Calculating inverter for motor

Quang_Lo · Oct 4, 2014

Dear All,
I have a problem. It's calculating inverter for 3 pharse non-synchronous. I mean that i don't know the formula. My colleagues show
Here we go :
P[SUB]1[/SUB]/P[SUB]2[/SUB] = (n[SUB]1[/SUB]/n[SUB]2[/SUB])[SUP]3

[/SUP]Please help me out
Thanks

Marvo · Oct 4, 2014

If the motor is driving a pump or a fan impellor them read up on the Affinity Laws to calc correct inverter specs.

Besoeker · Oct 4, 2014

Quang_Lo said:
Dear All,
I have a problem. It's calculating inverter for 3 pharse non-synchronous. I mean that i don't know the formula. My colleagues show
Here we go :
P[SUB]1[/SUB]/P[SUB]2[/SUB] = (n[SUB]1[/SUB]/n[SUB]2[/SUB])[SUP]3

[/SUP]Please help me out
Thanks

Rate the inverter for the motor being used.
It really is that simple.

SheffSimon · Oct 7, 2014

Besoeker said:
Rate the inverter for the motor being used.
It really is that simple.

It really isn't always that simple, length of cable run and other factors can also affect the choice of inverter size.

LeeH · Oct 7, 2014

SheffSimon said:
It really isn't always that simple, length of cable run and other factors can also affect the choice of inverter size.

Can you please expand.

I always spec a drive on its load and its available functions for task in hand.

The cable spec is done as normal.

Cheers.

Besoeker · Oct 7, 2014

SheffSimon said:
It really isn't always that simple, length of cable run and other factors can also affect the choice of inverter size.

If the motor is rated at 100A, would you really rate the inverter differently?
I wouldn't. But then I have only been in the VSD field for forty some years.
Maybe I'm just a slow learner.

Bobster · Oct 7, 2014

The only real effect on a drive with cable length is if it needs line filters for EMC compliance. Otherwise as Besoeker has stated, match the dive for the motor size.

Most drives will have 2 ratings heavy duty and normal duty, pick which is suitable.

Cable size is then specified using the normal methods.

SheffSimon · Oct 7, 2014

RoB2 said:
The only real effect on a drive with cable length is if it needs line filters for EMC compliance. Otherwise as Besoeker has stated, match the dive for the motor size.

Most drives will have 2 ratings heavy duty and normal duty, pick which is suitable.

Cable size is then specified using the normal methods.

Currently commissioning an MCC fitted with ABB ACS 800 drives, some of which ABB oversized wrt motor rating plate because of long cable runs.

LeeH · Oct 7, 2014

SheffSimon said:
Currently commissioning an MCC fitted with ABB ACS 800 drives, some of which ABB oversized wrt motor rating plate because of long cable runs.

Can you please explain why?

Besoeker · Oct 7, 2014

LeeH said:
Can you please explain why?

To make more money maybe.

SheffSimon · Oct 7, 2014

It was because they were very small (0.18kW) motors on long (300m) cable runs. Something to do with cable charging currents from memory.

Besoeker · Oct 7, 2014

SheffSimon said:
It was because they were very small (0.18kW) motors on long (300m) cable runs. Something to do with cable charging currents from memory.

Chokes would sort that out.

Darkwood · Oct 7, 2014

Motors with excessive torque demand on ramping start/stop set-ups may require larger inverters to avoid shock load tripping issues, I know 2 brands that recommend this, also excessive start/stop duty may warrant a larger inverter too.

2 reasons off the top of my head for getting larger inverters than motor rating.

Marvo · Oct 7, 2014

Occasionally our choice of inverter will be driven by a requirement for a wider range of carrier frequencies than normal.

Frequency of stop/start cycling would be an issue that we'd address with uprated forced ventilation rather than oversizing.

LeeH · Oct 7, 2014

Don't understand how you would get excessive start up current with a correctly designed and set up system.

Not saying you cannot, but just cannot see it in my experience of moving heavy loads with VSD's.

Darkwood · Oct 7, 2014

Marvo said:
Occasionally our choice of inverter will be driven by a requirement for a wider range of carrier frequencies than normal.

Frequency of stop/start cycling would be an issue that we'd address with uprated forced ventilation rather than oversizing.

Thats why I used the wording 'excessive' and 'may warrant', as you show other solutions exist but situation dependent as to best solution, I always check with tech' for best solution for my set-up.

Darkwood · Oct 7, 2014

LeeH said:
Don't understand how you would get excessive start up current with a correctly designed and set up system.

Not saying you cannot, but just cannot see it in my experience of moving heavy loads with VSD's.

I experienced it with high gear ratio sets with very short ramp times where drive dectects instantaneous over-current or shock load on motor during acceleration or deceleration, in some cases the monitoring circuits of the drives are very effective and can be too keen and when ncreasing ramp times or voltage boosting is not working the manufacturers recommend upgrading to larger drive.

This is a ramping related issue and I know of 2 drives that can give this problem, both tech' support suggest larger drives, I believe there is a fine balance between the trade off of protected outputs against short circuits and trip logic set-ups .... the better output protection you get the more chances that some arrangements may give a false trip.

SheffSimon · Oct 8, 2014

Besoeker said:
Chokes would sort that out.

Tried chokes, they didn't make an appreciable difference.

Besoeker · Oct 8, 2014

SheffSimon said:
Tried chokes, they didn't make an appreciable difference.

Appreciable difference to what exactly? What problem were you trying to fix?

If it's charging currents that are deemed to be the problem, and that's not an uncommon problem with PWM inverters, the fast switching edges that cause it are there regardless of the inverter rating. Oversizing the inverter won't change that.

You know the i=Cdv/dt thing.

Here's a measurement I took on a system where the fast edges were causing untimely motor failures:

The major divisions on the horizontal axis are at 2us intervals and the minor ones at 0.4us.

We are currently in the build process of a couple of VSDs that have a 300m cable run. We had the option of going for du/dt filters or sine wave filters. There was a significant cost difference which of course we highlighted. The customer opted for the latter.

In both the real life examples I have given, an increase in drive rating was never a consideration as a fix for cable length.
In fact, in my fairly long experience, it never has been.

But, as ever, I'm prepared to learn even if I am beyond my sell-by date.