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.