On Fri, 20 Jun 2008 13:21:40 -0700 (PDT), " email@example.com"
I haven't used the Teco invertors, but I can see no fundamental reason why a
properly designed invertor shouldn't cope with DOL starting and disconnecting
The Bauer invertor that I got from Brian Willis (Thanks Brian, I really owe
you), doesn't bat an eyelid at starting and disconnecting the motors on the
J&S grinder:- 2hp + 3/4hp + 1/2hp. Mind you, it is rated at 30A/23kVA and has
a 5hp motor permanently connected to the output in lieu of a set of reactors.
If the vendor is offering any sort of warranty at all, I would nail it to the
wall, configure it, turn it on and do my best to blow it up with every three
phase motor in my possession. You may need a set of reactors if the total
cable length is more than five or ten metres (stops the possibility of the
invertor seeing a leading power factor/capacitive load, this is what blows up
switching power semiconductors)
Bear in mind that it's 30 years since I got my electrical engineering degree,
but my positive experience counteracts this lack of uptodate knowledge.
The output of the inverter (VFD) needs to be connected to a motor
whenever it is switched on. The reason is that the correct opperation
of the unit requires the inductance of the motor to create the pseudo
Low-frequency three-phase supply from the hi-frequency PWM that the
VFD generates. Consequently, the manufacturers state - correctly -
that the output of the VFD should not be fed top the motor via any
form of switch/NVR Contactor, etc which might/will be used to stop/
start the motor.
However, one could always arrange for the VFD output to be connected
to a suitable 3-phase and earth socket, into which you plug the motor
of your lathe, mill, etc, whichever you need to run. Following
connection, you can then switch on the VFD mains supply and operate
the machine via the VFD controls or a remote control switch-box.
Before disconnecting the motor, you must make sure that the VFD is
definitely "off". Many retain the charge in their capacitors and can
show activity on the screen.
The manual should tell you the maximum length of cable allowed from
VFD to motor and you'll need to posotion the machines you will feed
from it to comply with this.
All the motors must have power ratings at or lower than the VFD output
capability. Especially important with some makes of VFD - eg Siemens
- the motors should have same number of poles.
It is, also, necessary to compromise on the parameters set in the
VFD. For example, setting the deceleration time to suit a vertical
mill - low inertia - may result in overloading the VFD when
decelerating a lrager mass on a lathe. This is the main reason that
VFDs are recommended to be used in the one motor/one application mode.
On Sat, 21 Jun 2008 10:34:15 -0700 (PDT), houstonceng
For example, you might have more than one machine.
In the first, neither the motor nor the control gear can easily be
rewired to 240v three phase and it doesn't require variable speed
(i.e. it has mechanical variable speed). This can be plugged straight
into the above invertor's output.
In the second, you might want variable speed, hence if you interpose a
415 in 415 out VFD between the invertor and the motor, this would
If you have a third machine that needs variable speed, a second VFD
with different parameters could be used for that one.
My thought was that the (series of) 415v VFD would be cheaper than a
dedicated invertor for each machine. With the added necessity for the
sine wave contraption, this may not be so.
The "Inverters" sold for supplying 3-phase motors from single-phase
mains are nearly always VFDs as well. The 240v AC single phase in /
415v 3-phase AC out "Inverters" sold by DD are.
A true Inverter creates AC from DC, and many are available on e-bay to
supply mains equipment for "leisure" purposes from 12v DC batteries in
caravans, boats, cars, etc. So - perhaps - DD thought you were
thinking of supplying a 240v mains input VFD from a 12v DC to 240v DC
In that case, I can see that they would recommend a sign wave filter
as the outpit of these "leisure inverters" is full of harmonics and
the VFD wouldn't like that.
I'd be interested to know more about doing that. I'm familiar with
some forms of voltage doublers, but don't know enough about inverters
to attack one.
Email me off list if you'd rather a private discussion.
It's actually very simple. An inverter simply converts the input to a DC
voltage, then chops it into a high frequency which is modulated with a
variable (much lower) frequency, potentially with three outputs
separated by 120 degrees phase shift.
The input can be converted to a voltage doubler just by connecting N to
the mid-point of the 2 DC buss caps and L to any of the 3 phase inputs.
The principle is that one of the two caps gets charged on one half cycle
and t'other cap on t'other half cycle.
Words to the wise: this invalidates any CE marking (and warranty) and
involves high voltages - simple is only simple if you know and
understand what you're doing.
Excellent, thanks for that. I used to design machine tools and in the
now fading past worked with 270kV DC power systems so I'm aware of the
'keep fingers out of the works' aspect. I just didn't know that aspect
of motor drive inverters. It would be a purely domestic undertaking so
I'm not bothered by the EMC/CE/warranty aspects - almost everything I
possess has been modified.....
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.