GB2432015A

GB2432015A - A method for comissioning a fan

Info

Publication number: GB2432015A
Application number: GB0521203A
Authority: GB
Inventors: Geoff Lockwood; Jeffrey Elkins
Original assignee: Ebm Papst UK Ltd
Current assignee: Ebm Papst UK Ltd
Priority date: 2005-10-19
Filing date: 2005-10-19
Publication date: 2007-05-09
Anticipated expiration: 2025-10-19
Also published as: GB0521203D0; GB2432015B

Abstract

An automatic fan commissioning method where the fan comprises an electric motor driving an impeller which moves air within a system and has a particular pressure/volume flow rate characteristic curve. In the method, the motor is operated with a pre-set reference input A and the speed of the motor is determined. Then, using the fan characteristic curve at least one of the pressure rise and volume flow rate is assessed for that determined speed. The characteristic curve is used to predict F the motor speed needed to achieve a specified required value for pressurise or volume flow rate and then the speed of the motor is adjusted to that predicted speed G.

Description

FAN COMMISSIONING

This invention relates to a method of automatically commissioning a fan incorporated in a system to move air within that system. The invention also relates to a fan comprising an electric motor driving an impeller and arranged to perform such a method of automatic commissioning.

In this specification, the term fan is used to mean the combination of an electric motor coupled either directly or indirectly to an impeller, to rotate that impeller and so drive air. Further, references are made herein to the speed of the electric motor and also of the impeller, by which is meant the rotational rate of the motor or the impeller, as required by the context.

A fan has a performance characteristic associated therewith, dependent upon the type of fan as well as its structure and that of a housing or duct within which the impeller is incorporated. The volume of air moved by a fan depends upon the resistance to the flow of that air: as the resistance increases, the volume flow rate reduces. This relationship is conveniently expressed by way of a fan performance characteristic curve. The resistance to flow depends upon the installation within which the fan is used and is referred to as the system resistance; it is usually very difficult to determine the system resistance.

In the case of a fan provided with a direct drive AC induction motor, there will be a particular performance characteristic associated with driving the motor at the nominal mains supply voltage and frequency. Thus, the characteristic curve indicative of the performance of a fan is normally shown for the nominal supply voltage and frequency.

In an actual installation of a fan, only rarely will the required performance coincide with the particular performance characteristic for that fan. As a consequence, the resulting performance of the fan is a compromise, leading to inefficient operation. The solution to this is to adjust the speed of the fan to meet the required duty, in terms of either a required volume flow rate or a required pressure rise across the fan impeller. As a consequence, it is a common practice to provide an electric motor of a fan with a speed control arrangement, which must be set appropriately to achieve the required operating performance for the fan.

The setting of the speed control can be done either manually or automatically. In the case of the former, the actual volume flow must be measured and then the fan speed is manually adjusted until the actual volume flow is approximately equal to the required flow. With an automatic system, a volume flow sensor has to be provided within the system and be connected to the speed control arrangement, whereby the control arrangement may adjust the motor speed until a pre-set volume flow is measured.

In either case, access is required to a portion of the system where there is non-turbulent airflow, in order to permit an accurate measurement of the airflow rate. A manual system requires considerable user input and technical expertise, but with an automatic system there is added complexity and cost to the control arrangement, as well as the need to provide an additional flow rate sensor. However, even with the automatic system, this can achieve only a pre-set airflow rate without taking into account the characteristic curve of the fan, its installation conditions and the system resistance.

It is a principal aim of the present invention to provide a method whereby a fan may be automatically commissioned without the need to measure actual airflow in a system, in order to optimise the fan operation by taking into account the fan characteristic and the system resistance.

According to this invention, there is provided a method of automatically commissioning a fan incorporated in a system to move air within the system, the fan having a particular pressure/volume flow rate characteristic curve associated therewith and comprising an impeller driven by an electric motor provided with means to control the speed thereof, which method comprises the steps of: a) running the motor with a pre-set reference input so as to drive the impeller and move air within the system; b) determining the speed of the motor when running with said reference input; C) assessing from the fan characteristic curve at least one of the pressure rise and volume flow rate at the determined speed; d) specifying a required value for the pressure rise or volume flow rate for the system, and then assessing from the characteristic curve a predicted motor speed needed to achieve said required value; and e) adjusting the speed of the motor to said predicted speed.

With the method of this invention, a reference characteristic curve of the fan may be determined empirically, before the incorporation of the fan into the system within which it is to move air. Once done, on first powering the fan in the system, it should be supplied with the same reference input as was used to determine the reference characteristic curve, to enable the commissioning of the fan. Once stable operation has been achieved, the speed of the motor is determined. From this speed, the duty point (that is, the point at which the system resistance curve crosses the fan charactenstic curve) can be assessed, and so the system resistance could be deduced from this, using the accepted square law that system resistance is presumed to follow. Thereafter, on selecting a required value for the pressure rise across the fan or a volume flow rate, a required motor speed may be predicted from the fan characteristic curve so that the speed of the motor may be adjusted to said predicted speed.

io The characteristic curve of the fan may be stored as a series of pressure values and corresponding value flow rates, for a series of rotational rates for the fan. The determined motor speed may be stored for comparison with the stored rotational rates for the characteristic curve, thereby to permit the derivation of the duty point for the fan, and from this at least one (or both) of the pressure rise and volume flow rate may be assessed.

