3 phase inverter query?

I've worked with Siemens 420 series inverters for the last fifteen years and when I've needed to make changes or fit new units I still referred to notes I kept because there are so many variables that needed tweaking.

Martin P

Andrew,

I would take the opportunity to find out if the other identical unit has a rating plate/model number on it.

The last inverter to go pop on me was an ABB. That was switched via the inverter, was twice the rating of the motor and likely just got very tired and expired.

I stuck an oscilloscope on my lathe's built-in VFD (can't find the blurb, think it's an 'inexpensive' Delta) and the output looks like this:

It's an approximation of a 50Hz sine wave built of 5kHz pulses varied in amplitude. It's only 50Hz on average. Notice that the peak to peak voltage is 840V. Zooming in from 2.5mS to 250µS you can see the sharp internals better:

So my particular VFD is pretty grubby. At 50Hz the waveform is full of spikes and harmonics extending from the 5kHz base to VHF. When the motor windings are correctly connected, the winding inductances smooth the spikes and the motor 'sees' 50Hz. What happens when the motor windings aren't connected properly is up for grabs. The spiky waveform is liable to resonate and create very high voltages. It's not hard to imagine a situation where a mis-connected motor winding accidentally behaves like a car ignition coil. There the contact breaker pulses all the energy stored in a condenser into the coil, causing it to poke out 10kV or so at the top. Even more volts when a spark lead is disconnected. (Ouch. You can guess how I know...)

I'm sure that not all VFDs are as spiky as mine. I guess that many of the cheaper single motor units are as bad because the spikiness doesn't matter much provided they're wired up correctly. It's possible to clean up the output of a VFD and I reckon units designed to drive several motors do just that.

Dave

I backpedal on my earlier comments.

My IMO Cub inverter does allow switching between inverter outputs and the motor. In fact it says the following about uising a magnetic contactor for switching:

An MC can be used at both the power input and output sides of the inverter. At
each side, the MC works as described below. When inserted in the output
circuit of the inverter, an MC can also switch the motor drive power source
between the inverter output and commercial power lines.
■ At the power source side
Insert an MC in the power source side of the inverter in order to:
1) Forcibly cut off the inverter from the power source (generally,
commercial/factory power lines) with the protection function built into the
inverter, or with the terminal signal line.
2) Stop the inverter operation in an emergency when the inverter cannot
interpret the stop command due to internal/external circuit failures.
3) Cut off the inverter from the power source when the MCCB inserted in the
power source side cannot cut it off for maintenance or inspection purpose.
For the purpose only, it is recommended that you use an MC capable of
turning the MC on/off manually.
Note: When your system requires the motor(s) driven by the inverter to be
started/stopped with the MC, the frequency of the starting/stopping operation
should be once or less per hour. The more frequent the operation, the shorter
operation life of the MC and capacitor/s used in the DC link circuit due to
thermal fatigue caused by the frequent charging of the DC Link. If this is not
necessary, start/stop the motor with the terminal commands (FWD), (REV)
and/or (HLD), or with the keypad.
■ At the output side
Prevent externally turned-around current from being applied to the inverter
power output terminals (U, V, and W) unexpectedly. An MC should be used, for
example, if a circuit that switches the motor driving source between the inverter
output and commercial/factory power lines is connected to the inverter.
Note: As application of high voltage external current to the inverter's
secondary (output) circuits may break the IGBTs, MCs should be used in the
power control system circuits to switch the motor drive power source to the
commercial/factory power lines after the motor has come to a complete stop.
Also ensure that voltage is never mistakenly applied to the inverter output
terminals due to unexpected timer operation, or similar.
■ Driving the motor using commercial power lines
MCs can also be used to switch the power source of the motor driven by the
inverter to a commercial power source.

Neil has just reminded me about how the company I worked for used their inverters, one piece of equipment had two three phase semi submerged pumps one running and one on standby i.e. after 24 hrs the running pump is stopped and the stopped pump is started up which then runs for 24 hrs when the process happens again, both pumps have contactors between them and the inverter and when the pumps are stopped the inverter is shut down to change over and the inverter is switched back on to ramp the pump back up to running speed.

Martin P

I will back Andrews emailer, i have been running an M300 for 8 years with original isolator, all contactors exactly as it was intended to be used ie with 3ph power.
Done nothing to mine except plug the inverter in to machine exactly as if it were 3ph cabling in any industrial environment.

To power inverter i just flick the 32A mcb c type switch and press Run thats it. Upon hitting Run hear all the contactors latch in. Now if put a 3 second delay ie slow start all it does is make the contactors chatter until voltage is reached.
All i do then is hit the spindle start, job done and have all the original needed controls ie foot brake, coolant, spindle forward with reverse and all switches. Using any other way highly dangerous.

As it happens just had a much smaller Mitsubishi D720-100 arrive this morning which will require a rewire and figuring out of the spindle start and foot brake or it wont get used.

