Use of drivers to start the electric motor
Today, the use of AC drivers in the industry is very significant. These drivers are used for speed control of AC electromotors and engine torque
The advantages of drivers are:
- Predominantly, electrical consumers in the industry are engines These engines are the reactive power consumer in addition to active power consumption This power will give rise to feeder cables heating, increase engine applied current, and reduce the level of voltage consumption as well as the reactive power consumption cost for an energy supplier. Capacitor addition to the consumption bus will be required to eliminate the above points. But the use of an inverter removes the need for a capacitor.
- The current usually reaches 5- 7 times the rated current of the motor at the moment electromotors start directly. This action causes an energy consumption increase, engine destruction, and electromotor bearing and coupling failure. Using the drive in the long-term will lead to energy conservation and no maintenance cost for equipment.
- Motors starting will need protective and controlling equipment. But these accessories are not required using an inverter.
- A 20 percent reduction in the engine speed reduces the electric power consumption by almost 50% and leads to a lower electricity cost.So, according to the above, the use of inverters seems necessary. But there are many kinds of inverters in the industry under current circumstances.
According to their applications and different purchase costs, we need to know their adjustments way and applications to use from kinds of products with various brands. There are general settings jointly in all these brands , but they have different grammatical codes. We can also adjust and apply other AC drivers by mastering a specific brand and principles of its regulation. Therefore, we will set up similar cases in this article.
Mode of inverters selection
Many factors will be impressive in the selection of an inverter, in addition to awareness of the electromotor power and applied rated current.
Type of engines application, including light or heavy loads of direct coupling to the load, or by the gearbox, constant torque, or variable load torque are the factors that will help us in determining the type of inverters.
It should be noted that this information will be available from inverter creators, and based on our user type, we can choose a proportionate inverter.
Joint settings in different inverters
- Inverter output frequency tuning: the output frequency rate from inverters to engines varies from 0 to 400 Hz in most inverters. In tuning this frequency, we give its working frequency to inverters from the engine specifications. This frequency is 50 Hz in the most consumed electromotors in the Iran power industry.
Of course, in some situations, we can give up to 20% of this frequency to the engine, namely 60 Hz. We have to refer to the engine’s datasheet and its manufacturer to increase the frequency from this degree. In some Siemens company electromotors, it can be given to the engine up to 90 Hz in a specified interval.
It must be considered that increasing the output frequency up to 50 Hz leadsto an increase in inverter output voltage. This voltage is usually for the three-phase consumer in the electricity network of Iran (380- 400 volt). The output voltage cannot be raised in the higher frequencies.It will cause warming of the engine core. Thus, the necessary precaution must be respected in this work.
- Acc Time (Acceleration Time): acceleration time is the time that was elapsed from low frequency (minimum) to the user regulated frequency (maximum). For example, if the minimum frequency is 0 Hz and the maximum frequency is 50 Hz, and Acc= 5 sec, it will take 5 seconds to reach the intended frequency. Care must be taken in choosing this time. Because reducing this time to a definite limit may cause an overcurrent fault.
- Dec Time (Deceleration Time): the deceleration time is at the inverter stopping time, which will be precisely the opposite of Acc Time.
The first point: Acc and Dec settings, namely acceleration or deceleration, can be linear or curved.
Second point: there are second and third acceleration and deceleration times in some inverter creator brands. Hence, it can be reached the maximum or minimum value at several different times.
The third point: if the rate of Dec Time is less than the default value for a quick stop, we may have an overcurrent or overvoltage fault. . Therefore, we will need to use an external dynamic brake resistor to prevent this and rapid deceleration at relatively heavy loads. That will thoroughly be explained in the drives stopping section.
Inverters application methods
It can usually be used from inverters in several ways:
- From the inverter keyboard (or keypad mode), which in this method, the inverter start order, acceleration, and deceleration, and round left or right the engine will be performed from keyboard or keypad.
- 2. Perform the above steps through the inverter command terminals. That, they are often separate from the power terminals.
- 3. Use the network platform, which usually commands are given through PLC and networking with the inverter.
Output frequency adjustment modes
Command and control of drivers usually can be done concerning the selected modes in several ways:
- 1. Through the device keypad: 1. as mentioned earlier, this mode is used for start order, acceleration, deceleration, and left or right-turning of the engine by inverter.
