Overload protective arrangement and method for reducing power consumption upon voltage fluctuations

The invention relates to an arrangement for overload protection and a method for reducing the power consumption at input voltage variations according to the independent claims.

Compressors for cooling are usually driven by electric motors. When it comes to power voltage drops, these electric motors can be overloaded and overheated by the stronger power. This can in extreme cases cause the engines to stall. This means an overload of the motor and on the other hand, a load on the supply network. Conversely burden driven by electric motors cooling arrangements the power grid during normal operation very irregular because compressors for cooling are usually operated in a relatively short duty cycle, for example, 15 min rest cool and 5 min. In particular, conventional motors, which have a high power consumption during start-up, have a destabilizing effect on the power grid. As a result, there may be power sags, through which the electric motors are again loaded.

The <patcit id="pcit0001" dnum="US3585451A"><text>US 3585451</text></patcit> discloses an arrangement for overload protection of an electric motor, which drives a compressor. The engine comprises three coils. Both the temperature and the flow through the individual coils is monitored continuously. If an overload is detected, the load of the compressor is disconnected from the motor. If persistent overload the motor is switched off and switched on again after it has been cooled. This overload protection protects the motor from overheating, but caused by the increased switching cycles a destabilizing effect on the power grid.

The object of the invention to overcome the known problems, in particular to provide the driving compressors overload protection.

This object is achieved with an arrangement for overload protection of an electric motor of a compression unit and a method for reducing the power consumption at input voltage variations according to the independent claims.

The arrangement for the overload protection of an electric motor of a compression unit with a compressor includes the motor, the compression unit, a mains voltage monitoring unit, means for reducing the load on the motor and a control arrangement. The control arrangement is connected to the mains voltage monitoring unit and to the means for reducing the load on the engine such that the engine disconnected from the load and / or the load is reduced when the mains voltage exceeds a predetermined first threshold value, preferably 80 to 90% of the rated voltage, particularly advantageously 85% of rated voltage, below.

The load of the engine caused by the compressor work, which generates the pressure difference between inlet and outlet of the compressor. To reduce the load on the motor can be reduced so that the pressure difference between inlet and outlet of the compressor. Advantageously, this is done through a by-pass between the inlet and outlet, which can be at least one valve is opened or closed.

Alternatively, the motor may also be connected via a clutch with the compressor. The load of the compressor can be completely separated from the motor adjusts via this coupling, in particular reduced, and / or.

Advantageously, the power voltage monitoring unit includes a voltage. With this, the mains voltage is continuously monitored. The response time of the voltage is advantageous as a maximum of 10 ms.

The control arrangement is preferably operatively connected to a means for turning off the electric motor. When the under voltage longer than a predetermined period of time, eg 10 to 60 s continues, the electric motor is switched off. This prevents the engine at a long-lasting disruption of network operation consumes power and the power thus continued to weigh. In addition, the motor is thereby protected from overheating.

More preferably, the motor with a device for reducing the power consumption is provided in the start-up phase. For this purpose, it is advantageous the use of thermistors, which can be connected in series in the main and in the auxiliary winding of a single-phase electric motor. In the auxiliary winding in series connected with the thermistor-start capacitor is advantageous whose capacitance value approximately 3 to 5 times greater than that of Betriebskondesators. The NTC thermistor of the initial current is limited by the starting capacitor, which reduces capacitive current peaks in the startup phase. Due to the large capacity of the starting capacitor, a large phase shift of the current between the auxiliary winding and the main winding is produced, which facilitates the starting of the motor. The NTC thermistor in the main winding reduces the initial current through the main coil, which also provides for a reduction in power consumption in the startup phase. When reaching the rated speed of the first start-up capacitor and to upstream NTC thermistors are disconnected from the circuit, only then, the NTC thermistor is bridged in the main winding by a switch.

Preferably, the control arrangement is so connected with the means for switching on the motor that the motor will only be switched on when the mains voltage is above a second predetermined threshold value, preferably 90 to 95% of the mains voltage. Otherwise, the switch on the motor is prevented. Thus, the mains voltage is not passed on.

Advantageously, the load is only connected to the motor when it has reached a certain predetermined speed. This reduces the power consumption at start-up also, so fluctuations in the mains voltage can be further reduced. The predetermined speed is preferably the rated speed of the motor, but it may also be lower, for example at 80% of rated speed.

The compressor of the inventive device is preferably incorporated in an air conditioner or a refrigerator, because the voltage fluctuations generated by refrigeration equipment due to their short cycle are relatively large. In cooling systems, therefore, the benefits of the inventive device is particularly large.

