At EMA, we perform preventative maintenance on a variety of Medium Voltage Drives in all kinds of environments. One of the more complex drives we run into is the Siemens/Robicon WC series drives. The “WC” stands for “Water Cooled” and they are typical on large horsepower applications. Because the heat load is so great on larger applications, liquid cooled drives are used to keep the footprint of the drive smaller. The disadvantage is that there is another potential point of failure within the system. From a preventative maintenance standpoint, this adds several additional steps to check the cooling system.
The cooling works as a closed loop system that functions by running a glycol/water mixture through the phase-shifted transformer and the power cell cabinet, which cools each power cell (we also repair these power cells). To further complicate matters, there is a second, redundant pump controlled by a pump controller. The pump controller’s settings can be changed to get equal run time on each pump but for PM purposes, there are two pumps that need to be looked at. In older generations, the pump control is built into the drive’s SOP.
We have outlined the importance of Medium Voltage VFD preventative maintenance and listed the checks necessary in previous blogs, but checking the cooling systems of the Siemens WC series drives requires a few more steps:
Check the Discharge Pressure Gauge: This gauge is typically labeled “PG2” and is usually on the left-hand side of the cooling cabinet. This gauge reads the discharge pressure of the pump. The pressure in this gauge should be less than 120 psi. A low reading indicates improper pressure coming from the pump and a high reading indicates a blockage in the system.
Check the Suction Pressure Gauge: This gauge is typically labeled “PG1” and is usually located near the floor of the back wall of the cooling cabinet. This gauge is reading the pressure of the suction side of the pump; it should be around -10 psi to +10 psi. A low reading on this gauge indicates one of the pumps may be overflowing the system, which could damage critical parts such as internal heat exchangers, deionizing canister, strainers, and more. A high reading of this gauge indicates a faulty check valve of the standby pump which can cause premature wear of the standby pump.
Check the Flow Meter: The main flow meter is usually near PG2 and indicates whether liquid is flowing. The flow rate varies with the pump but gives a strong indication of potential issues within the system.
Check the Coolant Tank Level: There is a “sight glass” level indicator on the side near the expansion tank. The liquid level is indicated by a color flag (see orange sight glass in the picture above). Your level of coolant should be between the high and low sensors.
Check the Conductivity Level: To even further complicate things, the glycol/water solution must be deionized by another tank. Because the liquid passes through all the power cells and the transformer, it cannot be conductive at all (for obvious reasons). The Deionizer tank has a resin that traps the ions in the liquid as well as acts as a filter. There is a conductivity probe that measures conductivity and will shut the drive down if it is too high. That gauge should be checked, and the conductivity level should be less than 0.999us/cm. If it is higher than that, the Deionizing tank needs to be replaced.
Check Coolant Inlet and Outlet Temperature: There are temperature readouts for both the inlet and the outlet temperatures. Environmental temperatures will affect this, but operating temperature should be around 85-90 degrees Fahrenheit.
Check the Check Valves: Each pump has a check valve to prevent back flow, which is important since only one pump runs at a time. The check valve cap should be removed to ensure there is a good seal between the valve body and the white Teflon seal. All brass parts should fit together with minimal play.
Check the Strainer: There is a strainer basket downstream of the pumps that filters out particles. The strainer basket can be removed after isolating it properly with the appropriate isolation valves. The strainer basket can be blown with compressed air if necessary to clean it.
Is your head spinning yet? Preventative Maintenance on the WC series drives can be quite tedious and time consuming. Not only do you have to understand the electrical but the plumbing side as well! If you have a Siemens/Robicon WC Series drive that needs maintenance, let us help you. At EMA, we have experienced service engineers who know what to check and how to check all elements of your drive; we’ll give you a comprehensive game plan of how to fix anything that might be wrong.
Don’t leave your expensive assets to chance, contact us today. No One ANYWHERE Is Better At Drives Than We Are!
