IMPROVEMENTS IN COMPRESSOR CONTROL

The invention relates to improvements in compressors and, in particular, to an improvement in a method of controlling variable speed dynamic compressors to avoid motor overload as a result of choking.

Variable speed dynamic compressors, such as centrifugal compressors, have been used to compress air, or other gases, since the 1960s. Centrifugal compressors, which comprise a cylindrical assembly of compressor blades mounted to an axis, are used in a wide variety of fields for a number of reasons. They are generally energy efficient, are low maintenance as they have few moving parts, and they usually give a higher air flow than a similarly sized reciprocating compressor .

The operating characteristics of a compressor are generally defined by the relationship between the pressure ratio and volume or mass flow rate. However, its useful operational range is limited by surge, choke and the maximum permissible compressor speed.

In variable speed dynamic compressors operating at a given delivery pressure, the air flow rate through the compressor can be increased by increasing the speed of the compressor. A choke condition is encountered at high flows, when an increase in compressor speed gives a diminishing increase in the rate of flow. When the flow at any point in the compressor reaches the choke condition, no further flow rate increase is possible. This condition represents the maximum compressor volumetric flow rate as a function of the

pressure ratio. Compressor designers have attempted to find ways of preventing choking during compressor operation to maximise compressor efficiency.

The inlet temperature of the air (or other gas being compressed) determines how much power is required to compress the air to a given pressure ratio, as it may take more power to compress a given volume of cold, denser air than it does warm, less dense air. Thus the power output of a compressor drive is a function of its speed and torque, the torque being a function of the compressor delivery- pressure and air inlet temperature.

An increase in the speed of the motor driving the compressor necessitates an increase in motor power, which leads to an associated increase in the motor winding temperature. One of the problems encountered in trying to protect compressors from choking and to avoid excessively high motor winding temperatures, is that it is difficult to measure power directly. One prior art approach is therefore to run the compressor within fixed speed and power limits. Intervention is necessary in the event of a breach of these limits and ultimate protection is provided by stopping the machine when the limits are reached. This is clearly not acceptable, however, for normal commercial compressor operation.

It is therefore an object of the present invention to provide a method of controlling the power by using the measurement of other parameters as a means for preventing choke by avoiding motor overload.

The invention therefore provides a method of controlling a compressor to provide compressed gas at a target delivery pressure and prevent excessive motor power consumption, the compressor being driven by a variable speed motor which has motor windings, wherein; gas inlet temperature, gas output delivery pressure, motor speed and the motor winding temperature are continuously measured during operation of the compressor; the gas inlet temperature is used to determine a predetermined maximum motor winding temperature limit; the maximum motor winding temperature is used to set a maximum motor speed limit; and the maximum motor speed, target and gas output delivery pressures are used to control the actual motor speed below the maximum motor speed limit.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: -

Figure 1 is a diagrammatic representation of a double PID loop used by the control unit of the present invention to control the operation of the compressor;

Figure 2 is a graph showing motor winding temperature/ power for different inlet air temperatures; and

Figure 3 is a graph showing motor winding temperature/ air inlet temperature.

In the present invention a dynamic compressor, such as a centrifugal compressor, is driven by a variable speed

- A -

motor, and its operation is controlled by a control unit. A human machine interface (HMI) is provided to enable certain parameters to be preset.

The purpose of a compressor is to supply air (or other gas) at a given delivery pressure P _d at a rate that matches the demand for air. To achieve this the speed of the compressor, and therefore its output flow, is varied. To protect the compressor, certain maximum limits are set on the winding temperature, power and speed of the drive motor. The maximum speed V _nJn3x is that which keeps the compressor operating within its preset limits and the minimum speed V _π i _m i _n is used when the compressor reaches surge point, and the compressor is off-loaded to reduce the delivery pressure P _d . It should be noted that these values are given as a working example for one particular compressor and motor combination. These will of course vary from compressor to compressor and motor to motor.

In the present invention, the control unit is programmed to use a double proportional-integral-derivative (PID) loop as shown in Figure 1, the individual loops of which are described below.

