EN 
UA 
RU Frequently asked questions about frequency inverters and soft starters.
In this article we will analyze the questions that users most often ask or search for on the web. The questions concern frequency converters and soft starters.
What voltage does a frequency inverter output?
At the output, a frequency inverter provides alternating voltage with an adjustable (variable) frequency. The frequency usually changes in the range of 0.1–600 Hz. But there are series of frequency converters that can output 1000 Hz (brand Veichi AC320 series) and more.
The voltage at the output of a frequency inverter is usually PWM. PWM (Pulse-width modulation) in simple terms is a set (sequence) of pulses that imitate a sine wave but with the required amplitude and frequency.
You can’t measure the voltage with a regular tester. More precisely, the tester will show some values, but they will be incorrect. Professional multimeters or oscilloscopes are used for this.
How to understand that the frequency inverter has burned out?
If for some reason you suspect that the frequency inverter has burned out, but you don’t know how to determine it exactly, here are a few indicators to understand that the frequency inverter has burned out (failed):
Voltage is supplied to the frequency inverter, but the LED or LCD display on the inverter does not light up
A start command is supplied to the frequency inverter, the display shows the status that the inverter is in operation mode, however, the motor does not rotate (if the motor is fine and the set frequency is such that it should rotate)
The circuit breaker installed before the frequency inverter trips or the fuse blows
burn marks on the housing and the corresponding smell
Start commands are given, but the frequency inverter does not rotate the motor
When voltage is applied to the frequency inverter or a start command is given, corresponding errors appear on the inverter’s display.
Disassemble the frequency inverter and inspect the boards for burn marks
Burned section of the INVT frequency inverter board
These are only the main indicators for understanding that the frequency inverter has burned out. But for simplicity and your convenience, contact the team of engineers at chastotnik.ua. We will help you quickly identify the reason why your frequency converter does not want to work.
What is better: a soft starter or a frequency converter?
This question has an unambiguous answer — definitely a frequency converter. A soft starter has very limited settings and functionality.
It is also not suitable for very frequent starts (10 or more per hour). It controls torque poorly, has a limited start and stop time — as a rule up to 1 minute, and contains few protection functions.
A soft starter is also not suitable for many applications, such as cranes, elevators, operation from generators, etc.
Whereas a frequency inverter can be used for very frequent starts, has full control over torque during acceleration and braking, many different settings, many protections, and is suitable for virtually any mechanism. It is also the main solution for the task when it is necessary to start a motor from a generator or a weak grid.
Soft starter (left) and frequency converter (right)
Is it possible to connect a frequency inverter without a load?
The answer is definitely yes. A frequency inverter is often connected only to the supply voltage in order to enter parameters for quick commissioning at the site, or to read parameters and view the error log. Even if you issue the 'Start' command with the motor disconnected, nothing will happen to the frequency inverter. At most, it will show an error that there is no load at the output. Some manufacturers, in particular, Veichi, even recommend turning on the frequency inverter first without a connected motor — to make sure that the inverter is in good condition (no damage during transportation, movement, etc.).
What is the problem with a soft starter?
Soft starters can also burn out over time. The reasons for failure are usually overheating of the power elements or a short circuit on the soft starter boards. Repair of soft starters is usually impractical due to their low cost compared to the cost of a frequency inverter.
How much electricity does a frequency converter save?
The economic effect from implementing a frequency converter largely depends on the application, and most importantly — on the correctness of the settings, and also that the motor is not under repair — not rewound. But, as a rule, energy savings can be obtained in practically any application.
The economic effect can be achieved by reducing the rotational speed of the motor to ensure the technological process — for example, maintaining water pressure or supplying air through a fan. As the set parameter (pressure, flow) is approached, the frequency inverter automatically reduces the motor speed to the level necessary to maintain the set parameter.
But also some frequency inverters (for example, inverters by Veichi, some series Dandoss VLT202, Schneider Electric and some others) make it possible to obtain additional savings even when operating at nominal frequency. This effect consists in automatically supplying the minimum necessary voltage to the motor so that it performs the required work. As a result, as a rule, energy savings can be up to 30–35%.
What are the disadvantages of a soft starter?
There are many disadvantages to soft starters.
Limited number of starts per hour — as a rule, 1 start every 10 minutes.
Very few protections for the electric motor.
No hard limitation of inrush current and acceleration/braking time — the fact is that due to its design, if a soft starter cannot start the motor, it automatically raises the current limit level and, if necessary, will increase the acceleration time. But if the start goes easily, the acceleration time can automatically be reduced.
No torque control. Acceleration occurs exclusively due to the increase of voltage and limiting of current.
Limited application — it is not suitable for all mechanisms. It is also not suitable for operation from generators.
Does a soft starter really work?
Yes, a soft starter works, but the scope of application is small. As a rule, these are mechanisms that are started and operate for a long time without stopping and without the need to regulate the motor speed. For example, a crusher, a conveyor, a heating pump, etc. And the grid must have a margin for starting, since the current when using a soft starter exceeds the nominal by at least 2.5 times.
What is the service life of a soft starter?
The service life of a soft starter is generally long because a soft starter does not operate often — usually up to 10 starts per day. And sometimes even less. Because of this, its wear is minimal. But in the event of abnormal situations, a soft starter fails due to the small number of protections in the soft starter. Therefore, as a rule, they need to be additionally protected by auxiliary devices. The manufacturer of soft starters usually gives recommendations in the operating manual.
In this article we tried to answer frequently asked questions. But if you still have questions, contact the team at chastotnik.ua — we will help, explain, and select.