Frequency converters 15 years ago were very expensive and rare starting systems, which for the majority of society were the so-called a "black box" to which power and control cables had to be connected. Currently, frequency converters, often commonly called inverters, are the most common form of starting and regulating the rotational speed of engines. The constantly increasing needs for automation of industrial processes, the increase in production rate and its efficiency are the main factors that influenced the development of the inverter market in Poland and around the world.
The technological leap in semiconductor components and their decreasing price also had a large part to play in this. Currently, frequency converters can be found practically everywhere: in industrial plants, cafes, sewage treatment plants, restaurants, department stores and residential houses. The first converter systems were created in the 1960s, but despite the passage of 50 years since their premiere, the basic principles of operation and the general block diagram have remained unchanged.
In this article you will read about it:
- Why have frequency converters gained so much popularity and are so needed in various industry sectors?
- What did the starting and regulation systems of asynchronous and induction motors look like in the past and what were their imperfections?
- How is the engine speed regulated and what are the 4 methods of starting it?
- You will also learn about how an inverter works.
Check out the inverters at the Onninen wholesaler
Motor speed regulation
Speed adjustment methods:
- slip change caused by the inclusion of a regulating resistor in the rotor circuit of the slip-ring motor. This regulation had a number of disadvantages: lack of full speed regulation, high energy losses, failure rate and a large number of resistors and contact elements, large limitations on power and the number of starts;
- changing the number of motor pole pairs. Also the lack of full regulation (step change in speed), the presence of switching systems;
- changing the frequency supplying the motor (difficult to implement).
Four methods of starting the engine
Starting methods with advantages and disadvantages:
- Direct start. One of the most common starting methods and one of the most undesirable, due to the very large current surge (typically the starting current is from 4 to 8 times the rated current) and the problems associated with it (mechanical stress, water hammer at pumps, jerks of conveyors, etc. .);
- Star/Delta Starting. Also a very popular boot method. The disadvantages include: still high starting current, low starting torque, failure rate of contact elements, and twice as many cable entries. Advantages: limitation of starting current over time, low installation cost.
- Starting with a soft starter. As the name suggests, soft starters are devices for soft starting and stopping of electric motors. Unfortunately, we are unable to regulate the engine speed using a soft starter. The starting current, depending on the starting method (voltage control, torque control in two or three phases) and the number of controllable phases, is from 2 to 6 times the rated current. Advantages: gentle current build-up (no mechanical stress, no current surges), adjustable starting time, high starting torque. Disadvantages: lack of control perfection in soft starters with controllable two phases, no speed regulation.
- Starting with a frequency converter. The best possible method of starting the engine due to: very low starting current (with appropriate configuration of settings, the starting current does not exceed the rated current), full regulation of the engine speed, full protection and supervision of the engine operation, reactive power compensation, saving electricity, reducing costs operational machines, minimizing failure rates, etc.
Check out LS Electric products at the Onninen wholesaler
Different types of frequency converters
Numerous problems and difficulties resulting from the basic starting methods of electric motors caused and forced the rapid and dynamic development, first of all, of thyristor soft start systems (soft starters), and then of frequency converters. These devices are the best way to regulate, protect and control electric motors. Typical, standard frequency converters are used to operate asynchronous induction motors.
However, you can find converters on the market with numerous control algorithms intended for synchronous motors (PMSM - permanent magnet synchronous motors), reluctance motors or BLDC (brushless DC motors with permanent magnets). The vast majority of the industry still focuses on induction motors due to their low price. As you can easily see, using frequency converters brings many benefits. Without a doubt, this is the best method of starting and regulating engines. Taking into account the continuous downward trend in converter prices and the linear increase in their capabilities over time, the demand for these devices will continue to increase in the coming years.
The main component blocks of the inverter
In every frequency converter , we can distinguish four main component blocks (see Fig. 1):
- Rectifier, i.e. the so-called entry stage;
- Intermediate circuit layout;
- Actual inverter element, output stage;
Check out LS Electric products at the Onninen wholesaler
How does a frequency converter work?
The main task of the converter's input stage is to rectify the alternating current drawn from a given source. Here, several rectifier designs should be specified: uncontrollable diode with various numbers of pulses (6, 12, 18, 24 and more); half controllable (diode - thyristor) and fully controllable, based on IGBT transistors.