The method for reducing the power consumption of an electric motor to a compression unit with mains voltage fluctuations, in particular in a reduction in the mains voltage, umfassent the steps of:<ul><li>Monitoring the mains voltage</li><li>Reduce the load on the engine by the compressor when the mains voltage drops below a first threshold value, so that the engine is operated approximately at idle.</li></ul>

The reduction of the load on the motor can be realized by the pressure difference between inlet and outlet of the compressor is reduced. Particularly advantageously, this pressure difference is reduced to zero, since almost the entire load is then taken from the engine. The reduction should advantageously within less than 30 ms, and particularly advantageously to occur within about 20 ms, thus the reaction time is short enough to counteract a sharp voltage drop and to prevent that the motor comes to a halt.

The pressure difference can be reduced by the opening and / or closing one or more valves in a by-pass between the inlet and outlet of the compressor. It is advantageous if the remaining paths of the medium to be compressed, such as coolant, to be closed by opening the by-pass. This results in a pressure drop in the cooling system let prevent.

Alternatively, it is also possible that the load is uncoupled via a clutch from the engine. Thus, the load is taken very quickly and completely from the engine, it is running in no time at idle.

Further, it is advantageous that the time during which the engine is idling, is measured, and that is turned off during idling of the engine after a predetermined period of time. This prevents the engine the mains voltage still burdened with prolonged voltage dip or that it is superheated. Advantage, this predetermined period of time is in the range of 10 to 60 s.

Particularly preferably, an electric motor is used, which can be supplied during the startup phase with a reduced current. Thereby, the system voltage is less heavily loaded when the motor is turned on, as would be the case in a conventional engine.

One way to feed an electric motor during the startup phase with a reduced current is provided when the switch-a thermistor is connected in series with the main winding of the motor. Thereby, the initial current is reduced by the main winding. also an arrangement of a thermistor in series is particularly advantageous switched to a start capacitor in series with the auxiliary winding. This arrangement should be connected in parallel to the operation capacitor, so that add the capacities, and thus during power a large phase displacement between the main winding and the auxiliary winding is produced. In order to prevent capacitive current peaks, the current to the start-up capacitor at the time of switching is limited by the thermistor and only increased by the heating of the thermistor. When in the course of the call goes to the rotor slip of the motor to zero, the arrangement in series with the auxiliary winding is first disconnected from the circuit and then bridged the NTC thermistor in series with the main winding by means of switches. Then the engine is in normal operation, the phase difference between the main and auxiliary winding is generated solely by the operating capacitor whose capacitance preferably approximately three to five times smaller than that of the starting capacitor.

Further, it is advantageous if the electric motor is connected only after reaching a predetermined speed with the load of the compressor. This engine also consumes less power during startup. The predetermined speed is preferably the rated speed of the engine, but it may also be lower.

The first threshold value for the pressure reduction is preferably between 80 and 90% of the rated voltage, more preferably at 85% of rated voltage. The second threshold for the reconstruction of the pressure is between 90 and 95% of rated voltage. This overloading of the power network is efficiently counteracted.

The invention is illustrated below with reference to embodiments and drawings. Show it:<dl id="dl0001" compact="compact"><dt>Fig. 1:</dt><dd>schematic representation of the arrangement according to the invention</dd><dt>Fig. 2:</dt><dd>schematic representation of an alternative arrangement,</dd><dt>Fig. 3:</dt><dd>schematic circuit diagram of a preferred electric motor</dd><dt>Figures 4 to 7th:</dt><dd>Diagrams of the power consumption of the electric motor and the pressure difference between inlet and outlet of the compressor,</dd><dt>Fig. 8 and 9:</dt><dd>Charts the course of the average current at a voltage drop and the pressure difference between inlet and outlet of the compressor</dd></dl>

In the figures <figref idrefs="f0001">Fig. 1 and Fig. 2</figref> are shown two alternative embodiments of the inventive arrangement. The electric motor 1 drives a compressor 2nd A medium, for example a cooling fluid, with a low pressure T is in the compressor to a high pressure H compacted. The mains voltage monitoring unit 3 continuously measures the mains voltage U and passes the information to the control system 4 on. In<figref idrefs="f0001">Fig. 1</figref> the motor is fixed to the compressor inlet E and the outlet A of the compressor can be closed with the valves 5, 5 'via the by-pass 7 briefly. In normal operation of the compression unit, the valves 5 'are opened and the valve 5 is closed. If the mains voltage U falls below the first threshold of 85% of rated voltage, causes the control device 4, the valve opens 5 and the valves 5 'are closed. The pressure difference between the pressure LP at the inlet E and the pressure HP at the outlet A is characterized within 20 ms, preferably to less than 2 bar, more preferably less than 1 bar, reduces the load on the engine 1 is thereby sharply decreased.