Pressure Control

The first PID loop uses the measured delivery pressure P _d as its control input and motor speed V _m as its control output. This PID loop is represented in the lower Section of Figure 1. The P and I terms (the D term not actually being required) are set at the HMI and the measured delivery pressure P _d is the process variable which is compared with

the target (desired) pressure P _t (also set at the HMI) . If the delivery pressure Pd exceeds the target pressure P _t , the motor speed V _m is reduced according to the PID equation. If the delivery pressure P _d drops below the target pressure Pt the motor speed V _m is increased up to the maximum motor speed V^ax-

Maximum Speed Control

The measured motor winding temperature T _mw is used as the control input (process variable) in the second PID loop to adjust the maximum speed limit V^ _ax of the motor (the control output) and thereby keep the motor winding temperature T _mw within the preset limits. This second loop is represented in the upper section of Figure 1. The P, I and D terms are also set at the HMI and the measured motor winding temperature T _mw is the process variable which is compared with the maximum motor winding temperature T _mwmax . This allows the maximum possible motor speed V^ _ax to maintain the required delivery pressure P _d -

Maximum Power Control

As stated previously, the motor power W _m required to meet a particular demand situation depends on the inlet air temperature Ti _n . As the maximum motor winding temperature Trnwmax is also a function of the air inlet temperature Ti _n the motor power W _m can be controlled using the motor winding temperature T _mw .

In the present invention the maximum motor winding temperature T _mwmax for any inlet temperature Ti _n is derived

from a proof of concept (POC) unit and is used to plot the graph of Figure 2 using steady state measurements at various pressures and temperature conditions. For a given air inlet temperature Ti _n this graph shows that there is a linear relationship between the motor power W _m and motor winding temperature T _mw . Furthermore, the power W _m required decreases linearly with air inlet temperature Ti _n .

There are also two fixed limits shown on Figure 2. These are defined by the physical design of the machine and are the Design maximum motor winding Temperature T _mwdes and the Design maximum motor Power W _mdes .

The graph of Figure 2 is used to plot the curve of Figure 3 by plotting mT vs Ti _n at the maximum design power W _mdes - At Inlet temperatures above the intersection of the maximum design temperature and power, the maximum temperature mT is limited by the max design temperature T _m w _des and the inlet temperature Ti _n has no influence. Thus the value of the maximum winding temperature T _mWmax may ^¬ be calculated for Ti _n below K _t from the following formula :-

Tmwmax ⁼ ^Tl • Ti _n + C

where m is the slope of the curve and is dependant on the air inlet temperature Ti _n and c is a constant dependant on the intersection of max design temperature and the inlet temperature K _t in figure 2.

Ifl ⁼ ( Tmwdes ^~ T _mw o°c ) / K _t = ( 150 - 130 ) /13 = 1 . 548 ( for this example ) c ⁼ T _mw0 ° c

= 130 (for this example)

It can be noted from Figure 3 that, to keep within the preset limits, above K _t (13 ⁰ C) it is the winding temperature 5 T _mw that is the limiting parameter, whilst below K _t (13°C) it is the power W _m which is the limiting parameter. However, given the near linear nature of the curves and given the T _mw /W _m cross over temperature K _t (13°C) it can be seen that the maximum motor winding temperature T _mwmax at low 10 temperatures can be reduced proportionally to the air inlet temperature Ti _n , whilst still maintaining an absolute maximum at other temperatures. Thus:-

I f Tin < K _t , T _mwmax = m . Tin + T _mw o°c

-L ~> θ _L Se J-mwmax ~ J- setmax

The invention is thus based on the principle that a reduction in the maximum winding temperature T _mwmax will cause a reduction in maximum speed V _nHn3x resulting in a reduction

20 to the actual speed of the motor V _m and therefore control the actual winding temperature T _mw which has the effect of controlling the power W _m . Hence the PID control loops are combined, as shown in Figure 1, to provide the overall control of the motor power W _m which enables the operator to

25 prevent choking. The first loop controls the speed of the motor and therefore the compressor speed V _m , up to the maximum speed limit V _max calculated by the second loop. The measured motor winding temperature υ _m is fed into the second loop, which compares it with the calculated maximum motor

30 winding temperature T _mwmax based on the measured inlet temperature Ti _n to provide the maximum working speed V _max which is fed back to the first loop.

The method of the present invention is based on steady state measurements and cannot be applied in run up conditions. However, if the power limitation is primarily to control motor winding temperature or choke condition, this would not be a problem. If there is another limitation due to acceleration, not covered by the VFD variable frequency drive current limit, then the ramp up time would need to be adjusted in the variable frequency drive.