variable speed drives

[Johan2]

Hi I am new in the air conditioning trade. Need help in understanding how the vsd controls the sec chilled water pumps and the Fan on the cooling towers. How does the drive know at which speed to run. And could you please explain to me how you connect the vsd electricaly please

Thanks Guys
From the new guy.

[Brian_UK]

Pump control is probably based on monitoring the pump pressure.

As valves close the pressure will increase thereby wasting energy. The pump will slow down to reduce the pressure back to that required.

Likewise the cooling tower fan will respond to water temperature.

As regards wiring, contact the manufacturer of the VSD for diagrams.

[Johan2]

Thanks Brain I will do so. Cheers

[Lowrider]

In a decoupled primairy/secundairy chilled water system the pumps aren't controlled based on the pump pressure but on pressure differential in the system. All the coils, eg CCU's, are fitted with 2way valves. The amount they are opened is a direct result of the load that is on the CCU. If the load increases, the valve opens, the pressure diff drops and the pumps will have to speed up to get the diff pressure back to setpoint. If load decreases the valves close or are less opened if u like. so the pumps will have to speed down to get the diff pressure back to setpoint.

As for the cooling tower, the Fanspeed is controlled based on the setpoint of the leaving water. Usually by a PID loops controlling the speed needed to get the leaving water on setpoint.

Hope this clears it up a bit.

[Johan2]

Thanks lowrider you gave me a good idea of what is hapening now.

[PRESS]

Hi Johan2.

Brian UK and Lowrider have said much there is to say. With secondary chilled water pumps you will have a pressure sensor set at a predetermined pressure set point. As the building temperature lowers the actuators on you air handling units or fan coil units will close slowly resulting in pressure build up in the secondary chilled water pipework. The pressure sensor will send a 0-10v signal to the drive which the drive recieves and adjust the frequency of the VSD accordingly. So the direct relation ship between these aparatus is that the lower the building temperature the lower the speed of the pumps and the higher the building temperature the higher the speed of the pumps.

When it comes to your cooling tower fan the drives recieves a 0-10v signal from the cooling tower leaving water which the drive processes to adjust the speed of the fan motor acordingly. Click here to see some danfoss literature on speed drives. You can also click here to access my contacts page if you want me to email you other VSD material I have.