Variable Speed Control

[O_E]

Hi Every Body,

I am very glad being a member of RE forum.

I have a question: I want to change my 3 kw scroll compressor to VARIABLE SPEED one.
Firstly I will add frequency converter to change main line frequency of the compressor.
But I don't know how can I control the system, P, PI, PD or PID ??
And what kind of hardware I need for this control ??

Thanks so much.. OE

[nike123]

Hi Every Body,

I am very glad being a member of RE forum.

I have a question: I want to change my 3 kw scroll compressor to VARIABLE SPEED one.
Firstly I will add frequency converter to change main line frequency of the compressor.
But I don't know how can I control the system, P, PI, PD or PID ??
And what kind of hardware I need for this control ??

Thanks so much.. OE

Why you whant do that?

[Brian_UK]

What is the compressor serving ?

[O_E]

This scroll compressor is serving for chiller system. And this is a project for PhD study. I need detailed controlling info if this system changes variable speed
also inverter system info.
(system; 3 kw scroll compressor, air cooled condenser and condenser in a channel which temp and flow rate can be change, 12/7 celcius water in evap, TXV and on/off control with carel mc2)

thanks

[SteinarN]

It is several ways to do the control logic. If you dont have a PLC to control the system I sugests you install a pressure sensor in the suction line, connect it to the VFD and set up the internal PI regulator of the VFD to calculate the necessarry Hz to maintain selected set point suction pressure. You will then get a constant suction pressure regardles of variations in water temp and flow. Alternatively you could install temp sensors in the entering and/or leaving water flow, connect to the VFD, and use one or both of them as the input signal to the PI regulator in the VFD. In that case you will be able to maintain a constant leaving water temp. The entering water temp will be more prone to fluctuation.

Dont use the D function in the regulator, only the PI function.

You also ned to take care of short cycling protection and define some parameters to stop the compressor at low heat load. All this should be possible to configure in the setup of the VFD without any need for additional hardware.

The only compressor specific hardware you need is a HP/LP safety switch controlling a contactor in the mains circuit and a circuit breaker for the VFD.


Do a search for VFD on the forum. You should be able to find some information here.

[nike123]

Do you want to change compressor with one real inverter compressor, or you just want control speed of fixed speed compressor with VFD?

If that is the case, you need to change TEV to EEV because TEV cannot control evaporator in all capacity range of inverter compresor.


Carel MC2 cannot do control of compressor speed continuously! It can control speed of condenser fan but control of compressor speed could be in only 4 steps (speeds).

[Erik Detroit]

Hi Every Body,

I am very glad being a member of RE forum.

I have a question: I want to change my 3 kw scroll compressor to VARIABLE SPEED one.
Firstly I will add frequency converter to change main line frequency of the compressor.
But I don't know how can I control the system, P, PI, PD or PID ??
And what kind of hardware I need for this control ??

Thanks so much.. OE

I've thought of doing the same thing for my home AC/HP system. The problem is that the compliant scroll design uses the centrifugal force to keep the scroll walls together. I've read some examples of guys who have done it and they were recommended to stay above 40hz by Copeland to keep blowby gasses to acceptable levels. (Don't take that statement as fact, as I really am not sure of the source)

I'm sure you could run a scroll at a lower speed, but will the oil still be picked up? Will the compressor isentropic efficiency suffer more than the efficiency gains in heat exchangers and blowers/pumps?

So for me, 60Hz to 40Hz it's not much of a variable speed system, and I'm thinking not worth the trouble.

[SteinarN]

Scrolls isn't the optimum commpressor to speed regulate due to a high minimum Hz. However it is possible to run Copeland scrolls up to 70 or 75 Hz. The voltage must be high enough tho. Select a 230V compressor if the mains is 400V. The VFD then can regulate the correct voltage, ie suply a 230V 50Hz motor with the correct 322 volt at 70Hz.

[O_E]

I've thought of doing the same thing for my home AC/HP system. The problem is that the compliant scroll design uses the centrifugal force to keep the scroll walls together. I've read some examples of guys who have done it and they were recommended to stay above 40hz by Copeland to keep blowby gasses to acceptable levels. (Don't take that statement as fact, as I really am not sure of the source) I'm sure you could run a scroll at a lower speed, but will the oil still be picked up? Will the compressor isentropic efficiency suffer more than the efficiency gains in heat exchangers and blowers/pumps? So for me, 60Hz to 40Hz it's not much of a variable speed system, and I'm thinking not worth the trouble.

