View Full Version : VSD Compressor

Fridgy Dave
08-08-2011, 06:21 AM
Hi All,
I have a Co2 rack that keeps lossing comp 1 to a VSD fault, the alarm comes up as overcurrent fault then shuts all the cases down in stages. Just wanted to know what causes the overcurrent fault?

750 Valve
08-08-2011, 01:09 PM
Where do you start....

What is the minimum hertz? Could the comp be stalling? What is the max hertz on the drive? Ramp up time? How close to FLA is it when operating in its normal window? What is over current set on the drive? Is drive setup for constant or variable torque? Could it operate outside its normal operating window? Is there a chance it could be liquid locked (trying to compress vapour into a discharge line or oil sep full of condensed liquid)? Bitzer comp? Emerson or Danfoss drive? How many LT comps? Subcritical I assume, DX or liquid recirc rack?

08-08-2011, 06:15 PM
Hi All,
I have a Co2 rack that keeps lossing comp 1 to a VSD fault, the alarm comes up as overcurrent fault then shuts all the cases down in stages. Just wanted to know what causes the overcurrent fault?

When was it Commissioned?.

09-08-2011, 03:25 AM
Hi Dave
as per 750 valve, check everything. What ever which way the drive is telling you there is a problem somewhere. Either the drive or the compressor.
Had a similar problem with an intermitent fault on a danfoss drive on a screw compressor, finally failed and the processor card on drive was faulty.

Fridgy Dave
09-08-2011, 04:14 AM
Thanks 750 I will check all of these and get back about it

Fridgy Dave
18-08-2011, 07:00 AM
The system is a dx system with 3 comps, it's an Emerson drive and it operates between 35 and 55 hertz.
The system has been having several issues due to high discharge trips, cases have been staging down then coming back due to flexible combiner and I think this might be related.
I think that the valves on the condenser heat exchangers aren't set up correctly and that once they have shut down they take to long to recover and the system faults on discharge, the head pressure doesn't seen to be constant as specially in times of lower load. Is this something someone has come across before??? The valves are alco electronic valves on the two heat exchangers

750 Valve
22-08-2011, 01:42 PM
Just as a matter of interest, how high is the discharge temp? Is there any desuperheating before discharge lines enter HX's?

I personally think this type of rack design is a PITA, the loads fluctuate far too quickly and the result is poor EXV control. The Emerson controllers seem to struggle more than the Danfoss EKC316A's as well. The newer design where the discharge is immersed in the liquid receiver and the vapour is taken from the top of the receiver to the HX's provides far smoother operation. Anyhow, that doesn't help you in this instance...

Have you checked actual logging of amps on E2 (I haven't done one in a while but am pretty sure the drives as displayed on the E2 can log amps).

Do the excess current trips coincide with the high head pressures? If so then this could be the major contributing factor - a lot more input power is needed to get up higher in the comp ratio. What is comp FLA and what is drive settings for current trip?

Also do you have any further heat exchangers or accumulators on the high stage rack(s)? If so can you afford to flood back a little now and then, by this I mean if you slow the alco valves down and they happen to overshoot every now and again and slop some liquid back is there something to catch it? I have seen these racks get into a hunting scenario and the end result is an elevated Co2 condensing pressure. I had success in relocating coil out probes in the past to nothing below 9 and 3 o'clock, to be a little less sensitive to liquid out of the HX's.

750 Valve
22-08-2011, 01:51 PM
Also make sure VSD comp ramp up time is short - like 2 seconds and under.

27-08-2011, 01:47 AM
Hi Fridge Dave.
how did you get on with the VSD, to add what 750 said, if you can in the motor input set up, input motor voltage higher, say 460 VAC, and the VSD will fire higher starter voltage and get motor started quicker.