SKIPPING FREQUENCIES FOR VARIABLE SPEED CONTROLS

BACKGROUND OF THE INVENTION

This invention relates to a method of avoiding objectionable frequencies for equipment driven by a variable speed motor, and in particular for motors driving equipment utilized in refrigerant systems.

Electric motors are utilized in refrigerant systems to drive the fans, pumps and compressors. As is known, in a basic refrigerant system, a compressor compresses a refrigerant and delivers that refrigerant downstream to a first heat exchanger. The first heat exchanger exchanges heat between the refrigerant and another heat transfer media such as air, and passes the refrigerant to an expansion device. From the expansion device, the refrigerant is delivered to another heat exchanger, and heat is again exchanged with another heat transfer media. From the second heat exchanger, refrigerant is returned to the compressor. Fans or pumps are associated with each of the two heat exchangers, and a motor is typically associated with each fan or pump. Further, a motor is provided to drive a compressor unit. Also, refrigerant system circuits can have other components such as for example fans or pumps driven by a variable speed motors.

Variable speed motors are becoming more widely utilized in refrigerant systems. A variable speed motor provides a designer with enhanced flexibility in system operation and control. For instance, the capacity of the refrigerant system can be changed by varying the speed of the compressor motor. Thus, variable speed motors and driven equipment can operate across a variety of operational frequencies. Typically, the variable speed motor starts from a frequency of zero and is ramped up toward a desired operational frequency. Thus, the frequency advances from zero upwardly to an operational frequency, which may be selected to achieve a desired cooling capacity, etc. Further, at shutdown, the frequency decreases from that operational frequency back towards zero.

A control for the variable speed motor may change the operational frequency, as conditions or load demands faced by the refrigerant system change.

One problem with the above-described systems is that for any mechanical systems, there are certain frequencies, which have undesirable aspects, for example,

as caused by either acoustic or mechanical resonances. Such frequencies could cause excessive vibration and internal pulsations resulting in component damage as well as undesirable noise potentially leading to customer discomfort. The above- described systems, with the motor frequencies starting from zero and advancing upwardly towards the desired operational frequency, may pass through these resonance frequencies both at start-up and at shutdown. Also, as the control changes frequencies during operation to satisfy external load demands, it may sometimes move the electric motor operation to one of the resonance frequency zones that should be avoided. The system resonance frequencies can also be excited by multiples of motor running speed frequencies, or by the running frequencies (or their multiples) of the driven equipment itself. It should be pointed out that the equipment running speed frequency can be different than that of the motor, if for example the driven equipment is attached to the motor via a gearbox.

This is undesirable, as excessive vibration, noise and pulsations may occur and result in damage of the system components.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, the undesirable frequencies for a particular component associated with an electric motor are identified. As motor frequency is varied, the control is programmed to avoid those undesirable frequency zones. In known control algorithms for an electric motor associated with a refrigerant system, the frequency is varied, and the resultant change from the refrigerant system operation is monitored. The control has a desired system operational feature. That desired operational feature may be the cooling capacity of the refrigerant system, as an example. In one well-known control method, the control does not necessarily determine the required operational frequency of the motor. Instead, the control varies the operational frequency and monitors the resultant change on the refrigerant system until a frequency is found at which the operation of the system is as desired. Typically, the frequency is varied in incremental steps. With this invention, the control will vary the operational frequency of the electric motor, but will skip operation in zones associated with the undesirable frequencies.

As mentioned above, the disclosed application for such a control and method would be for the fans, pumps and compressors driven by a motor in a refrigerant system. However, other system components may benefit from this basic control concept. The undesirable frequencies (frequency that would normally be associated with either acoustical or mechanical resonances) may be determined experimentally, in a laboratory for a particular type of equipment, or may be determined by various types of sensors mounted upon the component. As an example, sensors can be mounted on a fan housing, and sense one of the vibration characteristics. The frequency of the motor or the running frequency of the driven equipment or multiples thereof can be associated with the varying vibration level, and in this manner, the frequencies most subject to vibration and exceeding the desired level can be identified, and then avoided, or associated with a "higher slope" of ramp-up during the start-up, shutdown and frequency adjustment processes. The same reasoning would apply to measurement of excessive pulsations, as for example measured by dynamic pressure transducers installed into the piping adjacent to the system components.

Additionally, the system may self-learn during operation by comparing, for instance, vibration sensor measurements to acceptable values and the controller may include frequencies to be avoided to the skip frequency list in a stored database.

These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of a refrigerant system incorporating the present invention.

Figure 2 is a graph of one of the vibration characteristics versus the operational frequency of an electric motor. Figure 3A is a graph of the operational frequency over time in accordance with an inventive method.

Figure 3B is a flowchart of the inventive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 shows a refrigerant system 20 incorporating compressor 22 delivering a compressed refrigerant to a heat exchanger 24. The heat exchanger 24 is associated with a fan 26 for driving air over the heat exchanger 24. The fan 26 is associated with a motor, as known. A variable speed control C and a transducer T are associated with the fan 26. The variable speed control C drives the motor for the fan 26, and the transducer T may identify one of the parameters associated with vibration level at the fan.

