Inverters? Talk to me.

Most people I am aware of who have converted a Boxford have fitted a bigger motor. Do look at fitting it carefully though.I fitted a 1HP to my ME10 and just about managed to get fixing holes on the motor mounting plates. I only looked at frame size which was just about ok. I didn't check that against where the mounting holes would need to be. Some of my fixing blots had to be high tensile M4 which shows how tight it is,

The reason for going bigger is so that the motor can be slowed down and still provide the torque that was available with a belt reduction to the same speed. There is a catch however - motor cooling limits how slowly they can be run and significantly loaded for any length of time.

What I did to get round that to a certain extent was to fit a 6 pole motor but that means to get up to the usual speed it has to be driven at 75Hz. The manufacturers are very coy about what they reckon can be done but in this case they indicated unofficially that they doubted if it would be a problem. This was Brooks. If something went wrong they would deny saying it. Anyway I set mine to drive from 40 to 80Hz which should be bomb proof. It's easy to put the back gear in if I need low speeds.

It's possible to find a TEK catalogue that shows what they think their motor inverter packages can do via graphs. There is another one that doesn't give this level of detail which usually pops up in searches. All makers are more or less the same really other than dedicated inverter drive motors which up the costs a lot. I think there was an interesting post recently concerning the UK TEK agent. Maybe you should read it.

What do I think now I have done it? Mixed feelings largely down to noise. They are loud. The random switching in my Mitsubishi inverter helps a hell of a lot but it's still a lot noisier than the original single phase motor. I understand something can be fitted between the motor and inverter to fix this but have never managed to tie it down.

If I look at it logically as the ME10 has a countershaft it's easy to change speed so why do it. The back gear also adds more flexibility. I did feel the need for a more powerful motor though. I managed to let a bit of the smoke out of the original one. That was an interesting experience. I noticed a bit of a smell so stopped. The smell got stronger and stronger and eventually some smoke appeared over a period of at least 15min. It takes a long time for the heat to work it's way through the motor and a extremely long time for it to cool down again. The heat came about by simply reaming a rather large hole with a so so reamer circa 1 1/2 dia - should have put the back gear in.

John

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Shaun

You can use a 3 phase 415V input VFD to replace the existing starter and leave your motor connections as they are, for convenience simply fit the speed pot close to the VFD unit.

Any electrical equipment installed within spray booths needs to be specially rated to prevent explosive vapours being ignited so any non rated VFD will have to be mounted in a safe area away from the booth.

Emgee

Edited By Emgee on 09/03/2016 13:46:24

Shaun/InvoluteCurve,

Inverters are not really inverters - they're VFD's (Variable Frequency Drives) (the inverter is the back end of the VFD, the transistors that take the DC current from the converter/capacitor bank and switch it back to a rough AC wave)

also could be called variable speed drives, adjustable speed drives adjustable frequency.... you get the point. If you looking for one cheap, its worth checking all these names online when searching the classifieds.

I believe you just want a variable frequency drive with 3ph in and 3ph out - these are often the cheapest second hand since they're more commonly used in industry and worth less to the general public - if you see a nice inverter going cheap, its probably becuase its 3PH in and/or its 415v.

Buff up on your ATEX ratings to get one with the correct internal/external explosive atmostphere rating (any ATEX is not good enough, it must be the correct zone1/2 ratings) - or cheaper still, put it inside and ATEX rated enclosure and use ATEX rated cable glands etc). I dont know if you can buy ATEX inverters due ti the nature of the job they do.

Edited By Russ B on 09/03/2016 13:56:51

Hello John,

The device to fit between the VFD and motor to reduce noise is a three phase choke, that should iron out some of the harmonics from the output. They are primarily installed to reduce leakage on long cable runs that can cause false tripping.

Increasing the PWM frequency can also reduce noise in the motor. I have Allen Bradley drives installed on three machines, the motors do not make any discernible drive related noise.

I remember past VFD installations where the motors rang with a pretty unpleasant noise. I don't think it is as much a problem with modern devices.

