Inverter Drives for Motors

There have been a number of posts arising from members having difficulty with Inverter / Variable Frequency Drives.

These devices have a number of characteristics which can trap the unwary and it would be helpful for these to be explored and pulled together in one place, hopefully in terms that can be easily understood by less technically informed members of our community.

This might already be met by reference to a book or learned paper with which I for one am unfamiliar!

To set the ball rolling I list below some of the issues that I think worthy of highlighting:

1. Safety

a. nature of their output and requirements of cabling and earth bonding.

b. Power stored in the drive (power capacitors) and residual power after mains power is lost. How to ensure it is powered down before diving into the electrics.

c. Effect of using conventional 'No Volts release' starters with inverter drives - damage to capacitors, how to prevent machines from self restarting after power loss, how to wire in emergency stop switches.

d. Fire risks and fires from failing power capacitors. Non combustible mounting boards, dust free ventilation and distance from combustible surfaces.

e. Separation of power and extra low voltage (signal / control) wiring.

2. Compliance with Building Regulations, IEE Regs and insurers requirements.

a. Fixed wiring or plugged connections.

b. RCD protection and associated issues

c. testing and certification

3. Practical Application to Machine Motors

a. Appropriate motors and how to identify them.

b. Use of one drive to feed more than one machine (or not)

c. Use of multiple motors on one machine (eg suds pumps and power feed drives) operating at different rotational speeds.

d. Star or Delta configuration and what inverters will work with them.

e. How to control overheating of motors (ability to 'self cool' when revs are reduced, loss of torque if voltage is reduced at lower frequencies) Reliable rules of thumb.

f. How to establish and input control and protective settings appropriate to you motor/ machine setup

g. How to control speed at either the inverter or with remote controller (potentiometer)

h. Selection ( electrical, mechanical characteristics, 'suds proofing' and mounting speed control potentiometer on your machine. Suitable plug and socket connectors for ELV wiring.

i. How to use one inverter to feed more than one machine (or not)

j. How to establish safe 'overspeed' of your motor / machine by setting frequencies above 50 Hz. Appropriate rules of thumb. (Chuck explosion, grinding wheel failure, lubrication / bearing failure)

k. Appropriate acceleration and deceleration settings, electric braking and switching between forward and reverse.

l. Trouble shooting, Fault codes and how to address their cause.

m. Sources of inverters and pitfalls.

n. Need to reform capacitors and how to do this (when inverters have been unused for 2 or more years). Implications of not doing this.

Not necessary.

You MUST have the system installed and commissioned by an expensive, qualified Electrical Engineer and only he should mess with it in the event of a problem.

Suggesting anything else on a forum invites those who "interpret" what they read through the filter of their world view and invites disaster and death.

I just thought I'd throw that in before someone who's eaten an industrial H&S manual pitches up

More than 90% of what's listed above is covered if you can be bothered to read the manufacturer's Installation, Commissioning and Quick Start Guide manuals for your choice of inverter, including troubleshooting.
If you choose an inverter for which such documentation is not available all the basics still apply, read the manuals for some of the well manufactured, adequately documented inverters you didn't buy - ABB, Siemens, Mitsubishi, Jaguar etc. just ignore model specific info like actual connector location and labelling (get that from your manual) and parameters and their settings (get them from your manual too)

Hello John,

Your questions under 1 are best dealt with by reference to the manual.

Under 2, I don't think there are any special requirements that are different from any other consumer electronics.

Under 3, plenty of room for discussion, although many of the things you raise will be machine/motor/VFD/application specific. Some like replacing/reforming capacitors will be covered by the manual.

I appreciate that VFD manuals are huge and complicated - this is because VFDs are complicated and trying to simplify key points about the safety/EMI/programming issues always risks leaving something important out. It doesn't help that VFD manufacturers and professional installers don't always agree about best practice on things like EMI.

As a start, by the next issue of MEW which has the first part of a detailed and very well executed installation of a VFD in a Warco lathe by Barry Chamberlain.

