VSD's

[edexlab]

I'm looking to read up on the basics re VSD'S can anyone point me to some basic info on this , I have manufacturers booklets from work which are mostly about setting them up, but want a basic understanding of them from a grassroots level to help me understand how they work , I do understand the concept of how the motor speed is controlled by the output frequency etc but want to know more as the guys I'm working with know how to get them set up but seem to lack anymore knowledge than this and if there is an issue they just replace it.

 

[Besoeker]

I'll try.
Variable speed drives come in different flavours but you are obviously referring to variable frequency drives. VFDs

So the basics.
VFDs are used to control the speed of cage induction motors. I suppose you know that anyway.
How?
Well, by changing the frequency for one thing. A standard 4-pole motor on a 50Hz supply will run at about 1485 rpm. You probably know that too.

OK. reduce the frequency sent to the motor and the speed will reduce.
So that's the first part of strategy. Changing the frequency.

But you can't do that without changing the voltage too. Usually in proportion to the frequency.
50Hz, 400V. 1485 rpm
25Hz, 200V, 742 rpm.

Constant v/f ratio which is the common method of operation.
The why is just a little more complicated and perhaps not......
But I'll give that a go too.
Motors, like other wound magnetic things like transformers are designed to operate at a given flux.
Outside that region you get saturation and huge currents.

Flux is, at a basic level, volts times time. A volt-second area. If you reduce frequency you increase time. So, if you reduce frequency you also have to reduce volts to keep that volt-second area and thus flux, the same.

So voltage and frequency are proportional.

How does it get done?

The circuit arrangement.
Three main parts.

Input rectifier.
The input voltage is rectified. For three phase 400V that produces about 540Vdc.

Then DC link.
A big smoothing capacitor. Better VFDs will have input reactors and a DC choke.

The 540Vdc is presented to the third part.
The inverter.
This takes the Vdc and chops it up into three phases.
The output voltage is a series of pulses with an of ratio that equates to a sine wave. Low frequency is more off than on.
The inductance of the motor results in a reasonably good sinewave current.

Confused yet?

 

[dingledong]

Edexlab I think that deserves a thanks even 2 thanks for that really good effort

 

[edexlab]

I agree I didn't need as much of the theory personally but I'm sure it will help other people !
Cheers Besouker btw did you get any sleep last night!

 

[Besoeker]

I agree I didn't need as much of the theory personally but I'm sure it will help other people !

Sometimes difficult to know how to pitch it especially if you don't know what or how much the person already knows. Or doesn't.
I tried.
I sometimes try to help my wife with dressmaking. To the extent of converting units.
She is American and sometimes the patterns come in Imperial units - inches, yards. The material bought here is generally metric (SI) and is in metres and centimeters. I'm old enough to be familiar with both.
Same for cooking which she is very also good at. The challenge there is when units of volume and weight are mixed.
Her Teriyaki salmon is to die for.

Cheers Besouker btw did you get any sleep last night!

Besoeker if you don't mind!!

A little background.
I was commissioning some of our variable speed drives in South Africa. Gold mines so the security was a bit tight.
My security badge had visitor on one side and besoeker on the other. Afrikaans.

It was on my desk when I started posting and I thought it appropriate. Visitor.
We are all visitors. We are around for a while. We learn. We can offer or impart knowledge. Make friends. Irk others. Respect, envy, procreate. Enjoy what we have. Or despair because of what we don't

But our tenure is a limited term. Hence visitor.

 

[edexlab]

Ah I did wonder about the origin of your forum name

Thanks again for the reply much of it was very useful to me, I just found it frustrating that being new to using these drives nobody here could really explain much about them , it originated while commissioning a system in the factory before its stripped down and shipped , a vsd was tripping out, the error code said phase missing , it's been working and then had this problem, I was asked to check things through as a bit of a training exercise ie connections,continuity through relays etc and on the final circuit which showed a loose connection on the winding side of a fan terminal block, the fault continued for various times from 30s to several minutes, so it was changed for a new one , I thought someone might have checked the set parameters first to be sure it needed changing, I like to be sure I'm changing something for the right reasons as one day it may be a case of doing it on site and as the work is global this could be expensive!

 

[tomfj]

Schneider have a great site for giving you loads of info to study at your own pace with other topics other than electrical as well ,just search for Schneider energy university ,it has info on anything energy orientated HVAC vsds compressed air building automation boilers etc.it explains a lot of the parameters you input into a VSD just because they're on the sheet but don't know why

 

[edexlab]

I actually registered last week but haven't had chance to login and take a look yet , I got the impression it was mainly covering energy efficiency but I will have a look see.

 

[Besoeker]

I actually registered last week but haven't had chance to login and take a look yet , I got the impression it was mainly covering energy efficiency but I will have a look see.

Promoting product.
That's what manufacturers do.

 

[edexlab]

Yes as per usual just had a skim through now but there is some useful info also but I was right about the energy efficiency aspect .

 

[Marvo]

https://www.rockwellautomation.com/...Basics_practical_operation_Motor_starting.pdf