Michael

Edited By Michael Briggs on 09/03/2016 13:55:33

Quite. For continuous use at half speed Marelli rate their inverter motors at about half power as a result of lack of cooling. In other words the same torque as at full speed. You can of course install an external electric fan to supplement cooling but I haven't found it necessary with my intermittent use.

Best to retain the mechanical (belt or geared) speed change for times when you want extra torque.

Russell.

I think you will find that just putting your non Ex rated vfd into an ATEX approved enclosure isnt sufficient in terms of certified safety.

The cert for the enclosure is purely for that, that is on its own with no added internals.

To have the whole assembly certified costs a lot of money due to the testing needed to ensure the criteria are met.

I know this as I work in the industry...and have gone down a similar route for some Ex rated equipment for a client.

Best solution is mount your vfd as far as is practical from the booth, take into account cable lenghts etc..

And if your motor/fan is mounted in the both it should be ATEX rated....

Most of the above would apply to a business, in which case you may want to have a look at.      ...http://www.hsa.ie/eng/Topics/ATEX_and_Electrical_Apparatus/Atex_Regulations_-_Frequently_Asked_Questions/

If its a home job, then consider your own personal/ equipment safety....but you know that already...

Edited By John Rudd on 09/03/2016 14:17:02

Shaun, some invertors can be programmed with set points so once you find the speed for low flow a simple switch will select it. I think if you speak with some of the suppliers they will be able to suggest a suitable piece of equipment for your needs. It seems to me to be an easily implemented application for a VFD.

Martin

In answer to RussB, I wouldn't be without the variable speed. I just leave the belt on middle speed and ramp it up and down with the VFD. Mitsubishi inverters have an output which you can just feed into a moving coil meter which tells you the % of top speed, so a simple table converts that to spindle RPM. Be careful as the output is at a highish voltage. Better is a tachometer gismo from e_bay, very cheap.

I made my own tacho for the Centec as the pulley on the back is geared to the spindle so it had to have a factor in it to compensate

Edited By duncan webster on 09/03/2016 14:43:37

John Rudd,

Perhaps you're right, I don't work in electrical, (I'm in the Mechanical side of R&D)

I would guess if we design a new product, its electrics are standardized by the electrical department and then they have that cabinet tested/approved for mass production etc Hence, I don't see that side of things - we just do as we're told =)

Edited By Russ B on 09/03/2016 15:07:42

I think it is a good application for a VFD. Many VFDs can be programmed with set point speeds so a simple switch would be all that is needed to go from full fan to low fan speed. I would suggest contacting some suppliers and asking for their recommendations.

Martin

Hi Michael. I could increase the switching frequency but was a bit concerned about eddy currents and bearings etc. Another thing they take care of on inverter rated motors which also have a separately powered cooling fan.. I knew they were chokes but haven't found anything suitable. On ebay at the moment there are some but these appear to be for noise reduction etc at 50 or 60Hz. My set up isn't as bad as some I have heard. The random switching helps a lot.

If anyone has any links to the correct beasts at reasonable cost I would be interested.I'd guess that the prices might be unreasonable in my case.

Noticing a comment about 1/2 speed half power same torque so power is reduced by the speed reduction take care that's how they all behave who ever makes them. The inverter messes with the voltage to contain the current to what it would be in normal use. Power is I^2 R so unfortunately heating effects are more or less the same and the fan is running at 1/2 speed. There must be some more due to eddy current losses in the magnetics too but it seems these can be disregarded. I do have a motor I intend to fit with a separate fan but as there is little need I don't want to risk it on my lathe. My DW mill doesn't need it as it's even easier to change speed.

Mentioning DW there was some one about who had phoned a supplier wanting to get rid of both pulley speed changes and the "back gear" in them. Supplier fine, fit one of my 2,800 rpm motors and one of my inverters and it will do what you want. There are a number about like this. Fortunately the punter found that TEK catalogue I mentioned and then got extremely annoyed saying inverters are hopeless etc. Prior to finding the catalogue he wasn't really prepared to accept what people told him concerning torque and heating effects.