Some of the individual points you raise could be the basis for short articles on their own.

Neil

Nick, Neal I agree with you both but from previous posts on other threads it strikes me that many people are only partly aware of the issues.

Hopefully Barry Chamberlain's article will address some of the aspects I have listed.

Perhaps this thread will elicit observations from others on various points and maybe stimulate someone like Barry to produce a booklet in the Workshop Practice Series to supplement guidance on motors.

The use of inverter drives in industry has been very common for many years - I think I've been using them for 25 years or so.

Whilst the integration of inverter drives requires some degree of expertise, it is not beyond the reasonably skilled engineer ( legally known as a competent person ) assuming that they read and can interpret the supplied documentation, which is usually quite sizeable ( most inverter drives also come with a quick start document to allow knowledgeable people to short-cut reading the whole manual ).

Integration of the controls for an inverter drive will be covered within the manuals, but beware that one will have to change some parameters to get these controls working.

The primary issue - in industrial and domestic installations, is to make sure that the device is safely installed. Electrical installation must be carried out by a competent person who will understand the wiring requirements although from my experience, it is unlikely that they will understand the implications of guard closed limit switches and emergency stop push buttons. It is insufficient to connect these safety devices to normal digital inputs to the inverter drive since they pass through the software which runs the drive and are therefore considered to be UN-SAFE.

As discussed on several threads, it is possible to implement the standard no-volt method for removing power from the supply terminals to the drive and in some cases ( see the manufacturers data book ), it may be permitted to place the disconnect between the inverter output terminals and the motor. One should note that it may be necessary to feed-back an additional contact of the disconnect to shut down the drive.

Generally, disconnection of the infeed power will shut down the drive, but one must understand that there will be some stored energy within the drive and therefore the motor may run-on for a time even though the emergency stop has been actuated - an unsafe condition. Disconnecting the output of the drive from the motor can cause premature failure of the output electronics of the inverter rendering it faulty.

Since these issues have become really important in industrial installations, inverter manufacturers has added safety rated inputs to the inverter which disconnect the output power from the motor within the inverter rather than externally - such inverters allow repeated safe stopping through emergency stops and guard safety switches without risk to the user or the device.

Many inverter manufacturers provide units with the safe stopping facility and anyone looking to implement an inverter drive on a machine should contact the manufacturers for advice ( I would add that inverter manufacturers who are active within Europe will provide CE marked products - those from more doubtful sources might suggest that CE stands for China Export ).

Inverter / VFD IDC cables, this is a related question to the above however slightly OT, I have a few VFDs in my workshop now, and just purchased another one its a cheapo 2.2kw from HY, I intend to use this on my Drill press, I also have a 3kw one I'm going to install on the M300 at a later date, whilst playing about with the HY one, I noticed the front control panel clips out as a complete unit, it has 14 pin 2.5mm pitch IDC socket on the back of the panel and a corresponding plug on the VFD itself, my question is where can I buy a ready made 14 pin IDC 2.5mm pitch male to female cable, say 600mm long, this I will use to relocate the control panel onto the front of the machine, I know you can buy a remote pendant and or operate the VFD via switches, I just think this would be a neater and simpler solution.

Below is a link to a similar VFD with the cable!!

**LINK**

Shaun

You can get the connectors and cable from CPC. They are easy to assembly by gently pressing together in a vice.

Reference has been made to checking and certification of the installation of VFDs, one assumes that part P certification is being referred to here, my understanding of part P certification requirements are that if the equipment is connected to the electrical main by a simple 13 amp plug and socket then this negates the need for part P certification of that equipment. I agree with the comment that the installation should only be undertaken by what is deemed a “competent person” I have to admit that nowhere in any official documentation have I ever found a legally binding description of what constitutes a “competent person”. Perhaps someone could direct me to an official document that quantifies a competent person. The overriding point that must be paramount with any electrical equipment is if you don’t fully understand what you are doing, find someone who does.

Dave W

It means someone who has an official bit of paper that says they're a competent person. It simply means they can have completed a course and passed an exam. It says nothing about whether they actually understand anything.

