How to choose the right AC inverter

Having more options isn't always a good thing. Take AC drives for example: In the past, only a few companies produced AC drives, and they were mainly used for basic motor speed control. There are now hundreds of brands, multiple genres, and a wealth of communication and control options, all of which can confuse you.

Know your motor

Checking the nameplate on the motor is the first step in specifying the correct inverter drive. Most AC motors have a nameplate that will indicate the supply voltage, kilowatt size or power, and amperage. To convert horsepower to kilowatts, simply multiply the horsepower by 0.75. Note this information, as it is important to specify the correct drive. Note that you will need a DC drive for a DC motor.

There may be a lot of information on the motor nameplate, so if you are not sure what you are looking for, take a picture of the nameplate and email it to us.

Understand your requirements

This may seem like an obvious question, but knowing your requirements is key to ensuring that you don't have to pay too much for your inverter drives. Most modern variable speed drives have heavy and normal load ratings. These are often referred to as variable torque (ND) or constant torque (HD) applications. Basically, constant torque is like driving on a highway while your speed remains steady for hours. Variable torque is like driving in a city of constant stops and starts. Applications such as fans and pumps usually work properly because they slowly speed up and then run for hours. Things like conveyor belts and compressors are heavy because they can be started and stopped, and the load changes multiple times per hour.

Consider the environment

Take a moment to consider where the variable speed drive will be installed. If the environment is corrosive or very humid, then it is wise to install the inverter drive in the panel or buy a drive with a higher IP rating. IP stands for Ingress Protection and is usually a two-digit number. The higher the level, the higher the protection. Variable speed drives subject to flushing procedures or extreme conditions must be sealed to at least IP65 or IP67.

Control your drive

Consider how to control the speed of the inverter. Do you plan to use a keyboard, potentiometer or PLC to control the speed of the AC drive? If the variable speed drive is not easily accessible (that is, inside the panel), you may want to consider using a remote keyboard or PLC.

Determine your drive I/O requirements

Your new inverter drive will need to interact with other devices in the factory. Therefore, it is important to know how many inputs and outputs you will need and the types of inputs and outputs.

Digital inputs are used to connect the inverter drive to devices such as PLCS, switches and buttons. These signals can be used for start/stop, forward/reverse, external fault, preset speed selection and other functions.

Analog inputs, on the other hand, allow the driver to be controlled by a simple 0-10VDC or 4-20 mA signal. This can be as simple as a jar, or it can be feedback from a PID controller.

PTK inputs can be used to monitor simple temperature sensors in motor windings to avoid damage in case of overload.

Digital and relay outputs are useful when you want to notify external devices of their status, for example if a failure occurs or when a set speed is reached.

Communication requirements

Profibus, Modbus, Ethernet, CanBus... The list of available communication options is endless. Thankfully, most modern inverter drives, such as the Yaskawa A1000 and Control Techniques Unidrive SP, have plug-in extension modules that give access to most communication protocols.

programming

Programming a VSD can be a daunting task. Unfortunately, too many manufacturers focus on high-end applications and make simple application programming much harder than it needs to be. It is estimated that 60% of operators use only the first seven parameters. For simple applications, we recommend the Control Techniques M200 or the Danfoss Micro Drive series, which are very easy to set up and have preloaded parameters for standard tasks. For more complex tasks, we recommend using the Control Techniques M700 or Danfoss VLT AutomationDrive, which are also easy to install but offer more functionality than the simple M200 and Micro Drive AC drives.

Here are some selection principles;

1, according to the load characteristics of the selection of frequency converter, such as load for constant torque load can choose Siemens MM440 frequency converter, ABB ACS800 series frequency converter; If the load is fan, pump type load can choose Siemens MM430 inverter, ABB ACS510 series inverter and so on.

2, the selection of frequency converter should be based on the actual motor current value as the basis for the selection of frequency converter, the rated power of the motor can only be used as a reference. Secondly, it should be fully considered that the output of the inverter contains high order harmonics, which will cause the power factor and efficiency of the motor to deteriorate.

3, If the frequency converter is to run a long cable, the frequency converter should be amplified or installed at the output end of the frequency converter output reactor.

4, when the frequency converter is used to control several motors in parallel, it is necessary to consider the total length of the cable from the frequency converter to the motor within the allowable range of the frequency converter.

5, for some special applications, such as high ambient temperature, high switching frequency, high altitude, etc., this will cause the reduction of the frequency converter, and the frequency converter needs to be amplified for selection.

6, when choosing a frequency converter for high-speed motors, it should be slightly larger than the frequency converter for ordinary motors.

7. When the frequency converter is used for the variable pole motor, full attention should be paid to the selection of the capacity of the frequency converter so that its maximum rated current is below the rated output current of the frequency converter.

8. When driving explosion-proof motors, the frequency converter has no explosion-proof structure, and the frequency converter should be set outside the dangerous place.

9. When the frequency converter is used to drive the gear reduction motor, the scope of use is restricted by the lubrication method of the gear rotating part. Do not exceed the maximum permissible speed.