Retrofitting VFD

Variable Frequency Drives have high reputation in the industry for conserving energy. On the other hand it is crucial to check Variable Frequency Drive retrofit applications properly to make sure that the project would earn normal savings. The nature of the system Variable Frequency Drives applied confirms extent of possible savings.

Variable Frequency Drive has proven their value in various applications where energy can conserve by reducing the speed of a motor. With variable torque centrifugal pump or fan applications diminishing the motor speed would conserve more energy than you might expect, since energy use decreases by a cube function in related to reduction in motor speed.

Consider a Variable Frequency Drive Retrofit

1. Energy savings are a priority. Without this priority, the project will not worth it.
2. A new Variable Frequency Drive compressor is too costly. If limited capital, a retrofit might be able to explain.
3. There is no room or electrical capacity for a new, large Variable Frequency Drive compressor. New Variable Frequency Drive compressors need more advanced electricity than a conventional compressor of the same nominal motor power. This is because the compressor is not at its peak efficiency at great speed, and increased Variable Frequency Drive losses that are max at top speed.
4. New Variable Frequency Drive compressors don’t always integrate into customer’s master controls. Often, new Variable Frequency Drive compressors sold as stand-alone units in multiple compressor systems and the integration issues ignored. In cases where we attempt to integrate them, some models don’t interface well with a third-party master control and monitoring system. Some OEMs are getting better at supporting integration.
5. A suitable compressor for retrofit exists.

Recommendations for Variable Frequency Drive Retrofit

1. To properly integrate a Variable Frequency Drive compressor into a multiple compressor system, the Variable Frequency Drive compressor must have a “turn-down” (min-max speed flow range) of more than any of the fixed speed compressors in the system. That would allow the Variable Frequency Drive compressor to execute as the “fixed trim” or the unit that is always meeting the varying load. If the Variable Frequency Drive turn down is not large enough, the control system will have “dead-band” problems. One or more fixed speed compressors would load and unload at the same time the Variable Frequency Drive is “hunting” from maximal to minimal speed and vice-versa. If there are multiple Variable Frequency Drive compressors that together have the needed swing range, it is possible to execute them together as the “trim” capacity.
2. The retrofitted compressor is a positive-displacement compressor. While it is apparently able to run a centrifugal compressor in a small speed range, we don’t recommend it for a retrofit project.
3. The retrofitted compressor must have a known speed range. This is easier to regulate if the air end on the unit packed in a Variable Frequency Drive, gear or belt-drive package that has a known male rotor speed range. Typically, oil-flooded rotary screw compressors can run down to 900 rpm at 100 psig, often lower. Oil free compressors are more delicate to speed attribution, due to temperature increase with slip at lower speeds.
4. The retrofitted compressor should not have oil flow systems that demand full speed operation for proper oil flow. We are not aware of this in a standard package, but it might exist.
5. Motor can handle Variable Frequency Drive-operation. There are several things to look out for. First, the current draw will be slightly higher than it is now at 100% load, particularly at low frequencies, and the cooling will drop off. Some compressors have hardly evaluated motors and are running modulated, and with a Variable Frequency Drive, the 60Hz current will be higher than it used to (modulation will  turn out). Also, some motors are not well cooled now and will be vulnerable to overheating at minimal speed, particularly TEFC motors. An engineering evaluation needs to  do to see if the motor is suitable.
6. Adequate technical support is available. There is an electrical contractor who can act as an integrator, preferably with a controls programmer/engineer on staff and a compressor engineer available to properly design and commission the system. Some customers have the in-house resources to apply this type of project. Most do not.
7. Customer understands that master controls need to install or change. If already there is a “sequencer” in the system, it has to reprogram or possibly replace. It is important to have master control in absence of master control.

Implementing  Recommendations for Variable Frequency Drive Retrofit

Clear all local control issues with compressors: The compressors should able to run properly in “local” before they can run in “remote”.
The following issues analyzed and corrected in the case study, initial re-commissioning of master controls:

The variable rearrangement controls on the 400hp and 200hp compressor are pneumatically controlled with a mechanical regulator and not capable to control by a Programmable Logic Controller. We bring this up as the main reason for believing that the master control system turned off. These pilot valves can manually adjust to overlap in the master controller pressure range and can malfunction, wreking have on remote controller. The modulation control should adjust above the master control set point. The inlet modulation point will set even higher than that, but still within the great pressure capacity of the compressor and motor.

