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Drop-In Inverters: How Do They Work?


Son of Samurai

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By now, most of us are used to variable speed drive motor systems in appliances. The control board sends a PWM signal to an inverter, which tells the inverter how fast to run the BLDC motor.

While this is the type of inverter that's been used in appliances for years now, it's not the only kind of inverter out there. In fact, another type of inverter is starting to crop up in the appliance world, and it may very well become the norm for some applications. It's called a drop-in inverter, and we're going to look at what sets it apart from the PWM-controlled inverters we're used to.

First, let's take a look at a standard PWM-controlled inverter schematic -- this happens to be for a refrigerator.

GE-Refrigerator-Inverter-Compressor-Schematic.png

In this configuration, the inverter takes four inputs: a hardwired line and neutral, and then two connections to the main control board. The red connection on this schematic is the PWM signal line -- that carries the DC square wave that the control board uses to tell the inverter what to do. Then the white wire would be for DC ground.

Now let's take a look at the schematic for a drop-in inverter -- also in a refrigerator.

Service_Manual_-_700_pdf.png.e1c71eacb2dcee768477f411a3bd2ba2.png

It actually looks almost identical to the PWM-controlled inverter, but with a crucial difference: there's only one connection from the control board to the inverter. And that connection isn't carrying a DC signal -- instead, it carries a simple 120 VAC.

This is why these inverters are called "drop-ins". Instead of the main control board needing to have special programming so that it can send instructions to an inverter, it just needs to send 120 VAC when it wants the compressor to run, and take away the 120 VAC when it wants it to stop. The inverter has all the programming for running the motor by itself.

Right about now, the inquiring minds among you may be asking yourselves, "How is the inverter supposed to vary the speed of the compressor motor in response to sensor data? Isn't that the whole point of a variable speed compressor?"

Hey, good questions! The answer is that the manufacturer of this refrigerator isn't interested in granular control over the speed of the compressor. They're fine with it just running at full speed during operation, with no variance in response to thermistor readings or any other inputs. That's probably because they noticed that, in their models with PWM-controlled inverters, the compressor was almost always being run at maximum capacity. So the variable speed functionality wasn't used much.

But that begs another question: why even bother using an inverter and BLDC motor setup in that case? Why not just use an old-fashioned split-phase compressor?

This is because, even if the drop-in inverter runs the compressor at full bore during operation, it still has programming that lets it ramp up and ramp down the compressor when starting and stopping it. That takes a lot of strain off the motor, helping improve its lifespan and efficiency. So it's just a strict improvement over a split-phase compressor.

One more important thing to point out: we've just been looking at one possible implementation of these drop-in inverters. Some of these inverters have the capacity to take both a 120 VAC drop-in signal and also a PWM speed signal. As an example of that, here's a product manual for an Embraco CF05D drop-in inverter that has both functionalities (you must be a premium tech member of Appliantology to download it):

Hope you found this quick dive into inverter technology informative! If you'd like to learn more about the technologies that are fundamental to all appliances, click here check out our Core Appliance Repair Training course over at the Master Samurai Tech Academy.

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LearningTech

Posted

Something to point out. Just cause there are only 3 wires going to the inverter does not mean that its a drop in.

some like thermador/ libeir have 3 wires but are frequency controlled. 

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  • Team Samurai
Son of Samurai

Posted

8 hours ago, LearningTech said:

Something to point out. Just cause there are only 3 wires going to the inverter does not mean that its a drop in.

some like thermador/ libeir have 3 wires but are frequency controlled. 

That would be good to take a look at — do you have a model number for one of these units so we can pull the schematic?

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LearningTech

Posted

I cannot find the 3 wire version at the moment. I can see the kitchen i my mind where i last worked on one, but cant for the life of me remember their name. 

This is a 4 wire frequency controlled unit

 t18I*

 CN05B is constant 120vac from supply

and CN01 is  a variable frequency  control signal. 

Ill keep looking for the other version. 

 

1C10C01B-FD4D-4C65-81B3-C19358FD6813.jpeg

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LearningTech

Posted

@Son of Samurai

come to think of it  it may be the right unit is thinking of.  Just that its not wired  how the schematic shows. At least not all of them. 

Its wired with 3 inputs but the N is used at 2 points within inverter via a jumper or Y

7ADD4FA5-51FC-4C7F-954F-482E7E84A835.jpeg

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Son of Samurai

Posted

Can't really comment on it unless I can see the full schematic and board pinouts.

But you did make me think of another important point, which I've added to the original post. An inverter that has drop-in functionality (the ability to run the motor by itself with just the 120 VAC run signal from the main control) can also have the ability to receive a speed signal. This is all depending on the model of inverter. So calling an inverter a "drop-in" just means that it has the programming to run the motor on its own. It doesn't tell you if it can or cannot also accept a PWM speed signal.

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LearningTech

Posted

 

Uploaded 2 doncuments that have good reading/ info on differnt control methods.  In cludeing drop-in and Smart drop-in

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LearningTech

Posted

A graph representation of how the drop in inverters function, and can get benifits without  direct control from the refrigerators manufacture.

it can varry its cycle rate based off of the precence of the 120v switch on signal. 

0F9BEE07-36D3-4C68-9A21-84A4A9B31536.jpeg

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  • Team Samurai
Samurai Appliance Repair Man

Posted

According to Embraco's tech literature, inverters that have "drop-in" capability whether or not they accept a PWM speed control signal are referred to as "drop-in inverters." This is to distinguish them from the more common PWM-controlled inverters which do not have drop-in capability.  

These drop-in inverters have two drop-in modes: Default drop-in and Smart drop-in. Both are described below. 