The most common are designs based on diode rectifiers. These are, of course, the cheapest structures, but they also have the greatest impact on the THD (harmonic distortion coefficient). The diode rectifier is a non-linear load that has a strong influence on current distortion. To reduce the THD factor to an appropriate value, use one of the available THD filters: input chokes, chokes in the DC link, passive LCL filters, active THD filters. The rectified pulsed voltage at the output of the rectifier has a value of the order of 1.35 of the effective value of the phase-to-phase voltage of the network.
Then the signal goes to the converter's intermediate circuit, which usually forms a capacitor or a capacitor bank. The intermediate circuit has a double function: it smoothes the pulsation voltage received from the rectifier and at the same time stores the energy necessary to drive the motor. Very often, manufacturers also include two important elements in the intermediate circuit: a DC choke and a braking transistor (chopper).
- A DC choke is one way to deal with harmonic content. It also significantly improves the power factor.
- A braking transistor is necessary when it is required to stop high inertia on the motor shaft very quickly.
During the braking operation, the engine switches to regenerative operation. The engine becomes a generator that supplies energy to the converter. This energy is stored on the capacitor, which has a finite capacity. After exceeding the voltage limit on the capacitor, the converter cuts off from the motor with an error of too high voltage on the capacitor. In such a case, use the built-in (or external) braking module, which in turn is connected to braking resistor . After exceeding the charge limit, the transistor will transfer power to the external braking resistor. The energy will be lost on the resistor in the form of heat, and the converter will constantly control the motor.
Behind the intermediate circuit of the converter, there is the actual inverter element, which transforms the rectified DC voltage into AC voltage with the appropriate amplitude and frequency. The motor is connected directly to the inverter element. The main components of the inverter are controlled power semiconductors. They used to be thyristors, later due to faster switching time they were replaced with IGBT transistors (bipolar transistor with insulated gate). The operation of semiconductors is two-state, hence the often term "transistor switch".
The switching frequency of the transistors reaches 20 KHz (20,000.00 changes per second!). The semiconductors are switched using control signals generated in the converter control system. Control signals can be generated according to various algorithms and methods. Currently, the most common method is PWM modulation (pulse width modulation).
This method involves the control circuit determining the duration of the periods of switching on and off the appropriate pairs of transistors. Three inverter branches (two transistors per branch) generate eight possible open/close combinations of the semiconductor valves. This creates eight different voltage vectors at the inverter outputs (see Fig. 2). The remaining intermediate vectors are obtained by combining (summing) selected main vectors for appropriate times.
Depending on the switching frequency of the semiconductor valves, a sinusoid of a more or less gentle shape appears at the inverter output. Usually, the user has the option of setting the appropriate carrier frequency in the device itself. By setting the carrier frequency too high, too much heat will be released on the semiconductors, which results in a loss of converter power (manufacturers usually provide charts of the carrier frequency versus power in the user manuals). If the value is set too low, the engine may start to make noise.
Frequency converters - scalar and vector control
Each frequency converter currently produced has the option of both scalar and vector control. Scalar control, often called U/f (read U to f) control, is the simplest mode of motor control that has not changed since the 1970s.
The scalar mode is based on a constant voltage to frequency ratio. Without going into too much detail and higher mathematics, a constant voltage to frequency ratio ensures the creation of the rated magnetic flux in the motor. Therefore, the maximum torque can be achieved. Unfortunately, scalar control has its disadvantages. At low frequencies the torque is very low and often too small to operate with high inertias. Therefore, scalar control is mainly used for variable torque loads, such as pumps or fans. When there is a need to operate with constant torque loads or large inertias that require a large starting torque, vector control (described above as PWM modulation with voltage vector control) should be selected.
Vector control is more refined. Often, to use this control method, you must first provide all motor rating data, such as: current, voltage, power, number of revolutions, number of poles, slip, power factor. Then the converter performs the so-called operations. motor autotuning, i.e. it estimates and calculates other motor data automatically with and without motor rotation (winding resistance and induction, rotor time constant and many others).
With vector control, the converter uses a mathematical model of the engine implemented in the signal processor. Therefore, it is so important to always save all engine data in the converter (the more and more precisely, the better the engine control) and to periodically perform auto-tuning (engine parameters change over time and with changes in environmental conditions).