There seems to be some confusion about what the output of a VFD looks like. Here is a sketch of a typical VFD output stage, essentiallly a 3-phase bridge:

The 0V line is the same as the mains neutral and the DC_BUS is the rectified mains, around 320V for a single phase input. A motor is connected the the three terminals U, V and W.

Each pair of IGBTs can be in one of four states:

Both IGBTs off - the output will float and do it's own thing

Upper IGBT on, lower IGBT off - The output will be DC_BUS, minus the Vce(sat) of the IGBT

Lower IGBT on, upper IGBT off - The output will be 0V, plus the Vce(sat) of the IGBT

Both IGBTs on - exciting stuff, as the DC_BUS will effectively be shorted. Been there, done that as a result of a software boo-boo, short circuit current was on the order of 3000A.

Let's assume the the IGBTs are fed with 1:1 mark:space square waves, so the upper IGBT is on for t seconds while the lower IGBT is off, then vice versa. The output will be a 1:1 square wave. If this is filtered it will be a DC voltage at half the DC_BUS voltage, ie, mid-rail. This is essentially the "neutral" point of the 3-phase output. If all three arms are driven in this fashion then any motor connected will just sit there, as the same voltage will appear on each terminal of the motor, at zero frequency, and no current will flow.

Now suppose we ensure that the upper IGBT is on for longer than the lower IGBT; filtering will result in a voltage above DC_BUS/2. Similarly if the lower IGBT is on for longer than the upper IGBT the filtered voltage will be lower than DC_BUS/2.

If we're clever with the upper and lower timings we can get an output that, when filtered, looks like a sine wave. However, the output of the VFD (on V, U, W) is a PWM signal swinging from 0V to DC_BUS.. It is not a sine wave or anything like, as there is no filter on the output. Since the IGBTs are either full on or full off the dissipation in them is relatively low, compared to them being driven linearly to produce a sine wave output - essentially an audio amplifier.

So how does that explain the SoD waveform? The 0V line is not usually brought out, so I think SoD measured phase to phase. This will give a sinusiodal like waveform, albeit with the switching waveform imposed on it. The spikes and noise are a combination of two effects. One, there will be parasitic L and C which will give spikes during switching due to high di/dt and dv/dt. Second, unless one is using a high quality probe with proper "earthing", the probe itself will generate "spikes" and "noise". The bog standard ground lead you see on 99.9% of scope probes doesn't cut the mustard for high speed measurements. It has too much inductance.

Right, still got a long list of things to do this evening. The first of which is check on the medlars simmering on the hob. The plan is to make medlar jelly, but at the moment the liquid is looking like brown goo, not translucent red.

Andrew

Jon

Is your M300 just a standard machine ..no mods at all and is the inverter a 240v in and therefore 240v 3 phase out?

So you are running a 415v 3 ph machine on 240v 3ph ?

Oooo! Andrew's post has given me much to think about. Did I measure phase to phase or not? I can't remember and, although I took notes, they're not in my usual book. So it's possible I didn't measure what I thought I was measuring; the point about parasitic LC rings true. It sort of aligns with what I was saying about the possibility of a mis-connected motor winding producing high voltages though.

Never mind, it's all educational. Back to the drawing board!

Cheers,

Dave

PS Mentioning Medlar Jelly flashed back me back 50 years to Sunday dinner. Mum used to make it. One year it turned out horrible and she gave up. Pity.

Ian the electrical side is unchanged since the day it left the factory and served time in GKN.
Has a non dual voltage motor running at 400V.

No soft start, 1ph 240v to inverter giving 3ph 400v output is literally wired as a full on 3ph supply would be used or a static or rotary convertor.
I did have a 4kw 1ph 240v to 3ph 400v inverter this time 8 years ago and told to wire it direct to motor as a VFD. Manual and supplying company useless at helping wire in the important spindle start and foot brake then bowed down and said try it through control box/contactors etc. Not man enough to power a 2.2kw motor and lasted 1hr 23mins which they still owe me for.

To add to above i have always had electrical supply problems and now in process of changing the motor as first port of call. Original non dual voltage and always had doubts on it.
Cant get a twin pulley 28 bore 100m until Thursday and 4 extra days downtime. Now putting motor and backing stand off plate back on, found out and whipped my pulley off the 1hp Brook Crompton motor on ML7 i used to use then bore it out 28mm then fit on new motor to bore out and slot lathes original fitment.

Only then will i know whether the larger 100 case will fit in, originals on M300 a 90 case with 24 shaft.

I will be the only person that has ran same machine with a 5.5hp Rotary convertor 400v, 1ph 35Amp draw 240V 7.5kw digital inverters 400v and run 10A 220V 3ph for comparison as a VFD.
No interest in speed changes got a gearbox for that, its more to do with a question i have constantly asked for the last 8 years regarding the torque loss running 220v, we shall find out.