Anyone certified to Part P should understand the rules and regulations regarding electrical installation. I wouldn't expect them to have any knowledge about the internal workings of a VFD, or necessarily how to connect up the output.

Andrew

My advice would be to buy from a reputable source.

I recently bought one to power a dual voltage engraving machine and the supplier, Direct Drives couldn't have been more helpful in explaining all the ins & outs. Their webpage has a lot of information but when I got stuck, they quickly sorted out the details I needed and the inverter has worked perfectly since.

I should add that I have no connection with them other than as a satisfied customer.

Brian

Edited By BDH on 03/08/2018 17:36:19

This is the advice of the IET from electrical.theiet.org/bs-7671/building-regulations/part-p-england-and-wales/faqs/

I read this as meaning you can install your own socket or wired in connection for an inverter to an existing ring. the inverter itself isn't subject to part P as it isn't part of the dwelling itself, any more than a kettle or cooker is.

Q5: What types of electrical work are non-notifiable?

The following types of work are non-notifiable:

• Replacing accessories such as socket-outlets, control switches and ceiling roses
• Replacing the cable for a single circuit only, where damaged, for example, by fire, rodent or impact (1)
• Re-fixing or replacing the enclosures of existing installation components (2)
• Providing mechanical protection to existing fixed installations (3)
• Installing or upgrading main or supplementary equipotential bonding (4)
• Work that is not in a kitchen or special location and does not involve a special installation (5) and consists of:
  - adding lighting points (light fittings and switches) to an existing circuit (6)
  - adding socket-outlets and fused spurs to an existing ring or radial circuit (6)

Notes:

1. On condition that the replacement cable has the same current-carrying capacity, follows the same route and does not serve more than one sub-circuit through a distribution board;
2. If the circuits protective measures are unaffected;
3. If the circuits protective measures and current-carrying capacity of conductors are unaffected by increased thermal insulation;
4. Such work shall comply with other applicable legislation, such as the Gas Safety (Installation and Use) Regulations;
5. Special locations and installations are listed below;
6. Only if the existing circuit protective device is suitable and provides protection for the modified circuit, and other relevant safety provisions are satisfactory.

Special locations and installations (5):

• Locations containing a bath tub or shower basin
• Swimming pools or paddling pools
• Hot air saunas
• Electric floor or ceiling heating systems
• Garden lighting or power installations
• Solar photovoltaic (PV) power supply systems
• Small scale generators such as micro-CHP units
• Extra-low voltage lighting installations, other than pre-assembled, CE-marked lighting sets

Note: See Guidance Note 7 which gives more guidance on achieving safe installations where risks to people are greater.

Q6: How will Part P apply to DIY work?

Part P will apply to all electrical work in dwellings, whether carried out by professionals or DIYers.

Some DIY work will require the submission of a building notice to the local authority and the payment of a building control fee.

Some minor electrical work will not be notifiable (see above). Examples include adding a lighting or power point to an existing circuit, adding a spur to an existing circuit or replacing a light fitting.

No such thing. The Competent Person requirement is rather like that terrifying 5-way interlocked mini-roundabout in Swindon. It puts the onus on the driver to avoid accidents by forcing people to drive carefully without making assumptions about who is right and wrong.

If you're a business selling a service, any service, you're responsible for analysing the risks to your customer and then defining what you mean by a Competent Person in that context. No bureaucrat can help you; it's the businesses responsibility to define, recruit and train suitable staff. Could be an electrician, or a computer programmer, or an accountant, diver, or aircraft load-master.

Customers can and should challenge businesses to show what they consider a 'competent person' to be. Ask the right questions and you soon identify suspicious providers. The good-guys have clear answers ready while the duds huff and puff.

Like roundabouts the system is far from perfect. Whatever the law says, there will be outfits smart enough to do the theory, but too stupid to apply it. There are people who copy someone else's documentation without reading it. Or a trainee does the work while the Competent Person is unwell. However, the point is to reduce risk not eliminate it : a partial answer to the problem of incompetent work is better than not bothering at all.

