Member postings for Robert Atkinson 2

That looks like the original nuclear reactor SCRAM (shutdown) system. Allegedly the first reactor CP-1 had a control rod hung from a rope and the plan was to cut the rope with an axe if the reaction ran away. They still call a emergency reactor shutdown a SCRAM but there is no longer a Safety Control Rod Axe Man

Robert G8RPI.

Hi Mike,

I agree most industrial installations would have the E-Stop in the control line, but most industril machines will be 3 phase and higher current. If we were talking about using industrial quality contactors and switches from reputable manufacturers and suppliers I would be happier with just control line E-stops. However we are talking cheap all in one NVRs made in the far east and I have little faith in their reliability. There is virtually no cost or difficulty impact from putting one E-stop that is close to the NVR in the two phase power input before the NVR. Any additional E-stop(s) can be in the control (holding coil) circuit to simplify wiring and not have power on the wiring. My reasoning is if the NVR does not release when toy press the button you will go for the nearst E-stop which will work 100%. If a remote E-stop does not work you will again go back to the NVR and the primary E-stop. This may seem like overkill, but why not when it is very little cost in time or effort that may save a work piece, machine or bodily harm some time in the future.
One thing is not clear on my drawing, I say the input comes from a CB. Obviously ther shoyld be some kind of isolator between the power source and first E-stop. For single phase machines this would normally be a 13A plug. For larger machines either an industrial plug and socket ot a wall mounted isolator switch.

Robert G8RPI.

This is my recommended circuit for a KDJ17

It is fine for anything up to a few kW. 0.5mm "twin red" lighting cable would suit the wiring to the secondary E-stop(s)

Dave, I think it is quite reasonable to assume that mot of the KDJ17 NVR's sold on ebay are for replacement purposes rather than for manufacturing. For 3 phase you do have to use contactors etc but for safety critical circuits on production equipment you have to use suitably safety rated components. https://uk.rs-online.com/web/c/automation-control-gear/machine-guarding-safety/safety-contactors/

Robert G8RPI.

Edited By Robert Atkinson 2 on 10/05/2019 19:54:04

Hi Dave,
Yes I have looked at valve transmitters (2x4CX250) and similar with a 'scope. I use a Tektronix P6015 or HP 1137A EHT probe for that sort of work. For high energy e.g. connecting to mains, I use a Fluke ScopeMeter which has fully isolated inputs and properly rated, fully insulated probes. Using "normal" 'scope probes on mains is not immediately hazardous, but its certainly not safe. I've seen a scope lead go up in flames during this type of connection (no it wasn't me connecting it) The 'scope was destroyed by the current through the ground traces on the PCB. Connecting the 'scope common to neutral connects it to ground which will in almost all cases trip an RCD if fitted. The earth and neutral only need to be a few millivolts apart to drive the 30mA required to trip a typical RCD.

You may be surprised by the current waveform you see at the input to the VFD on load. If it is a non power factor corrected drive you will see narrow high amplitude spikes, not a clean sinewave current. These current spikes are "topping up" the charge on the DC capacitor.

At CAT III 1000V and 80M input impedance that differential probe is effectively isolated and is ideal for the job in conjunction with a current clamp.

Croc clips are a bit dodgy though.

Robert G8RPI.

Edited By Robert Atkinson 2 on 10/05/2019 12:26:06

If you put the POWER switching E-stop before the NVR (KJD 17) then the NVR will release when the E-stop is pressed so the motor will not re-start when the E-stop is reset.
If the KDJ 17 is so reliable why are there so many being sold? There are dozens just on ebay, they are not all for new build machines.

The bigger problem is the NVR contact is not failsafe. The only force opening it is a spring. If the spring breaks, coil armature or sear sticks or the contacts weld there is no force to break the contact. A properly designed E-stop has forced break contacts. The force on the button is directly transferred to the contacts in the opening direction. Short of the structural strength of the switch being exceeded the contact will open. With the E-stop in the NVR coil circuit all the NVR failure modes or the E-stop failure modes will cause it not to work.

Robert G8RPI

This is not strictly correct. For the sort of loads we are talking about here a decent E-Stop switch can handle the full load and still stop the motor if the no-volt release fails. 16A rated 2 pole E-stop switches are readily available. This primary E-stop is close to the NVR. Additional, remote E-stop switches can be wired in series with the NVR coil. The switch side of the coils should be in the neutral side so the wiring is not connected to a high current live. A short to ground will disable the remote e-stop (test them once a month if you are worried) but will not cause a fire as a live feed might. If you do put the remote switch connection of the holding coil in the live a fuse should be fitted at the source end. There is no advantage in fitting the E-stop local to the NVR in the holding coil circuit rather than the power. The suggested dual power and holding coil wiring costs no more and gives redundancy. Personally I would not trust a cheap Chinese NVR as my final safety device.

I have designed E-stop, guarding and interlock systems for robotic machines to meet European and North American safety standards so do know a bit about this.

Robert G8RPI.

As shown your set-up is likely to damage your oscilloscope if it dosen't electrocute you. It will certainly trip any RCD you have on the circuit because you are connecting the neutral to ground via the oscilloscope.
As a minimum you need to use proper 600V CAT II high voltage 'scope probes and an isolation transformer. The proper way to do this is with isolated differential probe, a specialised oscilloscope with isolated inputs or a power analyser.

It's really not something you should be playing around with unless you really need to and take precautions.

Robet G8RPI.

Not quite.

