Re: Two-speed motors with inverters (again)

On 26 Mar, 13:22, Richard Edwards <poundea...@xxxxxxxxxxxxxxxx> wrote:
On Thu, 26 Mar 2009 05:22:46 -0700 (PDT), timle...@xxxxxxxxxxxxx
wrote:



On 26 Mar, 12:02, pentag...@xxxxxxxxx wrote:
On Tue, 24 Mar 2009 18:05:12 +0000, Mark Rand

<ra...@xxxxxxxxxxxxxxxxx> wrote:

SNIP

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

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,

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

    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.

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

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



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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