Default drop-in gives speed control based on the temperature control. This temperature input will be 120 VAC voltage and can come from either the main computer (which, in turn, is switching the 120 VAC based on thermistor reading) or directly from the temperature control (if it's an AC cold control). 

In addition to temperature control, Smart drop-in provides for more nuanced compressor speed control by accepting a defrost input. Again, this defrost input is 120 VAC and can be switched by either the main computer (which is monitoring the defrost thermistor) or directly by a defrost bimetal switch that is switching 120 VAC to the inverter.  

All of these compressor run algorithms are programmed into the inverter.

In addition to the run graph you posted above for the CF10B Default drop-in, here's the run graph for the CF10B inverter in Smart drop-in mode for comparison You can see that the additional defrost input (Smart drop-in) allows for more granular control of the compressor speed than input from the temperature control alone (Default drop-in): 

Embraco_CF10B_Inverter_Run_Graph_for_Smart_drop-in.png

On 4/27/2023 at 11:22 PM, LearningTech said:

it can varry its cycle rate based off of the precence of the 120v switch on signal. 

Yes and this 120 VAC signal doesn't need to come from the main computer (though in practice, it usually does).

Here's a sample control configuration for the CF05D inverter in Smart drop-in (the Smart drop-in run profiles for the CF10B and CF05D inverted are identical). Note that the two 120 VAC signals can be switched directly by the defrost control and the on/off thermostat without needing input from the main computer. It's also important to note that the connector location of the 120 VAC input to the inverter determines what the inverter software does with this input in controlling the compressor motor speed. Example: 120 VAC input at CN205 triggers the defrost algorithm; 120 VAC at CN201 triggers the temperature control algorithm. 

Embraco_CF05D_Drop-in_Inverter_Wiring.png

Note that the defrost and temperature controls have separate inputs to the inverter. Each input triggers separate algorithms for running the compressor. 

For each of these inverters, the CF05D and the CF10B, there are two drop-in control modes: Default drop-in and Smart drop-in. Here are some excerpts from the Embraco CF10B inverter manual that you uploaded. First, a description of Drop-in mode in general:

Quote

 

4.2 Drop-In control mode

The Drop-In mode is a CF10B Inverter control mode, where single thermostat contact is used to set the compressor running conditions. Drop-In mode allows the application to any refrigeration system with a simple ON/OFF thermostat, without needing a rotation control signal through serial or frequency communication. The compres- sor speed will be adjusted automatically by the Inverter, in accordance to the thermal load variation. This solution has 2 versions, the Default Drop-In and the Smart Drop-In.

 

Here is the description of Default drop-in. No defrost or temperature inputs as in Smart drop-in, just a 120 VAC off/off signal from the main computer:
 

Quote

 

4.2.1 Default Drop-In
Not recommended for new designs

This solution was designed with a focus on efficiency, where the control logic is divided in two main parts: when the compressor is energized by the first time (pull-down) and when the compressor is cycling (aſter the thermostat has switched off the compressor for the first time).

First time Pull-down
Aſter 3 minutes of intermediary speed, the speed is increased to maximum and it is kept at this rotation until the thermostat opens, switching the compressor off.

Normal cycling
Compressor speed increases and decreases proportional to thermal load variation during compressor running time. Optimum speed will be targeted to minimize energy consumption. If thermal load remains constant for a period longer than 20 minutes, the compressor speed is increased.

 

And here's the description for Smart drop-in:
 

Quote

 

4.2.2 Smart Drop-In
Recommended for new designs

The Smart Drop-In was designed with focus on cooling capacity, but always consid- ering good system efficiency. This solution provides a customization tool that allows the routine to be parameterized and adjusted for each refrigeration system.

The logic is divided in four mains parts: Pull-down, Stability Routine, Heavy Duty Routine and Defrost Routine. The Stability, Heavy Duty and Defrost Routine begin to run in parallel aſter Pull-down is completed.

First time Pull-down
Whenever the inverter is powered up, Drop-in is set to the pull-down state, where the compressor runs on the maximum allowed speed, generating more cooling capacity to reduce the pull-down time. This state is kept until thermal load reach stability.

Stability Routine
The stability cycling is the main routine of Smart Drop-in. This routine will select the best speed to run the compressor, in order to achieve the target cycle duration. The target duration is set by the system’s manufacturer through the customization tool via computer.

Heavy Duty Routine
The heavy duty is a routine running on the background, that keeps checking the com- pressor’s load to identify disturbances and exceptional cases of the system. Based on inverter electrical parameters variation, which represents the thermal load curve, it takes decisions of change or not the speed.

Defrost Routine
This routine is used for greater accuracy in detecting defrost, reducing the time of defrost (e.g. Hot-Gas) and accelerating the recovery in the post-defrosting (e.g. Hot- Gas and Heater).

 

 

 

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AQAppliances

Posted

I wonder what the "You Control" input is. It appears that there is a micro-USB port.

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  • Team Samurai
Samurai Appliance Repair Man

Posted

That’s the DC port for the optional PWM speed signal generated by the main computer board. Drop-in inverters can accept either a motor speed control signal (PWM) or a simple AC signal line. With the PWM signal, the main computer controls the motor speed (“You Control”). With the AC signal line, the drop-in inverter controls the motor speed using it’s built-in software. 

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dEON Chambers

Posted

In my mind I see these invertors as a temp band aid until the OEM part is in hand.  With so many parts randomly being on backorder this may help the customer temporarily get by.  I currently can't think of an example where I would leave it in a unit as a permanent repair.   

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  • Team Samurai
Samurai Appliance Repair Man

Posted

29 minutes ago, dEON Chambers said:

In my mind I see these invertors as a temp band aid until the OEM part is in hand. 

These drop-ins are already being used in OEM appliances. The second schematic I posted in the OP was from a Subzero 700TC.

 

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