VFD -under volt at source fault

[Gareth Blackstock]

So earlier this year I obtained a large machine with a couple of 3 phase motors in it, and the rest single phase. I bought a VFD, correctly wired it up, and while the machine ran with some functions, it also reported "undervolt at source"

I had bought a VFD with a maximum output of 380 volts. Checking over the machine wiring, I found it had 415volt circuit breakers. However, the biggest draw from the 3 phase motor I found was .75amp. Perhaps I mistakenly believed the 380volt VFD would still run the machine.

So, does the "under volt at source" report on the VFD state I do not have 240volt at the source, or is the VFD indicating I need a higher voltage output VFD? The source is a 15 meter extension lead plugged into the house, the machine is in the shed. Would this distance be enough to lower the voltage at the shed end?

I found a higher rated VFD for sale locally, a 4kw, 6hp, 415volt. Would this be the solution?

Hopefully there are some knowledgable VFD people here?

Thanks, Gareth

 

[Gareth Blackstock]

I just now found the specs of the VFD I originally bought, it is rated as 220volt output

• 
  Model: A2-8075
• 
  Power: 7.5 kW
• 
  Input Voltage: AC 200-260V
• 
  Output Voltage: AC 220V
• 
  Input Current: 35A
• 
  Output Current: 50A
  
  Output Phase: 3 Phase
  
  Horsepower: 10 HP
  
  
  
  
• So definitely under powered I think...
  
  
  
  
  
  
  

 

[DPG]

Doesn't look suitable to me.

Also, is the socket in your house (and the extension cable) capable of supplying the necessary current??

 

[Gareth Blackstock]

Doesn't look suitable to me.

Also, is the socket in your house (and the extension cable) capable of supplying the necessary current??

 

[Gareth Blackstock]

Hello,
I am confident the power to the shed is suitable as i run everything in the shed without issue, from several banks of lights, compressors, welder, etc. It would be normal household amperage...

Perhaps if i had a sparky wire in a 30amp line from the house meter box and run it to the shed... lots of money for unknown result to my issue...

 

[snowhead]

Is the machine running O.K and doing what it's supposed to?
It's voltage of the motors that matter not the voltage of the breakers.

You'd need to check the voltage at the input to the VFD whilst the machine is running,
Then check the setting in the VFD for Input / Source voltage to see if the alarm level is set too high.

 

[Gareth Blackstock]

The machine is running at half capacity. Not sure why, i assumed this was because not enough voltage getting through.

I will check the inputs into the vfd, and check if thr vfd alarms are set too high.

Thanks for the tips....

 

[James]

it looks like you have a misunderstanding of what a vfd can do.
it converts you 240v ac into dc at 240*1.414=339v DC

then it chops that dc voltage up and converts it into 3 different sine waves (phases)
unfortunately it cant increase the voltage so you end up with 240v phase to phase voltage

a 400v output vfd will normally have a 3phase (or at least 2 phase) input of 400v ac that it converts to a dc bus voltage of about 560v to 580v

it can then chop that up to generate a 400v 3 phase supply to a motor.

a single phase in 240v vfd is used to run a 3 phase 240v motor, it will not run a 3 phase 400v motor at anywhere near its rated output.

what you need to get is a rotary phase converter, this is basically a motor and a generator combined in the same unit/
it can step up the voltage from 240v to 400v however your current at 240v input will be many times what you get to the machine.

example
10kw 3 phase 400v machine
current per phase (assuming perfectly balanced phase currents, 100% efficiency and power factor of 0.85)
17 Amps per phase
as long as the distance is not to far it could probably be supplied with a 2.5sq mm cable.

if we then convert that to a single phase supply
assuming a 95% efficiency in conversion

it works out at around 52 Amps and would require 10sq mm cable at the minimum.

this is not something i would expect to run from an extension lead down the garden.
you have not stated the power rating of the machine but you could scale it from the above calculations.

i.e. if your machine is only 3kw then the single phase current will be 30% of above 52A x 0.3 = 15.6A

 

[Gareth Blackstock]

Thanks for explaining the workings of a VFD... and you are right, I did misunderstand how they work. So looking at your calculations and examples I reckon the cost of getting a phase converter is prohibitive considering this machine is going to be for non-business use. Bugger.

At the moment the machine is connected and runs well on 240 volt, the breakers for the 3 phase motors have been tripped and are not connected. I think I will forget the VFD route and simply re-wire for 240 volt use. later on I will replace the 3 phase motors with single phase.

Thanks all for helping me out,

Cheers, Gareth

 

[Darkwood]

You cannot simply replace the 3 phase for single unless you have the available power supply, the requirements on your supply with still be as heavy on supply demand, you are trying to run a 3ph motor with a designed torque output to suit the machine off a single phase supply, this will heavily burden the single phase supply given it need to achieve a 3phase equivalent however you attempt to do it, swapping out a 3 phase motor for the equivalent torque output single phase version will also require more input current to achieve the same output torque in a much less efficient way.
Example -
You have a 7.5Kw motor (10hp)

A 3 phase 400v motor would have a full load current demand hovering around 15amps per phase
A 1 phase 230v motor would have a full load current demand hovering around 53amps on one phase.