Dave

Edited By SillyOldDuffer on 03/08/2018 17:51:54

When I was still working for money they used to send a man round every now and again to PAT test all the 'portable' equipment. All he did was plug it into a box of tricks and if all green lights he'd attach a sticker and go on his way. When I asked him what he was testing for he maintained he was checking for earth leakage. He couldn't understand that for eqipment with a plastic earth pin (double insulated), the concept of earth leakage is a little difficult. Never even looked at the state of the cable, checking for damage/abrasion. Not a very competent peron in my book, and this was a big company.

Edited By duncan webster on 03/08/2018 18:02:54

Dave I like your reference to the “magic roundabout” I lived near Swindon and was around at its inception. My point about Competent Persons was that they are referred to in regulations but never defined.

Duncan in respect of PAT testing if you read the government schedule that relates to it there are two levels, the so called full electrical test and the lower level of a visual only check, the requirements for each are defined. It is not compulsory to carry out full PAT testing as some would have us believe, the relevant gov document outlines what and when is required. You seem to be of the opinion that it is a con and I concur.

Dave W

The trouble is that while you may think its a con in my experience any portable equipment I took to a customers site had to carry a current tag stating that it was fit for use, if it didn't carry the tag it wasn't allowed on site. Building sites were very strict on this down to extension leads and computer power supplies. The company I worked for had everything necessary PAT tested as they couldn't afford for us to be stopped.

Martin P

Many moons ago, I did a bit of PAT testing. The test instrument simply asked different questions as to the type of item being tested as it worked through its program. That included “is the item double insulated”, when the options were ‘yes’ or ’no’ and the reply took the instrument onto the next appropriate step.

The one problem item, I recall, was twin cored extension leads connected to earthed items!

All plugs and leads had to be inspected, as part of the check. Appropriate fuses were yet another issue, items often found with 13A fuses ‘protecting’ low current leads.

Of course, most new items come with a moulded plug these days, but not so, back then.

I think the relevant legislation is PUWER, Provision and Use of Work Equipment Regulations (or at least it was when I was still working), which require there to be a system in place to check that equipment is safe to use. I have absolutely no problem with this, but the guy doing it should know that you can't check earth leakage on a plastic earth pin, and he should check the cable, in fact I'd say the latter is the more important, as you can easily have bare conductors or damaged insulaton and not find either with a test box. Having the same regime for desktop computers which live in a nice friendly environmet and power tools used in a workshop is nonsense, but that's how some companies do it.

Just thought I would add that the IEE Regs wouldn't be involved in this as they mainly deal with fixed wiring installations. The competent person bit has been argued constantly in the electrical world. I've seen qualified sparks who are really good at their job and others I wouldn't let loose with a colouring book and crayons.

The rules (applied in line with the guidance and intent of PAT) differentiate between a computer that probably gets unplugged once or twice in its operating life and a power tool that may be unplugged a dozen times a day.

Way back when we had someone testing static equipment and putting 'do not use after' stickers on things like the photocopier's power lead. I insisted they use 'tested date' stickers so intelligence could be applied to what needed to be retested and when.

Neil

I'm afraid that you won't get electrical installations signed off by Building Control unless you can produce a certificate to show it was done (or at least signed off) by a Registered Domestic Installer. Just telling them you know what you are doing won't cut it. And if you don't get installations signed off by Building Control it is likely to be picked up by the buyer's solicitor when you sell the house.

To become a competent person seems to cost you £500 or so and requires practical assessment etc. In my case, although I've spent most of my career developing products like VFDs, chargers, power supplies and even RCDs, I am not considered a Competent Person. So I simply get it inspected, tested and signed off by someone who is. And to be fair, the fact that I've designed products doesn't necessarily mean I am capable of installing them safely taking account of the regulations that have been developed over decades as the result of much empirical and theoretical knowledge. You need to read up on the latest version of Part P and also the guides to how to apply it in practice to stand a chance of getting it right.

Murray