Two-speed motors with inverters (again)

OK, harking back to an older thread, I've now got a 3hp, 415 in & out 3-phase inverter (Siemens Micromaster 420) to run the head motor on my mill. The motor is 2-speed, 2700/1420, 3 / 2 hp.
Should I a) run it permanently on the high speed setting b) retain the 2-speed switching?
Would the inverter be seriously 'confused' by two different loads without resetting the parameters? That I suppose will really depend on how the setup will behave at motor speeds below the existing 1420
If I retain the 2-speed switching, is it vital not to switch speeds when the inverter is on? with the present belt-change system the top spindle speed is about 4880 rpm, I can't see myself wanting to push it much above that even if that was a good idea.
Thanks
TimL

On Fri, 20 Mar 2009 15:18:41 -0700 (PDT), snipped-for-privacy@btconnect.com wrote:
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Nobody's replied as-yet, so I will...
My suggestion and personal action would be that, unless the manual for the specific inverter specifically warned against switching in the output or multiple motors, I would keep the 2 speed switching in place and configure the inverter for the highest of the two outputs.
I would keep the wiring from the inverter as short as is reasonable to avoid stray capacitance and not worry about DOL starting and switching. This is based on the current experience with the grinder and inference from my original training/degree as an Electrical & Electronics engineer.
I wouldn't offer you a guarantee, but I wouldn't worry about doing it for myself. AFAICT, there is a lot more urban myth than circuit theory involved in peoples opinions on the matter.
Regards Mark Rand RTFM

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Mark,
The warning re: switching inverter outputs is presumably to prevent the very high back emf generated when the inductive load of a motor winding is switched damaging whatever is used in the output stage. Now I'm sure that modern inverters have some form of suppression, and I would hope that the semiconductor switching elements are well over specified, however personally I'd be nervous doing it for good technical reasons.
AWEM

On 24 Mar, 08:26, "Andrew Mawson"
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If switching *off*, surely the back emf is confined to the motor side of the switch? Switching the speed arrangements while running might be another matter, I can see that might not be a kind thing to do. The inverter instructions are not full of obvious 'don't' warnings, just the usual diagrams of how it *should* be done. I'm inclined, for the moment, to wire the mill head supply, complete with switches etc, direct into the inverter output and put the inverter controls in an obvious accessible position to reduce the chance of inadvertently reaching for the motor controls first. I'm bound to make that mistake at some stage, I'll take that chance in the hope that Mark is right ;-)
I'll report progress in due course.
Many thanks Tim

On Tue, 24 Mar 2009 06:19:09 -0700 (PDT), snipped-for-privacy@btconnect.com wrote:
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You're not allowed to be logical in these religious debates :-)
The maximum voltage that the inverter could see on any phase due to disconnection is twice the normal peak voltage, BUT (and this is important) the switching speeds of the outputs are so high that reflected transients of this magnitude would be possible at the inverter outputs purely due to the change in impedance between the supply cables and the motor windings.
The IGBTs on the output stages have flywheel diodes, either integral or very close to them in order to protect the expensive output stages from these transients. They will limit the output voltage to no more or less than the DC bus voltage. The other function they serve is to inject current back into the DC bus when the inverter is slowing the motor down. In this case the peak value back emf of the motor is higher than the DC bus voltage and excessive current is then dumped into the internal or external braking resistor to control the DC bus voltage. The same mechanism will come into play when switching speeds or disconnecting the motor. When DOL starting the motor, the normal current limiting protection of the inverter will come into effect. For a simple/little inverter, the inverter may trip out on an over current fault. On a more intelligent/bigger inverter, it will drop the output voltage to limit the load current until the motor has picked up speed.
Anyone want to step in excommunicate me? :-)
Mark Rand RTFM

