A couple of voltage waveforms with different turn-on phases are provided below. In fact, it can be pricey to run it in the desert, but the inrush amps when the compressor started could not be handled well at all by his inverters. This typically is 5 to 7 times as high as the operating current.
When using a mechanical switch, you have no control over the turn-on phase. The problem with the condensing unit is that running it was not free. Each subsequent measurement uses a 10-degree higher turn-on phase. In some devices, inrush current can be surprisingly high (10x or higher than their steady current). This compressor normally shuts off … However, the RLA is not really there to tell you what the compressor amp draw should be. Changes in condenser and evaporator pressures change the work load of the compressor, which changes the actual operating current. http://www.emersonclimate.com/en-US/products/electronics_compressors/Pages/securestart.aspx, and to a FAQ page that may help with some further questions you might have about the product. The Long Beach chapter of the Refrigeration Service Engineers Society (RSES) is a non-profit educational group comprised of volunteer members who strive to provide quality educational meetings and seminars to improve the technical knowledge of people working in the HVACR trade. An AC6801B AC source can be used to characterize the inrush current of a device quickly, helping you design a device with inrush current that you and your customers can trust.
To control the voltage turn-on phase and thus predict the inrush current, you can use Keysight’s AC6801B AC source. Configuring the AC source for inrush current measurements is a two-step process.
But also to use common sense; if it's only 70 degrees outside, your amps really shouldn't be near the RLA. I have a friend who, other than his condensing unit, was entirely off-grid. The same model compressor in two different applications can have a different RLA in each application. It's called SecureStart and is made by Emerson. Some of his lighting is 48vdc so there is no loss through the inverters converting power to AC. He had a 1kw wind turbine and 6.5kw of solar panels, batteries to store generated power, and a pair of inverters. Now he is enjoying all the air he wants for free.
Another possibility is to turn on a small part of the device and synchronize the rest of the turn-on to the AC line, taking advantage of the phase with the lowest inrush. The 12 VDC power supply tested delayed the turn on of its output. Typically, large inrush current lasts for a few cycles before returning to a steady-state current. At zero volts a sinewave has its maximum rate of change, this change causes an inductive load to create their largest inrush current. The steady state 2.26 A peak current drawn from the AC source.
Figure 2. A series of measurements are made to determine the maximum inrush current. AC6801B AC source measurement panel displaying AC current peak, peak hold, and rms. Trying to figure out what a compressor’s amp draw should be can be a bit confusing. Trying to inch my way off grid and at least save some power in the mean time I have 240v air compressors that have starting inrush current of at least 45 amps according to my laggy amp meter.
Figure 3. Inrush current can be limited by designing a device with lower reactance. This is well below the 600 mA design requirement and much lower than the 1.6 A seen in Figure 3 without any load switches being used.
The test setup below tests an external switching power supply with an AC source and a 30 W load. A peak hold measurement determines the inrush current while a peak measurement is used for steady-state current.
A third possibility is adding a negative temperature coefficient (NTC) current limiting device to your design.
A simulated graph of inrush current vs. phase for an inductive load. Devices with capacitive input will have low inrush currents when the voltage is turned on at zero volts. Some of his lighting is 48vdc so there is no loss through the inverters converting power to AC. Test setup to measure the inrush current of the 12 VDC supply. Please click here to register! In power supplies the energy requirement is primarily caused by load capacitors, in transformers by magnetizing energy. He had a 1kw wind turbine and 6.5kw of solar panels, batteries to store generated power, and a pair of inverters. Need to reduce compressor inrush current? Typically, large inrush current lasts for a few cycles before returning to a steady-state current. Figure 6.
The design of the device will affect the phase at which the maximum inrush current occurs. I put one on my friends unit and we switched over to his solar system and it worked great! Or we may wish to reduce inrush to save the customer from demand charges by the utility company.
The added load meant he also had to add a few additional solar panels, but a small expense in the overall scheme of things.
On complete systems submitted to UL, the rated load amp value will be determined by actual system tests. As the NTC device warms up, its impedance is reduced. In some devices, inrush current can be surprisingly high (10x or higher than their steady current).
The turn-on phase can significantly affect the inrush current. Equipment manufacturers label their equipment with the compressor RLA (rated load amps). The high inrush current of devices results from the higher energy required to turn on. Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on.
Most hermetic compressors only have the LRA (locked rotor amps) on the compressor data plate. The voltage applied to a 12VDC power supply and the current it draws with two different time scales is shown. Various turn-on phases of an AC voltage.
To avoid damage, many devices include a protection circuit to limit the inrush current. Excessive inrush current can damage components and pc boards designed for lower steady-state currents.
On the right, after 1.3 seconds, the power supply turns on and pulls steady-state current. This is because at a phase of zero and 180 degrees, the voltage is turning on at zero volts. Most electronics contain switching power supplies as they are incredibly efficient. To avoid damage, many devices include a protection circuit to limit the inrush current. Figure 8. Some devices will have to be designed to limit the inrush current. Figure 7.
Measuring inrush current is always interesting. Graphing the inrush measurements versus phase for the 12 VDC power supply reveals a trend.
thank you very informative my instructor will be pleased, Nice post on RLAs and LRAs for compressors. Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.
It is there for the purpose of sizing the wire, controls, and overcurrent devices for UL listed equipment. We were discussing compressor draw with our electronics teacher and he pointed out that the reason you have so much inrush current at startup is because until the rotor starts to move and you actually have some induction taking place, you're essentially applying line voltage to a resistance of about 1 ohm, the typical run winding.I tell the students to compare the actual readings to the data plate and not to get to concerned unless the amps are more than 10% over the RLA.
... Reducing starting inrush current can definitely be a cost-saver under these conditions especially if the inrush is not diluted by other large loads. It was originally designed for their Copeland line of compressors, but works well with any brand. It is also important to consider what phase the AC voltage is at when it is applied to the DUT.
Excessive inrush current can damage components and pc boards designed for lower steady-state currents.