Measurement of energy by means of a frequency converter

Abstract

A method and system for the measurement of energy in an alternating-current motor drive containing a frequency converter (10, 11) or an equivalent stepless rotational speed regulator provided with semiconductor switches, in which regulator the power taken by the electric drive from the mains is determined, and in which, based on the power taken by the electric drive from the mains, the energy taken from the supply mains, especially the accumulated energy consumption after a given instant is determined.

Description

The present invention relates to a method and system for the measurement of energy in an alternating-current motor drive containing a frequency converter or an equivalent stepless device for rotational speed regulation provided with semiconductor switches.

As is known, using an electric drive provided with rotational speed regulation it is possible to save a considerable amount of energy as compared to the traditional constant-speed motor drive, where the required regulation of the process is implemented by throttling with a valve or by applying the On/Off principle. Such processes include e.g. control of surface level or pressure.

The extent of energy savings can be estimated in one of the following well-known ways:

1. By verifying afterwards from the electricity bill how much lower the energy consumption has been (ΔE) after the installation of rotational speed regulation (T1) as compared to earlier history data (FIG. 1).

2. Mathematically by estimating on the basis of process data the required power before and after installation of rotational speed regulation, by taking into account the dissipation power of the motor and frequency converter and calculating the size of the electricity bill from the consumption indications thus obtained.

Often there are several energy consuming installations connected to the same energy meter, external factors such as loading conditions do not necessarily remain the same etc., so an energy saving estimation thus obtained may be very inaccurate.

Many frequency converters have an integrated power measuring system that indicates the power supplied to the motor and, on the basis of this, also the accumulated energy consumption. The measurement is normally based on a measurement of motor current (Im) and intermediate circuit voltage (Udc) as illustrated in FIG. 2. Based on these signals, the control unit of the frequency converter is able to calculate the output voltage (Um) as well as the magnitude of the phase shift angle (φ_m) between voltage and current. On the basis of these data, it is possible to calculate the power supplied to the motor Pm={square root}{square root over (3×)}Um×Im×cos φm   (1) and further the energy consumption as the time integral of power Em=∫Pm×dt   (2)

whereas the power and energy taken from the supply mains, for which the client actually pays to the electricity company, are not measured by prior-art frequency converters.

The object of the present invention is to create a motor drive which is implemented using a frequency converter or equivalent and wherein the power and energy taken from the supply mains by the electric drive in question are also measured. This is achieved by providing the frequency converter with an integrated system of measuring the power and energy taken from the supply mains. In this way, the advantage achieved by using a regulated electric drive can be measured directly without any error caused by external ambiguity factors. When the frequency converter is additionally provided with means allowing data storing and processing actions to be performed e.g. by using an application-specific program, it will be possible to display momentary and accumulated savings e.g. on the frequency converter's display panel both as an energy indication and in terms of money when the price of energy is known.

The measurement indicates at least

• the instantaneous value of the power taken from the supply mains
• the consumption of energy taken from the supply mains since a given instant of time
• the amount of energy saved as compared to a given reference (direct unregulated motor drive).

The energy meter of the invention, which is integrated in a frequency converter or equivalent rotational speed regulation device, can be utilized in many new electric drive and business concepts.

The details of the features of the method of the invention and the system applying it are disclosed in the claims to follow.

In the following, the invention will be described in detail with reference to an example and the attached drawings, wherein

FIG. 1 represents energy consumption as a function of time after the introduction of rotational speed regulation, as compared to earlier history data,

FIG. 2 represents prior-art measurement of power in a frequency converter, and

FIG. 3 represents energy measurement according to the invention.

FIG. 2 presents a prior-art three-phase voltage-controlled PWM frequency converter, which comprises a mains bridge (rectifier) 10 for rectifying the three-phase alternating mains voltage consisting of phase voltages U_R, U_S, U_T to produce an intermediate-circuit direct voltage U_DC and a load bridge (inverter) 11 for inverting the intermediate-circuit direct voltage to produce a variable-frequency three-phase alternating voltage consisting of phase voltages U_U, U_V, U_W. The frequency converter may feed e.g. a three-phase cage induction motor. The load bridge 11 is a full-wave bridge in which a control unit 13 controls the phase switches of each phase by pulse-width modulation. The mains bridge 10 may be e.g. an uncontrolled full-wave bridge as presented in the figure. According to prior art, the direct-voltage intermediate circuit is provided with a capacitor C_DC for filtering the direct voltage, and an inductor L_AC at the input connections for filtering the mains current. The frequency converter is controlled by a control unit CONTROL1.

The frequency converter in FIG. 2 comprises integrated means as described above for the measurement of output current and intermediate circuit voltage. On the basis of the results measured by the aforesaid means, it is possible to determine the power supplied to the motor and, based on this, also the accumulated energy consumption from equations (1) and (2). The output current Im can be measured e.g. as shown in the figure by means of current transformers IU, IV and IW, and the intermediate circuit voltage U_DC can be measured by measuring circuits contained in the control unit CONTROL1.