On Tue, 24 Mar 2009 18:05:12 +0000, Mark Rand
SNIP
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I don't have the necessary Papal authority but, with appropropriate humility:-
In relation to the earlier comment. Because it's a three phase load mechanically switched, disconnection of all three winding is unlikely to be simultaneous (within microseconds). At least one of the windings will remain connected for long enough to deliver its transient voltage back to the inverter.
I agree with your second paragraph but the maximum voltage statement is a bit optimistic.
The maximum size of the transient is can be extremely variable and, while most of the time about 100% overvoltage is a reasonable expectation it is far from being the worst case.
For instantaneous disconnection, the peak value of the the inductive transient is I x sqrroot L/C. (L is the winding inductance, C is the total winding plus wiring capacitance).
If disconnection occurs at the zero voltage maximum current time this will only be limited to twice the normal peak voltage if the motor winding inductance plus stray capacities is resonant at less than twice the supply frequency.
With typical motor installations the resonant frequency will be far higher than this raising the possibilty of extremely high transient voltages. The saving grace is that the actual mechanical switch interruption is not instantaneous - there is a finite arcing time which slows the rate of current reduction and this, together with other losses reduces the corresponding peak voltage.
Because of these (and other) variables it is nearly impossible for a manufacturer to arrive at an absolute worst case transient voltage that is independent of installation. Because of this, most VFD manufacturers either ban, or severely restrict VFD to motor switching.
They nevertheless they have to accept that blown fuse between the VFD and motor must not blow up the VFD. So VFDs are designed with sufficient safety factors to ensure that MOST of the time, they will not be damaged by load disconnection.
In practical terms VFD to motor switching is undesirable. If you need to do it, it will normally be OK but you are stretching manufacturers safety factors up to their limit
Jim.

On 26 Mar, 12:02, snipped-for-privacy@yahoo.com wrote:
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Having opened up the head switching arrangement, I've concluded that it should be fairly straightforward to bypass the forward-off-reverse switch and reassign it to doing the same job on the inverter. There's just about room for a speed pot between the two switches, making a fairly neat mod. I'll then just have to try very hard to remember not to switch the 2-speed switch until the motor has stopped ;-)
Thanks Tim

On Thu, 26 Mar 2009 05:22:46 -0700 (PDT), snipped-for-privacy@btconnect.com wrote:
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May I suggest that you remove the Knob from the two speed switch and put it nearby. Use it (with motor stopped) then remove it. Chances of changing whilst running are then very remote.
Richard

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That's certainly a very valid suggestion, may not be all that easy to apply in this particular case. I'm hoping to get enough productive work done this morning to give me time to have a go at it this afternoon.
Tim

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In Tim's place ? You have got to be joking, once removed and put nearby is akin to crimes against the state.
I can see it now, where did I put that - oohh what's this I've found, just needs a bit turning, milling and tapping, moves to mill, oohh what's this I have found just needs a bit welding - oohh what's this I've found just needs a bit.........what was I doing ?
Unless you have been to Tim's place think yourself lucky he can find his mill on two consecutive days <g>
Tims' saving grace is that he can only extend in one direction, he has the railway track on one side, the canal on the other and the tow path at his back. Heaven help Rail track if he decides he wants some long steel bars for a job and remembers where he's seen them.
John S. [ Who's recently bought a new brush <bg> ]

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Have you found some floor to sweep, John <g> That reminds me, I haven't seen the S1 bridgeport lately, I wonder where I put it? The new mill is 8 inches or so taller and deeper front to back than the old one, although the table seems to be exactly the same the travels are greater. The extra height limits where I can put it, and those extra inches of depth make a big difference in my place. If only I could persuade 'er indoors that she didn't need her part of the building..... <BG>
Tim

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...pah ..a mere 8 inches ... I've just had to totally alter the workshop layout to accomodate a Beaver Partsmaster that'll be arriving soon. Needed something like an extra 3 foot in each direction and the ability to walk round the back over and above the Bridgeport Interact it's replacing. Oh and the vendor threw in a full size robot arm foc - well not easy to refuse is it! Perhaps I can program it to make the tea?
AWEM

On 28 Mar, 13:53, snipped-for-privacy@btconnect.com wrote:
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He bought some on Ebay, with a load of wheels ;)
Dave

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Don't know why I didn't think of it before. Go onto Ebay and buy something cheap with loads of floor showing, when it gets delivered you have loads more floor space. Simple.
John S.

On Sat, 28 Mar 2009 14:34:05 -0700 (PDT), John S
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The problem is that floor space is so much cheaper when it's a long way away from home. The transport cost will probably scupper this otherwise sensible idea.
Charles

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That's the beauty of it Charles, get something cheap with loads of floor an low shipping costs. Under the long distance selling regs everything shown in the picture is yours.
John S.

On Sat, 28 Mar 2009 16:14:26 -0700 (PDT), John S
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Yabbut you can't go sending the floor back once you've filled it up with stuff. (unless it was defective of course)
Mark Rand RTFM

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That's the beauty of it Charles, get something cheap with loads of floor an low shipping costs. Under the long distance selling regs everything shown in the picture is yours.
John S.

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Bit of a bugger if the floor's a different level from yours - you'll have a step <G>
AWEM

On Sat, 28 Mar 2009 16:17:36 -0700 (PDT), John S
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I think there's a floor in your argument <G>
Regards, Tony