In the method of this invention, the required value for the pressure rise or volume flow rate may be specified by way of a manually adjustable control or other input device such as a keyboard. In the alternative, the required value may be set by a sensor monitoring a parameter associated with the system through which the fan is to move air.

The steps of the method of this invention are preferably performed under the control of a program running in a micro-processor or micro-controller (both of which are hereinafter referred to as a micro-processor, since a micro-controller is typically a micro-processor including integrated functions such as program memory and input/output devices, and possibly also having a reduced instruction set) associated with the motor or with a control unit therefor.

The method of this invention particularly lends itself to use with an electronically-commutated DC motor (usually referred to as an EC motor) arranged to drive the impeller and wherein the motor speed is controlled by adjusting a control signal applied to the electronic commutating circuit of the motor. Thus, the pre-set reference input in the case of an EC motor is a reference control signal applied to the motor commutating circuit. By contrast, in the case of a conventional DC motor or an AC motor having a suitable speed control means, the reference input is that where a pre-set voltage or frequency (for an AC induction motor) is applied, and typically is with the full normal drive voltage or maximum frequency applied -i.e. with the speed control means set to its maximum value.

The electronic commutating circuit may include a micro-processor to drive the commutation of the motor on the basis of the supplied control signal.

In this case, the same micro-processor may serve also to run an appropriate program to perform the steps for commissioning the fan.

This invention extends to a fan comprising an impeller driven by an electric motor provided with means to control the speed thereof and intended for incorporation in a system to move air within that system, the fan being provided with a commissioning arrangement adapted to perform the method of this invention as described above, thereby to set the speed of the motor to achieve a required fan duty.

By way of example only, one specific embodiment of a method of this invention will now be described in detail, reference being made to the accompanying drawings in which:-Figure 1 illustrates a characteristic curve for a fan and system resistance curves; Figure 2 is a flow chart of the method of this invention, as applied to the commissioning of a fan comprising an EC motor directly coupled to an impeller, for moving air through a system; and Figure 3 diagrammatically shows a part of an EC motor including a micro-processor for controlling the motor speed.

Before the implementation of the method of this invention, it is necessary to establish the characteristic curve for the fan, by measuring the volume flow and pressure rise achieved by the fan and the corresponding speeds of the impeller, by varying the system resistance while driving the motor with a reference power or control signal. These parameters are stored within the control system for the fan, as a definition of a reference fan characteristic curve.

The values of these parameters will be similar for the same motor and impeller combination. International standards define allowable tolerances for these values and so it is possible for a mass-produced fan to have a reference curve determined with a nominal supply voltage applied. In the case of a fan including an EC motor, the reference curve will be determined by a pre-set control signal applied to the motor, and in the case of a DC motor, with a nominal voltage applied. A typical fan curve for an EC motor as used in this example of the invention is shown in Figure 1.

When the fan is used in an installation, there will be a unique system resistance curve which will be a square law of the form y = x2. The so-called duty point for that fan in the system will be where the system resistance curve crosses the fan characteristic curve, as shown at point D in Figure 1. At this duty point, there will be a volume flow value A and a pressure value B, achieved with a fan speed value C. The fan will be running at some point on the reference curve, depending upon the system resistance. The impeller speed is determined from the speed sensor associated with the motor and that determined speed is stored in the control system. Using the stored values of the reference curve, the value of the volume flow and pressure can be looked up for this impeller speed, and then the values stored. In effect, therefore, from the stored reference curve and the determined impeller speed, the system resistance has been assessed since that resistance follows the square law as discussed above and as shown in Figure 1.

Turning now to Figure 2, this example of the commissioning method of this invention will now be described. At stage A, the fan is run at its pre-determined characteristic and when the fan has settled at a steady speed, that speed is read and stored at stage B. The control system reads from a local selection point C a required volume flow rate or pressure and stores that at stage 0. The system could instead read the required values from a remote selection point E. The volume flow is directly proportional to speed but the pressure development across the impeller is proportional to (speed)2. Using these relationships, the impeller speed necessary to achieve the required volume flow or pressure can be predicted and these speed values are stored within memory.

For example, a speed of perhaps 1,000 rpm may be measured at stage B and using the reference curve, this can be determined as equating to a volume flow of 60 I/s. If a volume flow of 30 I/s is required, and taking the proportional relationship between volume flow and speed, the speed of the fan should be reduced by 0.5 so giving a predicted speed of 500 rpm.

In the alternative, the measured speed of 1,000 rpm may equate to 100 Pa, using the reference curve, though a pressure of 50 Pa may be required.

io According to the pressure law, P2 = P1 x (n21n1)2 and this may be rearranged as: n2 = n1 (P21P1) -2 where: P1 is the original pressure determined from the reference curve.

P2 is the required pressure.

n1 is the determined speed.

n2 is the assessed speed to obtain the required pressure.

