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Emerson
 
Modulation Mechanism
| Digital VRF |
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Inverter VRF |
| During the normal compression process, the 2 scrolls are always held together with the optimal force in the vertical direction. However, through some mechanism, if the fixed scroll is made “unfixed” and is lifted by only 1 mm, there would be no gas compression even though the motor and the orbiting scrolls are moving. This is the simple mechanism of the Digital Scroll. The fixed scroll on the top is moved up by 1 mm (in order to get no compression) and pushed down and meshed with the orbiting scroll (in order to get compression). The up and down movement of the fixed scroll is achieved through a pressure differential/spring arrangement inside the compressor and is actuated by an external solenoid valve. A 220V/240V to the external solenoid valve makes the fixed scroll go up by 1 mm and 0V to the solenoid engages the 2 scrolls back again. The capacity is a time averaged summation of the loaded state and unloaded state. By varying the loaded state time and unloaded state time, any capacity (10%-100%) can be delivered by the compressor. |
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An inverter uses a variable speed motor on the compressor (which could be AC or brushless DC motor) and an external inverter drive. The goal is to provide a signal to the motor so that it can operate at variable frequency. For the AC inverter technology, the compressor motor is typically induction type (non synchronous machine). The drive for the induction motor ranges from simple (constant V/Hz) scheme to sophisticated vector control. For the DC inverter technology, the compressor motor is typically brushless permanent magnet type (synchronous machine). The inverter drive for the permanent magnet motor is more complex – the stator rotating magnetic field has to be positioned relative to the rotor position throughout the 360 degree rotor turn and determining where the rotor magnets are in space relative to the stator dynamically at every instant of time is very challenging. For a high end DC inverter drive, the inverter output waveform control is variable frequency switching PWM. |
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| Digital VRF |
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Inverter VRF |
At full load conditions, the Digital Scroll operates like a standard scroll compressor. This means that there are no additional energy losses while operating at full load. This makes the full load COP of the Digital Scroll system superior to any other VRF technologies in the market. |
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Even at the full load condition, the power to the compressor is delivered through the inverter electronics and there are always inverter losses associated with this. This loss can range anywhere between 5% to 10% and is dependent on OEM and their capability on the inverter technology. |
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| Digital VRF |
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Inverter VRF |
| The Digital Scroll technology has a very high Integrated Part Load Value (IPLV) number (3.56) and it has been tested in an approved government laboratory in China. |
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Emerson has done benchmarking of the inverter technology to understand the Integrated Part Load Value (IPLV) performance. The performance of inverters vary widely depending on the type of compressor used (AC scroll or DC scroll) and the type of inverter design (that determines the inverter loss).
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VRF systems, but their very nature of providing variable capacity output, are able to maintain room temperature much more precisely than splits/chillers/ducted system and is common to be able to control temperature within a 0.5 deg C tolerance band. The Digital Scroll VRF and the inverter VRF are very similar in this feature. |
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| Digital VRF |
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Inverter VRF |
The Digital Scroll compressor can change capacity output instantaneously – for example, it can move from 10% capacity output to 90% output instantaneously. This is possible as the loaded time and unloaded time can be changed without any operating issues to the compressor. The response time is more dictated by other system components – whether the expansion valve can open/close that quickly and so the response time is more limited by the capability of the accessory parts. |
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The inverter compressor has to go through the intermediate stages as the speed requirement changes. For example, if the speed has to go up from 1,800 rpm to 6,000 rpm, the compressor has to go through all the intermediate speeds before it reaches the maximum speed. This takes some time and so the response time to changes in room demand is slower for the inverter system as compared to the Digital system. |
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| Digital VRF |
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Inverter VRF |
The loaded and unloaded cycle operation of the Digital Scroll helps in better dehumidification. Example: 50% modulation with a 10 seconds cycle time. Under such conditions, the loaded time (where the mass flow is for full capacity and suction pressure is low) is 5 seconds and the unloaded time (when the mass flow is low and suction pressure is high) is 5 seconds. If we average the suction pressure over this cycle time (by weighting it with the mass flow) – the average suction pressure is lower. This results in better dehumidification at part load. |
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In the inverter system, at low modulation, the compressor operates at a lower frequency. This reduces the mass flow of refrigerant and results in a higher suction pressure. This results in a higher Sensible Heat Factor (SHF) and so moisture removal becomes a challenge. |
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| Digital VRF |
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Inverter VRF |
The Digital Scroll system generates negligible electromagnetic interference as the loading and unloading of the scrolls is a mechanical operation. This unique feature not only eliminates the need for expensive electromagnetic suppression electronics, it also adds to the reliability and simplicity of a Digital system. |
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Inverter systems need EMI filters to suppress the emissions from the electronics. These electronic circuits add the complexity and cost of the inverter systems. |
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There is a significant difference between the Digital Scroll technology and the inverter technology in terms of the hardware and electronics complexity. Digital Scroll is much simpler to apply as compared to the inverter system. Following pictures brings out the difference clearly.
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| Digital VRF |
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Inverter VRF |
During the loaded state, the oil circulation rate of the Digital Scroll is around 1%. During the unloaded state, no oil leaves the compressor and the circulation rate is 0%. This means that when the time averaging is done, the averages oil circulation rate of the Digital Scroll is always less than 1%. |
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When the inverter compressor increases in rotational speed, the amount of oil leaving the compressor also increases. This has to be watched very carefully as large amount of oil can leave the compressor if it is running at high speed for extended period of operation. Inverter systems typically uses oil separator and an oil return cycle to return oil back to the compressor. |
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Over the past 9 years, over 500,000 Digital Scroll has been operating around the world in various applications of VRF, rooftops, chillers, close control machines, refrigeration marine containers etc. The reliability history has been very good at 99.996%. |
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| Digital VRF |
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Inverter VRF |
| The standard Digital Scroll operates between 10% to 100% capacity. In order to provide the feature of overcapacity, Emerson has introduced the Digital Scroll heating that uses the Vapor Injection technology. Vapor Injection allows the Digital Scroll to deliver overcapacity when the demand so arises. |
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The inverter compressor operates on the principle of variable speed and when the room demand increases, the speed of motor rotation increases to deliver the additional cooling/heating capacity. |
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