This would run power through all the e-stops. A combination of your scheme and Johns is the best compromise. If you have a E-stop right next to the KDJ 17 wire it in series with the input as you describe. Then wire any additional E-stops between A1 and 24 as John's diagram shows. It's a debatable point but I would swap the live and neutral on Johns diagram so you don't have 240V live with a high current fuse running to all the e-stops.

The problem with Johns circuit is that it relies on the KDJ 17 working properly. If the closest E-stop is wired in series with the supply then if you press the normal stop and it doesn't work you will naturally hit the nearest E-stop which will. The remote E-stops still rely on the KDJ 17 but you generally won't be using them because the normal stop was broken.

Robert G8RPI.

Well I was trying to keep it simple and it's not relevant to the original question. Lets not go into vector control other than to say it certainly does not have a fixed output voltage and any set-up should not exceed the motors rated maximum voltage regardless of the control scheme.
Questions on vector control could fill several new threads.

Robert G8RPI.

Edited By Robert Atkinson 2 on 09/05/2019 07:31:30

While this is generally correct, you would not normally see 230V or 240V line to line quoted in the UK. The output voltage of a VFD is not normally fixed, it varies with the speed with the voltage dropping with the speed reducing. This is to stop excessive current flow. This called V/Hz control and maintains a constant ratio between the two. for a 230V 50Hz motor the ratio is 4.6V/Hz so if running at 3/4 speed the frequency is 37.5Hz and voltage 172V. Note that while this imples that running at 1.25x speed the voltage would be 287V the voltage is not normally incresaed above nominal unless the motor is specified for inverter use at higher speed. This is becuse the higher voltage (and induced transients) can cause insulation breakdown.

In any case the power delivered should not exceed the motor rating (average and overload).

Robert G8RPI.

Edited By Robert Atkinson 2 on 08/05/2019 22:30:25

The motor is wired Star (Wye). Is your inverter configured for Star or Delta? If it's Star just leave the motor as is and leave the neutral unconnected. If you want Delta you will have to re-arrange the windings as Phil described above.

If you are unsure, leave it Star for now. the downside is slightly lower power and top speed but less stress on the inverter.

Robert G8RPI.

HI,

8A x 230V x 3

= 5.5kW seems high but they are taking the worst case of high motor load and low input voltage (230/240V is nominal voltage). If the voltage was specified as 398V (415 for 240V phase to phase) then you would multiply by root 3 instead of 3. This gives the same answer 5.5kW

Don't worry about it I keep having to explain this to supposed qualified electronics engineers.

Robert G8RPI.

Edited By Robert Atkinson 2 on 08/05/2019 20:06:04

Indeed, I've seen similar issues. In your case it was almost certainly EMI fliter capacitors charging. The main supply capacitor charging would be at least 10s of milliseconds. It's the main supply capacitor charging that drives the need for slower circuits breakers, typically type C.

Robert G8RPI.

No you don't in this case. It is all referenced to 240V Phase to Neutral voltage this includes the root 3 factor from the Phase to Phase voltage (415V). If I multiplied by 1.72 again the power would go even higher.

I do know about these things, it's part of my day job designing aircraft electrical systems. I'm currently working on a system that is the inverse of a VFD. It takes variable frequency from a engine driven generator and produces fixed 400Hz at more than 10kW.

Robert G8RPI.

Edited By Robert Atkinson 2 on 08/05/2019 19:55:33

Yes you do, there are 3 phases, they should have quoted the per phase current. see next post for root 3.

Robert.

Hi,

230V x 13.3A x 3ph = 9.177kW. I assume that the 13.3A they quote is for all three phases added together (which would give a more reasonable 3kW) but this is NOT the normal convention which is to specify the current for 1 phase and assumes the 3 are equal.
1 to 3 phase conversion is of course possible. As virtually all the modern drives convert the input AC to DC before converting back to AC it doesn't matter what the input phase is. In fact as long as you down-rate the power so the input rectifiers is not overloaded and the assuming the DC bus capacitor is up to it you can jus stick single phase into most drives and they will run fine. If like most, it has a connection to the DC bus available you could run it off about 20 car batteries and a bunch of solar cells. Or more realistically use an external single phase rectified and capacitor to run a 3 phase unit at full rating. Obviously if the VFD has an input transformer or AC fan this approach won't work.

Robert G8RPI

Well your first example makes no sense at all with "Input Current: 19.6A (Single Phase), 13.3A (Three Phase)" so at a nominal 230V the input power is 4.5kW on single phase and 9.2kW on 3 phase! At 200V it's 3.9kW 1Ph & 8kW 3Ph. (single phase nearly matches the overload figure).

At least the Eaton figures make sense. Without qualifying what load and input voltage the input current apples to it's impossible to be sure what it means. For practical purposes the motor rating (in kW) / 200 is a good estimate for full load input on single phase. so for 2.2kW you are looking at about 11A so a "13A" outlet should be OK unless you are overloading the motor.(the 200 is based on 230V, 0.95 power factor, 90% efficiency and rounded up).

Robert G8RPI.

SWMBO would not thank me for washing her hair with dirty hands and I don't have enough hair to make a difference.

Another reason for the input current of 23.2A specified for the Eaton DE1 2.2kW drive linked to by Carl **LINK** is the overload rating and input voltage range. It says 2.2kW output so about 95% efficiency gives 2.31kW. Minimum input voltage is 200V 2.31 /200 = 11.55A. The overload rating is 200% for 1.875s 2 x 11.55 = 23.1A no missing power or incorrect data

If running at nominal 230 V this becomes 10.05A full load and 20.1 at 200% overload. Thus a "13A" outlet would be adequate.

Power factor is less of an issue with the better modern drives as the input may have PF correction.

Robert G8RPI.