I have read some research paper, according these, scroll compressors are able to operate between 25-60 Hz unless any problem (include lubrication, vibration etc.) with 30% performance improve.
By the way, do you think still apply VFD to your scroll comp. And what kind of control do you think?

It is several ways to do the control logic. If you dont have a PLC to control the system I sugests you install a pressure sensor in the suction line, connect it to the VFD and set up the internal PI regulator of the VFD to calculate the necessarry Hz to maintain selected set point suction pressure. You will then get a constant suction pressure regardles of variations in water temp and flow. Alternatively you could install temp sensors in the entering and/or leaving water flow, connect to the VFD, and use one or both of them as the input signal to the PI regulator in the VFD. In that case you will be able to maintain a constant leaving water temp. The entering water temp will be more prone to fluctuation. Dont use the D function in the regulator, only the PI function. You also ned to take care of short cycling protection and define some parameters to stop the compressor at low heat load. All this should be possible to configure in the setup of the VFD without any need for additional hardware. The only compressor specific hardware you need is a HP/LP safety switch controlling a contactor in the mains circuit and a circuit breaker for the VFD.Do a search for VFD on the forum. You should be able to find some information here.

Thanks for very useful reply, but I couldn't find useful VFD driver info in my searches. Generaly VFD information says you can use PID control. You said use only P and I function (not D function), I couldn't understant exactly.
I wonder if I need any calculation (except set output water temp.) for the system after connect sensor to VFD input regulator. I don't have enough electronic and control background.
Please, could you explain detailed information about control of compressor, VFD, sensors, EEV ( a scientific research was used TXV with variable scroll comp effieciently) in this kind applications.
(this compressor is ZR28K3 PFJ 522 with R134a in 50 Hz 220 V)

[O_E]

Do you want to change compressor with one real inverter compressor, or you just want control speed of fixed speed compressor with VFD? If that is the case, you need to change TEV to EEV because TEV cannot control evaporator in all capacity range of inverter compresor.Carel MC2 cannot do control of compressor speed continuously! It can control speed of condenser fan but control of compressor speed could be in only 4 steps (speeds).

I don't want to change compressor. I will use VFD and I don't know how can I control it. You said compressor speed can be control in only 4 speeds (steps) by carel mc2, so should I change this for PI,PD,PID (I don't know which one I will use) control or not ?
And some researcher had used TXV with variable speed scroll more efficient than EEV with fuzzy control. Also I want to compare fuzzy control with conventional one (PI,PD,PID).If you have much more infor please give me.

[SteinarN]

(this compressor is ZR28K3 PFJ 522 with R134a in 50 Hz 220 V)

The PFJ motor code tells it's a 1-phase motor. Not possible to speed regulate with a VFD or any other method.

To not use the D-function you set zero or off to the D-function value in the VFD.

I suggests you download the full installation/configuration manual for a VFD and study it thoroughly. You also need some electrical knowlegde to be able to set up such a system.

If you use the internal PI regulator of the VFD you dont need any other calculations. Set the suction pressure if you use a suction pressure sensor. Or set the temperature if you use a temperature sensor.

If my momory serves me right Copeland scrolls can operate between 35 and 60 or 70Hz.

It shoud not be any problem with using a TEV on a system like this operating in a capasity range of roughly 50-100%.

[tedre]

Hi, I'm still new to this forum.
If, O-E want control water temperature. On single phase compressor, you can't use VFD. I think you have a other way to control. You can use hot gas bypass ( inject hot gas to inlet of the chiller coil).
What you guys think?

tedre

[The MG Pony]

Tedre It is a school project so lets let him learn!

Ensure to use a balanced port TXV as it will take in stride the regulation of the compressor.

Your control system is to act as a analog feed back loop, so as one set point changes as does the other to balanced it, that is the true goal of this is to have it balance, With that said what is the best point to measure? I would think the water temp as it is our final goal.

What of the condenser though? As the compressor slows we are allowed to flood more of it to yield better sub cooling at no cost of head pressure due to reduced capacity! I think wise to place a VFD here feeding off head pressure