The variable rearrangement controls on the 200hp set low, the compressor would never promote 100% flow. It ran at 50% flow or off. That should adjust up, above the auto control setting.

The inlet valve on the 200hp compressor should not close. When the compressor check to unload, the blow down valve vented the sump, the inlet valve mechanism change to convenient region, but the inlet valve itself did not close. Thus, the compressor delivered between 25 to 40% flow supposed to fully unload. The rearrangement controls stuck at50% and the blow down valve wide open. This will repair.

Analyze one of the 150hp in modulation controls, and the pilot valve would force the compressor to run 0%/100% back and forward. It could not even control the modulation valve in a stable way. All electro-pneumatic components on the two older (150 hp) compressors should replace.

Motor greater current was an issue, so we trimmed the top end by permanently adjusting the variable displacement controls to trim capacity by about 6% and current by about 4%.

Modify and check fixed speed compressor controls for proper remote control: The following changes suggested. All of these done before in our case but need to verify previous to going into remote.

Two-situation local/remote switch at each compressor, or controller set for “remote” control.

Remote start and load, and feedback for standby, running, and alarmed. These can either be with relay dry contacts or with bits on the communications network.

Verify that remote load and start work previous to going into full auto. We installed simple two-position switches that mimicked the control system for testing. Verify that motor current/power comes up to the full load range when remotely loaded, and drops to the no-load range when remotely unloaded.

Select the Variable Frequency Drive Properly

We recommend a line filter to avoid noise reflected back to the customer’s electrical system.

Include interface controls and PID board. The minimal interface is start/stop and pressure set point. The minimal feedbacks are electricity, speed and alarm.

Select a constant torque Variable Frequency Drive. Positive displacement compressors are constant torque, actually slightly increasing in torque at lower speeds due to slip. Some Variable Frequency Drive designed for variable torque applications like fans and centrifugal pumps. A constant torque Variable Frequency Drive will handle higher current at minimal speed.

Select a Variable Frequency Drive that isn’t just the lowest cost and is locally supported.

Install the Variable Frequency Drive Properly

Enclosure and ventilation issues. Variable Frequency Drives in dusty environments do not do well. They might need to place in a clean Motor Control Center or enclosed properly (dust-proof with cooling fans).

Distance from motor to Variable Frequency Drive. Special wiring might be needed.

Motor issues to consider:

If the motor is TEFC and/or in a hot area, we recommend improving cooling and installing a motor RTD and alarm.

Check motor suitability for Variable Frequency Drive operation. Some motors do well running on a Variable Frequency Drive and some don’t. Consult your motor supplier.

Install a shaft-grounding brush or ring on the motor to prevent eddy currents being induced in the bearings and premature bearing failure.

Control the Variable Frequency Drive Properly

Analyze compressor starting and ending the compressor through the Variable Frequency Drive directly. It can either do through the existing starter in-line with the Variable Frequency Drive or with the Variable Frequency Drive itself. However, fewer components is simpler.

Give the Variable Frequency Drive a remote pressure set point from the master controller. Let it control pressure on its own PID card and transmitter.

Locate the Variable Frequency Drive transmitter at the same position as the master controller transmitter.

Integrate all useful Variable Frequency Drive viable data into the master controller. At minimal, current (for running and overload condition) and speed (for control of other compressors) is necessary.

Test the Variable Frequency Drive Compressor Before Master-Control

Min and max speed operation. Are all temperatures, current/power and pressure in allowable and expected range?

AC current into drive should roughly 50% linear with speed. Power element will drop, so it might be as high as 60%. Current to motor needs within motor service element. Overloads might need to adjust.

Oil temperature differential should not change a lot. The oil flow will drop the same percentage as the heat rate. Excessive differential is not good, indicating low oil flow.

Check Variable Frequency Drive Compressor Controls in Local Control

Make sure the gain and timer set properly on the PID board, particularly the proportional control settings. The pressure and speed should stable within 2 psi of set point with varying load.