Refrigerant passes from the heat exchanger 24 downstream to an expansion device 28, and then to another heat exchanger 30. The heat exchanger 30 is associated with its own fan 32. A variable speed motor control C and transducer T are also associated with the fan 32.

The refrigerant passes from the heat exchanger 30 back to the compressor 22. As is known, a motor drives a compressor unit 22, and a variable speed control C and a transducer T are associated with the compressor 22.

While refrigerant systems such as are utilized for air conditioning typically have fans moving air over the heat exchangers, other refrigerant systems may be utilized with fluids other than air. As an example, the assignee of the present invention has recently developed a system wherein a refrigerant system is utilized to heat water. In such a case, at least one of the heat exchangers would include a pump moving water over the heat exchanger, rather than a fan moving air. The present invention would extend to such systems.

As shown in Figure 2, if one were to plot the operational frequency of a motor versus one of the characteristics associated with the vibration, pulsation or sound level within a component associated with the motor, there would be typically one or more "resonance frequencies" at which the vibration/pulsation/sound level increases dramatically. As shown in Figure 2, these frequency zones are designated as X ₁ and X ₂ . The present invention seeks to limit the operation of the motors in or near these frequencies, or to "skip" these frequencies. Figure 3A is a control diagram of the present invention. As shown, the control may operate by moving through a series of incremental steps A, B, C, and D. The control moves to one of these steps, and operates the refrigerant system. The

operation of the refrigerant system is monitored, and if the refrigerant system is operating as desired, the control will remain at that operational frequency. However, it is typical that the control must vary the operational frequency, and over time certainly will often need to vary the operational frequency when external load demands change or the indoor space is reaching the desired conditions. As shown in Figure 3A, when the operational frequency is varied, it is varied in steps that avoid the resonance frequency zones. Thus, if the control starts the refrigerant system 20 operating at the frequency A, and determines that the operation of the refrigerant system 20 does not correspond to a desired state to satisfy cooling requirements, it will advance to frequency B. Again, if frequency B does not provide the desired result, the control will increase frequency to C, From frequency C, a shorter incremental step to frequency D may be utilized. This is an overly simplified explanation of the controls, which may be known in the art (other than the inventive addition of skipping through the zones X ₁ and X ₂ ). Typically, the incremental steps might be smaller, and/or of different size, and there may be several between each of the resonance frequency zones. However, the Figure 3A does provide an understanding of the operation basics.

In this manner, while the motor frequency will pass through both zones X ₁ and X ₂ during start-up, shutdown or frequency adjustment, it will only be in those zones for a brief period of time. Thus, the excessive vibration, noise or pulsation will not be felt for any undue length of time.

Moreover, as the control C controls the speed of the motor during operation, the speed may be varied dependent on operational conditions. That is, a worker of ordinary skill in the art would recognize various reasons for which variation in the speed may be desirable. As one example only, as the desired capacity for the compressor changes, it would be desirable to vary the motor speed for the compressor and consequently perhaps fan or pump speed as well. The controls C for this invention are programmed (as described below) to avoid operating in the zones X ₁ and X ₂ , regardless of whether operation in such zones may be dictated by the operational conditions.

The zones X ₁ and X ₂ may be determined in any one of several manners. In the illustrated embodiment, the transducers T are utilized to find the undesirable

frequencies (as mentioned earlier the undesirable frequencies may be associated with system or component resonances but can be "undesirable" for other considerations as well) by monitoring at least one of vibration, pulsation, sound or other characteristics on the several system components. Alternatively, the resonance frequencies can be determined experimentally for a specific family of components or type of the equipment and then pre-programmed into the operating logic of the controllers C.

Another method would be to utilize a system that will "self-learn" the frequencies to be avoided. Another method might be to vary the speed during initial operation to "hunt" for the resonance frequencies to be avoided and then input these frequencies into the system controller such that they can be avoided. Such cases may surface whe.n system natural frequencies are installation dependant or cannot be generalized for an entire product line. The 'hunt' for these undesirable frequencies may be repeated on a regular basis to detect whether there has been a change in these resonance frequencies over time.

The transducer T can be an accelerometer, and can be mounted on the fan or compressor housing, on interconnecting pipes, on the heat exchangers, etc. Other types of transducers such as proximity sensors, velocity pick-up vibration sensors, etc. can be utilized as well. Further, pulsation/acoustic measurement transducers such as a dynamic pressure sensor as well as other types of sound measurements, which may be remote to the component at issue, can be utilized. Furthermore, for redundancy purposes, multiple transducers can be employ to determine undesirable operational frequency zones.

Figure 3B is a flowchart of this invention, and shows the start-up or shutdown procedure, as well as the continuous operation while avoiding the "skipped" frequencies.

Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.