John

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Hi Involute Curve

If the spray booth you refer to is in someone shed then what your suggesting is doggy to say the least, but if it's in an industrial environment then I suggest you you seek expert advice from someone who is conversant with electrical equipment in class 1/2 zones

H

yep it is a class 2 spray booth with water wash curtain and filtered incoming air, and air fed masks etc etc etc...... all electrical equipment is installed outside this booth, the inverter or VFD will be installed by a qualified electrician, the fan / motor was removed from a much larger booth the motor and fan are separated, hence the belt drive system, I just wanted a bit of advice on which type of VFD best suited this motor, (the 380V question was more a curiosity) in the past I've fitted VFD's to my lathes and milling machines but that's at home with single phase input supply, a simple reset of the jumpers and or resoldering the motors tails to Delta format inside the motor to suit this............

anyhow thanks for the advice.

Shaun

Hello John,

I would try to keep a change in switching frequency away from the limits of the settings to play safe. Allen Bradley recommend de rating the drive at higher frequencies.

I tried to paste a de rating graph but was told my posting was about 180,000 characters too long! Best to have a look at the manual for your drive.

If you get a situation where you think the motor has got too hot it may be better to run it unloaded and let the fan cool it rather than stop it dead.

A useful thing to do is set the drive display to indicate current for a while so you can see how the motor is being loaded.

Regards, Michael

Edited By Michael Briggs on 09/03/2016 18:04:55

The manual for my IMO cub has a little chart that says upping the frequency from 0.7 to 15kHz )(i.e. beyond what we more mature folk can hear) will: reduce noise, improve the current waveform, increase leakage current and increase electrical noise.

They then elucidate that lower frequency increases harmonic components of the output current waveform and increase the temperature of the motor, they say if driving at 0.75kHz, derate torque to 85%.

Raising the frequency increases power loss in the inverter, raising its temperature, but they state internal functions compensate and protect against this.

This would seem to suggest a middle-range frequency is a good start. The default is 1KHz,

It also has different sound tone settings that can reduce offensive noise without changing the frequency (if below 7khz) Guess this is similar to your random switching and uses asymmetric or irregular switching patterns.

Neil

I don't have a significant problem of any sort with my inverter set up but I'd love a link to a suitable choke to fit especially if it costed a tenner.

John

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Got mine from Drives Direct, full story here http://www.model-engineer.co.uk/forums/postings.asp?th=112156

Hywel

Edited By Hywel Evans on 09/03/2016 23:08:55

I wouldn't increase the maximum frequency, If possible, it's meant to run at it's default maximum for a reason, to ensure a long life. If your inverter also has a fan then this makes it a little easier to justify. If your simply increasing the frequency to ensure that the fan is up to speed, this isn't strictly necessary, you could always fit an additional independent fan onto the motor which can operate constantly regardless of your motor speed.

Michael W

Well I've been on a researching binge and it doesn't look at all difficult so I've gone ahead and ordered an inverter! I went with the invertek optidrive e3. Looks simple, lots of instructional videos on YouTube, fairly inexpensive and it's made in UK! I just hope I haven't overlooked anything?

Motor: Brook Crompton, star or delta wiring, 3/4hp, 50hz, 2.4a, 1425rpm

Inverter: Invertek Optidrive E3 0.75kW 230V 1ph to 3ph

The only parts that looks tricky is the reversing switch, and I seem to have 3 white wires going into the motor? Not sure how I'm gonna know which is which?

You will not be using the reversing switch on the output of the inverter. The three wires from the motor go straight to the inverter output. It does not matter which way you connect them at first. If the forward and reverse are the wrong way round just swap over any two of the three wires. You will probably have to change the links in the motor connection box to change it from star to delta configuration. Post a picture of the connection box and members of this forum will tell you what to change. If you still want to use the old reversing switch it should be possible to use it to control the low voltage control signals to the forward and reverse control inputs on the inverter.

Les.