Instead of a by-pass 7 between the inlet E and the outlet A of the compressor 2 of the engine 1 as of may <figref idrefs="f0001">FIG. 2</figref> be seen also connected via a clutch 6 to the compressor. 2 When the mains voltage U falls below the first threshold value, the clutch 6 is released and thus separated the compressor 2 from the engine first

In both cases, the engine 1 is only re-connected to the full load of the compressor 2 when the mains voltage U rises above the second threshold value of 90 to 95% of the rated voltage. If this is not within 10 to 60 s is the case, the engine 1 is turned off and only switched on again when the mains voltage U exceeds the second threshold value. This prevents on the one hand, that the network is passed on, on the other hand, the engine 1 is thus protected from overheating. After switching on, the motor 1 is only connected to the load of the compressor 2 when it has a certain speed, preferably at least 80% of the rated speed is reached.

The circuit diagram of the preferred electric motor 1 is in <figref idrefs="f0002">Fig. 3</figref> shown. The engine 1 includes a main winding 11 and auxiliary winding 12 in series with the primary winding 11, a thermistor 15 is switchable. In series with the auxiliary winding 12, an operation capacitor 13 is connected. At the same time, a start-up capacitor 14 connected to a second negative temperature coefficient resistor 16 in series in series with the auxiliary winding 12th The switches 17 and 18 are operated by the control assembly 20th

When the motor 1 both NTC thermistor 15 and 16 and the starting capacitor 14 are connected in the circuit. The NTC thermistor 15 reduces the power consumption of the main winding 11 at the moment of start the NTC thermistor 16 those 14 through the starting capacitor The starting capacitor 14 by a capacitive current peak when switching is prevented. By the two capacitors 13 and 14, the phase shift of the currents through the main winding 11 and auxiliary winding 12 is large, which favors the starting of the motor. As soon as the rotor of the slip motor to zero, the start-up capacitor via the control arrangement 14 first taken via the switch 18 from the circuit, then the thermistor 15 via the switch bridged 17th Then the engine is in normal operation, the phase shift in the auxiliary winding is produced by the operation of capacitor thirteenth

The current curve I at power of a conventional electric motor 1 to a compressor 2 is in <figref idrefs="f0003">Fig. 4</figref> shown. The pressure difference between the pressure LP at the inlet E and the pressure HP at the outlet A of the compressor 2 is at the switch-zero. The operating capacity of the engine 1 is 35 microfarads, the initial capacity of 88 microfarads, the supply voltage U is 228 V. The peak current is 86 A. In<figref idrefs="f0003">Fig. 5</figref> the same motor 1 is turned on, but the pressure difference between inlet and outlet E A of the compressor 2 is 5.2 bar. The peak current is 85 A, the start-up process takes about twice as long. With such an arrangement, the network is heavily loaded when powered up.

<figref idrefs="f0004">Fig. 6</figref> shows the analogous situation <figref idrefs="f0003">Fig. 4</figref> with a motor 1 as shown in <figref idrefs="f0002">Fig. 3</figref> described, the mains supply voltage U 229 V. The peak current is only 28 A. If the pressure difference between inlet and outlet E A of Kompressords 2 at the beginning of the switch is 3.5 bar, the peak current is increased to 51 A that duration of the start is approximately doubled. The current waveform I and the course of the pressure LP at the inlet E and HP at the outlet A of the condenser 2 are made of<figref idrefs="f0004">Fig. 7</figref> visible.

From these pictures it can be seen that the peak current can therefore be significantly reduced if the engine 1 is on the one hand only started when the load on the engine 1 is small, and on the other hand, the voltage supply to the motor 1 according to <figref idrefs="f0002">Fig. 3</figref> is connected.

In the <figref idrefs="f0005">Figures 8 and 9</figref> the consequences of a voltage drop of 230 V are displayed on 160V. In<figref idrefs="f0005">Fig. 8</figref> is a conventional device without visible overload protection. The voltage drop of the engine 1 falls completely and does not run again even with the normalization of the mains voltage U in. When restarting the engine 1 is in accordance with<figref idrefs="f0003">Fig. 5</figref> require a high starting current.

In <figref idrefs="f0005">Fig. 9</figref> the pressure difference between the pressure LP at the inlet E and the pressure HP at the outlet A of the compressor 2 by the voltage drop within 30 ms is reduced to about 1/3 of the initial pressure difference. The engine 1 is characterized by a reduced current I on. When the mains voltage U increases again to the rated voltage, the pressure difference is increased again and the engine 1 is running at the normal power consumption.