In the example presented in FIG. 3, as a solution according to the invention, the frequency converter additionally comprises integrated means for measuring the voltage Uv of the supply mains and the current Iv taken from the supply mains, on the basis of which it is possible to determine the power taken from the supply mains and further, from equation (3), the energy consumption. The current Iv of the supply mains can be measured e.g. as shown in the figure by means of current transformers IR, IS and IT and the mains voltage by means of measuring circuits provided in the control unit. The power Pv can be calculated directly according to equation 3, where φv is the phase shift angle between the mains voltage and current. Pv={square root}{square root over (3×)}Uv×Iv×cos φv   (3)

The measurement of supply power can also be implemented on the basis of measured motor power by performing a series of calibration measurements with different loads, measuring the ratio k between the motor power Pm indicated by the frequency converter and the power Pv taken from the mains as measured by external measuring devices, which ratio is then programmed for use as a correction factor in the application program memory of the frequency converter. In this way, the frequency converter can calculate in real time the instantaneous value of the power Pv taken from the mains and therefore also the accumulated value of energy taken from the mains. Pv=k×Pm   (4)

In the control unit of the frequency converter, data storage and processing operations are additionally performed using e.g. an application-specific program. Thus it will be possible to display momentary and accumulated savings e.g. on the display panel PANEL1 of the control unit of the frequency converter both as an energy indication and in terms of money, the price of energy being known.

For example, in a pumping process, where the delivery is traditionally regulated by means of a throttle valve, the following procedure can be observed:

• • 1. The power taken from the mains by a motor provided with direct mains supply is measured at different delivery points of the process when the delivery is regulated by means of a throttle valve. The measurement may be performed using either a separate power meter or a frequency converter according to the invention when the output frequency has been set to be the same as the frequency (50 or 60 Hz) of the supply mains.
  • 2. The throttle valve is removed or set to a completely open position, and the power consumption is measured at the same delivery points with the motor working under rotational speed control.
  • 3. The consumption indication from measurement 1 is set as a reference to which the frequency converter continuously compares the indications given by the internal measurement during the process and, based on the difference, calculates the accumulated energy saving. Based on this reading, the frequency converter can also calculated the saving directly in terms of money.

It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the claims presented below.

Claims

1. A method for the measurement of energy in an alternating-current motor drive containing a frequency converter (10, 11) or an equivalent stepless rotational speed regulator provided with semiconductor switches, characterized in that in the regulator, the power taken by the electric drive from the mains is determined, and based on the power taken by the electric drive from the mains, the energy taken from the supply mains, especially the accumulated energy consumption after a given instant is determined.

2. A method according to claim 1, characterized in that the power taken from the supply mains is determined by measuring the input current (Iv) and voltage (Uv) of the regulator.

3. A method according to claim 1, characterized in that the power taken from the supply mains is determined on the basis of measured load power by performing a series of calibration measurements with different loads, measuring the ratio (k) between the power (Pv) taken from the mains as measured by external measuring devices and the load power (Pm) indicated by the regulator, which ratio (k) is then specified as a correction factor the power taken from the supply mains being thus Pv=k×Pm   (4)

4. A method according to claim 1, characterized in that the energy measurement determines the consumption difference as compared to a reference situation, e.g. a situation where the rotational speed of an electric motor is not regulated.

5. A method according to claim 1, characterized in that, to measure energy savings, the motor drive fed by a frequency converter is at first operated at constant speed to measure a reference consumption, and next the motor drive is operated under rotational speed regulation, with its internal energy measurement calculating the consumption difference directly as compared to constant-speed regulation.

6. A system for the measurement of energy in an alternating-current motor drive containing a frequency converter (10, 11) or an equivalent stepless rotational speed regulator provided with semiconductor switches and a control unit (CONTROL1), characterized in that the control unit measures the power taken by the electric drive from the mains, and based on the power taken by the electric drive from the mains, the control unit determines the energy taken from the supply mains, especially the accumulated energy consumption after a given instant.

7. A system according to claim 6, characterized in that the system comprises current and voltage measuring elements (IR, IS, IT) fitted at the input of the regulator, by means of which elements the power taken from the supply mains is determined by measuring the input current (Iv) and voltage (Uv) of the regulator.

8. A system according to claim 6, provided with output current and voltage measuring elements (IU, IV, IW) for the measurement of load power taken by the motor drive, characterized in that the control unit (CONTROL1) determines the power taken from the supply mains on the basis of a series of calibration measurements performed with different loads, in which measurement series the ratio (k) between the power (Pv) taken from the mains as measured by external measuring devices and the load power (Pm) indicated by the regulator, which ratio (k) is specified as a correction factor, the power taken from the supply mains being Pv=k×Pm   (4)