From this, n2 is determined as 707 rpm.

Referring now to Figure 3, there is shown in diagrammatic form an EC motor directly coupled to the impeller of a fan. Mains at 50 or 60 Hz is supplied on wires 10 to a DC converter 11, which provides DC to the permanent magnet motor M provided with an electronically-commutated rotating field under the control of a micro-processor 12. A control signal 13 is supplied to the micro-processor, which then controls the electronic commutation of the motor in order to achieve a fan characteristic corresponding to the supplied control signal 13.

A speed sensor 14 is associated with the motor M and provides a signal indicative of the rotational speed of the motor. Typically, the speed sensor 14 may comprise a Hall-effect switch which is triggered by the rotating permanent magnet rotor of the motor.

In order to start the commissioning process described with reference to Figure 2, a suitable control signal 13 is supplied to the micro-processor 12 to set the reference curve for the fan. Thus, the control signal should be substantially the same as the reference signal that was supplied when determining the reference curve.

Once the commissioning has been completed and the required speed has been predicted, either from the required volume rate or the required pressure as has been described above, the control signal 13 is adjusted to give the assessed speed. As these values have been stored in memory, they will be retained on subsequent starting of the fan and fan control, even should power to the system have been switched off or otherwise lost since the fan last ran. In this way, the fan will run at the required duty point, on restarting, and the control signal may be adjusted dynamically to ensure the appropriate speed is achieved, as shown at stage G on Figure 2.

If the system resistance is changed in some way so that the fan should be re-commissioned, then the control system should be set to a self-commissioning routine as shown at stage H of Figure 2. The method as shown in Figure 2, and corresponding to stages A to G, is performed except that following stage F, the self-commissioning is turned off, to ensure that on next start-up, the stored values will again be employed. -10-

Claims

CLAIMS

1. A method of automatically commissioning a fan incorporated in a system to move air within the system, the fan having a particular pressure/volume flow rate characteristic curve associated therewith and comprising an impeller driven by an electric motor provided with means to control the speed thereof, which method comprises the steps of: a) running the motor with a pre-set reference input so as to drive the impeller and move air within the system; b) determining the speed of the motor when running with said reference input; c) assessing from the fan characteristic curve at least one of the pressure rise and volume flow rate at the determined speed; d) specifying a required value for the pressure rise or volume flow rate for the system, and then assessing from the characteristic curve a predicted motor speed needed to achieve said required value; and e) adjusting the speed of the motor to said predicted speed.

2. A method as claimed in claim 1, wherein the characteristic curve of the fan is determined empirically before the incorporation of the fan into the system.

3. A method as claimed in claim 2, wherein the characteristic curve of the fan is stored as a series of pressure values and corresponding volume flow rates for a series of rotational rates for the fan.

4. A method as claimed in claim 3, wherein said determined motor speed is stored for comparison with the stored rotational rates for the characteristic curve thereby to derive one of the assessed pressure rise or volume flow rate.

5. A method as claimed in any of the preceding claims, wherein the required value is specified by way of a manually adjustable control or other input device.

6. A method as claimed in claim 5, wherein the required value is specified at a point in performing the method prior to the assessment from the characteristic curve of a predicted motor speed needed to achieve said required value.

7. A method as claimed in any of the preceding claims, wherein the characteristic curve is used to assess a plurality of predicted motor speeds for different fan duties for the assessed system resistance, said plurality of predicted speeds then being stored so that an appropriate speed may be selected for said required value.

8. A method as claimed in any of the preceding claims, wherein operation of the method may selectively be enabled or disabled, when the motor starts to run after a period of inactivity.

9. A method as claimed in any of the preceding claims, wherein the steps of the method are under the control of a program running in a micro-processor associated with the motor or a control unit therefor.

10. A method as claimed in any of the preceding claims and in which the fan has an electronically commutated DC motor arranged to drive the impeller, wherein the motor speed is controlled by adjusting a control signal supplied to the electronic commutating circuit of the motor.

11. A method as claimed in claim 10, wherein the motor includes a commutation circuit having a micro-processor to effect speed control of the motor on the basis of a supplied control signal, and said micro-processor also runs an appropriate program to perform the steps of the method and generate said control signal.

12. A method of automatically commissioning a fan incorporated in a system to move air within the system and substantially as hereinbefore described with reference to the accompanying drawings.

13. A fan comprising an impeller driven by an electric motor provided with means to control the speed thereof and intended for incorporation in a system to move air within that system, said motor being provided with a commissioning arrangement adapted to perform a method according to any of the preceding claims, thereby to set the speed of the motor to achieve a required fan duty.

14. A fan as claimed in claim 13, wherein the motor comprises an electronically commutated DC motor.

15. A fan as claimed in claim 13 or claim 14, wherein there is provided at least one of a local control and a remote control, with respect to the motor, for the required value of the pressure rise and the flow rate to be achieved in the system.

16. A fan as claimed in any of claims 13 to 15, wherein there is provided a separate control unit for the motor within which there is a micro-processor running a suitable program to perform the steps of the method and to supply a motor speed control signal to the motor.