If you cannot run this through a VSD because of power restrictions then you certainly cannot swap out the motor to a single phase

It also likely your motor is a 3ph 400/690 star/delta hence it is running at a massively reduced torque output as the windings are been supplied half the voltage at full frequency, had it been a smaller motor it may have been a solution to alter the winding arrangement as in a 230/400 volt motor but the motor itself would probably be too small for the machine designed torque requirements.

It seems that you simply are trying to run a industrial designed machine off a domestic supply and unless you have that available power your only other option excluding the one already given is a stand alone generator to provide that power, if this is a trial of curiosity then try hiring a suitable generator to test the machine with.

 

[Gareth Blackstock]

So would your calculations restrict replacing even a very low amp 3phase with a single phase? When I initially queried this forum last december with the prospect of replacing the motors, I received some positive feedback on replacing my low amp motors. I got the impression that if the 3phase motor is a low enough amp, then a single phase can safely cover it. The machine I have is not a heavy power consumption user, I am guessing the 3 phase motors are to ensure consistent running speed as the machine processes cine film, and any variation in speed will effect the final product.

At the moment the machine is running 240volt house mains on bare essential motors, without blowing fuses or dimming lights. I was planning on wiring up the remainder motors on a separate circuit with their own switches.

As I am not a business I have to explore work arounds. I have tried getting sparkies to attend and look over the machine but I am too far out of town, and it seems not many electricians come across VFDs to offer opinions.

This thread was my original, and lists the motor specs.

Adapting a 3 phase industrial machine to single phase

Hello, I am hoping this is in the correct forum. To start, I acquired a film processing machine that runs on 3 phase. Now before getting the machine I fully understood it's power requirements and the cost of getting 3 phase installed at my house. I figured as processors are mostly a few...

www.electriciansforums.net

 

[Darkwood]

I didn't design the machine but it clearly had a 7.5 Kw (10hp) motor fitted for its needs, unless you are utilising the machine for different purposes to its design which in turn reduce the load requirements then you will need the same torque output as before which will reflect the issues I expressed in previous post.
You are also drip feeding information to us which may be crucial to the advice we give, you now express that variants in speed has a detrimental effect on product, this would then indicate why a VSD has been used, it is likely if speed is so crucial the VSD control wiring maybe looking at a closed loop feedback system to ensure speed to maintain as loading on the motor varies throughout the process, if may also account for a larger motor been fitted than expected so load variants are easily dealt with by the motor without it passing it through as speed variants if it does not have closed loop system.
The type of questions I need to ask you simply won't have the understanding to answer with all respect and is also why electricians you call in are somewhat in the shade, this is electrical engineering type of knowhow and not your general sparky if you get me, the forum provides info only based on the info you provide, if you miss out info' you never considered to be relevant or simply didn't know then the answers the forum give may not be the correct ones for your enquiry.

 

[Darkwood]

I will note here, the link you provide to a previous discussion shows much smaller motors than the VSD you have bought, you can oversize a VSD and de-rate for the motor in programming it up but you will run into operational issues if the VSD is several orders bigger than the motor it covers, why have you got a 7.5Kw VSD?

I took that you were controlling a 7.5Kw motor with it which now reviewing your previous posted thread I question is the right drive at all.

 

[Gareth Blackstock]

You have not read my replies, I did not say the machine had a 7.5kw motor fitted, I said I had bought a 7.5kw VFD originally. I did not drip feed information, I was taking a guess that the electrical load of all the machine’s motors are quite low, perhaps the use of 3 phase was to ensure no detrimental effect was caused by power drop that would affect the final product. I never said the VFD was attached for this purpose.

I think you have read my replies too quickly or are putting 2 and 2 together and coming out with 5. I initially bought what I thought was the right powered VFD, and as it was reporting “under volt at source” I considered that my house supply was not offering enough amps, so I simply wired the machine to 240v with the intention of replacing the very small load 3 phase motors with single phase, and running other motors manually when needed.

I recently saw a higher powered VFD for sale locally and that’s why I posted the question, to get advice if the higher powered one would solve my initial under volt question.

I have provided as much info as I have been able to, as the machine has no identifying spec plate, which ever electrician worked on it last has left no details of anything. So I have had to try and work it out backwards, looking at the motors and tracing the wiring back to the controllers, panels, and switches. I have even contacted the people who built the machine, they also were unable to offer any paperwork, I have contacted businesses who use similar machines, no paperwork there either. So apologies if I have been light on the info, and apologies for being too simple to answer your questions, as it seems I will get this machine fully running without your immense help.

I will also note here, I have been repairing cine film cameras for a number of years, and frequently assist people with their repairs from across the globe, and it seems I understand better than you, that people who don’t work on cameras are ignorant of cine camera terminology…. And that’s o.k, that’s why they are asking me for help.

Don't bother responding, I won't return...

Thanks to the other members for helping.

 

[DPG]

Out of interest, what type of circuit is your outbuilding fed from?

Interesting that you repair cine equipment. Must be quite a niche market now that. Can you still get 8mm film processed these days?