hz to 120hz on a 4 pole

My mill has a 3hp 2 pole geared down 2:1 and i regularly run it at 15hz with plenty of power and also at 120hz so spindle is about 2800 rpm

My lathe however is always run at 40-60hz and i use the gearbox to change speed with sometimes a tweek to the frequency to fine tune for threading or vibration probs

[Simon Williams 3Participant @simonwilliams3]

Running the motor slower than its rated speed usually reduces the available torque. It depends on the drive set up to what extent, but it's probably pro-rata. The cooling of the motor is also reduced. The quantity of air shifted by a centrifugal fan drops as the cube of the ratio of the speed ratio, so in theory it's easy to get into problems with the cooling. In reality the current reduction as the motor is slowed mitigates this to some extent – probably (as the heating effect is I squared R) the actual loss of overall cooling could well be linear with the speed ratio.

Except that these motors on little machines worked not too hard are by and large immensely over-sized, and so running a machine at 10% of its normal rated speed is probably perfectly safe. You'll likely smell the toast if the motor gets hot before any real damage is done.

I've done exactly what you've done on five machines, by which I mean graft a VSD onto a motor running at the correct original rated speed. No magic smoke yet! I've also got an S7 I fitted a two pole motor to, I run that at all sorts of speed reduction and increase. Famous last words, but no smoke yet. I've also got a mill-drill I use the speed control instead of changing the belt positions (within reason) – works a treat.

For a horizontal mill you're usually looking for lots of torque and low surface speed with big cutters.  So the ideal thing would be to be able to run at slow speed with the motor developing its full power – so the 6 pole (or even 8 pole) motor has it.  So if you've got to buy a new motor, go for the slower one.  But if you've already got a suitable motor fitted at 4 pole, I'd happily run it at 50% speed at see how it behaved.   It's not going to overheat instantaneously, so check it as you use it, and see if all is OK. 

 

Edited By Simon Williams 3 on 24/11/2017 20:18:16

[Clive FosterParticipant @clivefoster55965]

Slower is usually the best way to go as it lowers the speed at which power loss becomes a problem.

In general a VFD will drive a motor respectably close to constant torque over about ± 1/3 rd of the normal speed range. If you just fit a VFD and stick within that range it will almost certainly be perfectly satisfactory in ordinary use. Outside that range torque drops off and you have to start thinking seriously about what your power needs are or what compromises you need to make compared to using mechanical speed changers.

So you can expect to run a 6 pole motor from around 670 rpm to 1330 rpm without worrying about what's going on whilst corresponding figures for a 4 pole motor are 1000 to 2000 rpm.

As Ian and Simon say you can run the motors quite happily well outside that ± 1/3 rd range but you must think about the effect of power changes. Running lots faster than nameplate speed significantly increases the power at the driven spindle or whatever which could be rather dangerous. Under half power at half speed will just stall the cutter if overdriven. Approaching double power at double speed is likely to be shrapnel time. High speed usually goes with small cutters anyway so there is no need for extra oomph.

Clive.

Edited By Clive Foster on 24/11/2017 20:15:28

[Mark RandParticipant @markrand96270]

Posted by Clive Foster on 24/11/2017 20:14:11:

As Ian and Simon say you can run the motors quite happily well outside that ± 1/3 rd range but you must think about the effect of power changes. Running lots faster than nameplate speed significantly increases the power at the driven spindle or whatever which could be rather dangerous. Under half power at half speed will just stall the cutter if overdriven. Approaching double power at double speed is likely to be shrapnel time. High speed usually goes with small cutters anyway so there is no need for extra oomph.

Clive.

It should be noted that you can only get more than rated power at higher speeds if you can also supply more than rated voltage to the motor. The maximum current which gives the torque, is pretty well fixed. But in order to drive that current through the motor at more than rated frequency (giving increased power) the VFD needs to provide higher than rated voltage.

[Mark Davison 1Participant @markdavison1]

Thanks folks. I think i will go 6 pole if i go through with this. I have a spare 4 pole 3 phase motor from a drill, but it is 400 volt and cant be reconfigured for 230v so i will have to buy a new one.

A previous owner had fitted a home made 2 speed gearbox to achieve lower speeds but that required a considerable reconfiguration of the machine, something i’m probably going to reverse to put it back to standard. This will not leave room for the gear box.

Edited By Mark Davison 1 on 24/11/2017 21:24:54

[MuzzerParticipant @muzzer]

Just to clear up some misconceptions above, there is no reason for the torque to fall off between base speed and zero rpm. In fact it is perfectly possible to maintain the rated torque all the way through zero and out the other direction to base speed in reverse (and beyond). The only limitation is the cooling, so if you have a motor with self cooling (fan rotor on spindle), operation at low speed should be limited.

If you are going to buy a new motor, buy a larger one so you have a good thermal and mechanical margin. I fitted a larger (2.2kW), higher speed (2880rpm) motor to my milling machine to replace the original Bridgeport pancake motor (something like 1kW at 1400rpm). I got it (new) from John Stevenson and it cost less than £100 – no idea where he got his from. It can generate the same torque and power at the same speed as the original, yet it can also manage twice the speed and twice the power. Having said that, and as noted, none of us are likely to maintain a high spindle load for very long at all.

Murray

[not done it yetParticipant @notdoneityet]

You might consider pulley size changes, if the machine is belt driven. Changing gearbox internals is somewhat more difficult!

If you indicate the manufacturer/model, someone may even have altered another example already.

[Niels AbildgaardParticipant @nielsabildgaard33719]

There is some usefull expirience here

http://www.homemodelenginemachinist.com/showthread.php?t=25236

many poles and Hz

[Mike PooleParticipant @mikepoole82104]

Using an inverter to run a motor at other than base speed you will run into two areas, running below base speed you can run at full torque with a flux vector controlled inverter to quite low rpm and if the inverter has the facility for an encoder then 100% torque can be developed at 0rpm. The downside is that as power is torque x rpm then at half revs you will only have half power. On the over speed direction you will hit the constant power problem which means that although the rpm will rise the torque will fall. Both operating areas can be useful but be aware that there are limitations.

Mike

[Neil WyattModerator @neilwyatt]

A simple but useful 'trick' is to set the rated power of a 50/60Hz motor to 60Hz. It increases the full torque speed range by 20% as it won't click into 'constant power' mode until it hits 60Hz.

Neil

[Author]

Posts