Testing variable speed drive installations

Thursday, 2nd July 2009

Testing Variable Speed Drive Installations

On variable speed drive systems there is usually a regulator fitted specifically to serve as a safety valve in the event that the variable speed drive fails. This is the safety valve that the flow chart refers to when testing variable speed drive systems.

When the flow chart refers to "Record as =STD" you have two choices of recording and either are acceptable. When recording this, first you need to go to the Standards Manual and look up the standard for reserve at receiver for the number of clusters the machine has.

Now you have two options:
  1. You can simply write < "number" or >="number" in the box indicated on the flow chart eg for a 20 cluster machine you would write in either <600 or >=600 depending on whether or not the vacuum changed.
  2. Alternatively you can just write in =STD, also depending on whether or not the vacuum changed.

So going back to our 20 cluster example above. If you allowed in 600 litres through the air flow meter and the vacuum dropped you would write in either =STD or >=600.

The choice is yours.

The reason we allow the standard reserve air in through the air flow meter is to determine whether or not the machine has adequate reserve. The assumption is that if the vacuum doesn't drop when you open the air flow meter to the standard reserve air figure then the machine has sufficient reserve. If the vacuum does drop then it is likely that there is inadequate reserve. We do this because when on variable speed drive we cannot determine what the actual reserve figure is.

I've been asked if it's OK to first allow in the standard reserve figure to determine if you have sufficient reserve but then allow in more air until the vacuum drops so that you can determine how much reserve there may be. While this is OK to do, under no circumstances should you state categorically that you have measured the actual reserve for this machine. All you have done is try to indicate how much air can be allowed in before the drive begins to fail to respond adequately.