- 2. Through the command terminals: one watt is performed using inverter analog input 0-10 VDC by potentiometer usually 10 KΩ.
- 3. Through the analog input 4-20 mA: this mode is usually used in pumps and fans speed control with PID controller. The sensor that produces 4-20 mA is mainly a pressure train.
- 4. Through the digital input terminals as up and down: in this method, the inverter must be run first, and then, digital inputs, which the user defines, are used for acceleration (up) and deceleration (down).
- 5. Output frequency variation through the network
- 6. Frequency variation through digital inputs in the form of multi-speed: this method principally uses three steps of frequency hopping. Which can be switched from one frequency to another according to user needs and order to the predefined inputs.
General parameters in driver settings
- 1. Acc Time and Dec Time
- 2. Engine power output
- 3. Rated engine voltage
- 4. Rated engine frequency
- 5. The number of engine poles
Highlight: the output current in more inverters to adjust the engine power is set by assumption. But it is changeable by the user.
Drivers stopping manner
It can usually be adjusted in several ways:
- 1. Ramp to stop: in this function, deceleration is performed through the time setting by Dec Time.
- 2. Free run to stop or coast stop: this mode is usually used in heavy loads with high inertia and self-heaviness of load stops the engine.
- DC BRAKE TO STOP: this mode is commonly used in heavy loads using dynamic brake resistance for braking. The performance manner is as follows: an intelligent switch connects the resulting energy from the engine to a brake resistor as soon as the feeding stops from the inverter to the engine. Therefore, we can stop the engine faster than in Dec Time mode. The brake resistor values and characteristics are available in the device catalog according to the inverter model and its power.
Starting torque adjustment (Torque Boost)
This work in the AC drivers is mainly possible in two ways:
- 1. Automatically starting torque adjustment:
this performance mode uses up constant loads which need the static torque, and the inverter applies a constant voltage to frequency ratio (v/f) to the engine. Which its performance is quite linear. For example, if an inverter in voltage of 400 volts exerts 50 Hz on the motor, its ratio will be 400/ 50= 8. For instance, this ratio for voltage of 200 volts is 25 Hz, and 200/ 25= 8. Therefore, in both cases voltage to frequency ratio is constant.
- 2. Manually starting torque adjustment we need a high starting torque at the movement initial time for heavy loads. Thus in these cases, we can do this work as a percentage for more flux production, which leads to high torque. In some AC drivers, both applied voltage and frequency percentages are changeable and provided to the user. This adjustment is practical for heavy loads starting at low frequency (beginning) by putting on a higher voltage percentage.
Protection parameters tuning
These parameters have been set as a default in most AC inverters. But some require user adjustment, and here we notice some of them:
- 1. Phase control at the inverter input and output
- 2. Percentage of overload level from engine regulated current rate
- 3. Overcurrent protection time tuning
Important note: in the electromotors setup by AC drive, we must pay attention to this point that the final voltage in the inverters to apply to the engine is equal to the input voltage of the power supply. Hence, if we put on a three-phase voltage of 380 V to the inverter input, we will finally receive the output voltage of 380 V. The input voltage in single-phase to three-phase AC drivers is 230 V. Then, we will eventually have access to three phases with a voltage of 230 V. Thus, we should notice the final voltage in the engine assembly.
Application of the variable frequency drivers(VFD) in Sima Foulad Jahan Company
- 1. For speed control and soft launch of four electromotors after area scissors to the capacity of 7.5 kilowatts.
- 2. To control the one pre-roll electromotor to the capacity of 5 kilowatts.
- 3. For control of one tail breaker to the capacity of 7.5 kilowatts.
- 4. For speed control and soft motion of electromotors of cranes longitudinal and transverse movements in Sima Foulad Jahan to the number of 15, and capacities of 2.2 and 1.1 kilowatts.
- 5. For speed control of one furnace needle to the capacity of 7.5 kilowatts.
- 6. For speed control and soft launch of roll guns before raffing the five electromotors with 55 kilowatts capacity.
- 7. To control and soft launch three electromotors before the guillotine to the capacity of 15 kilowatts.
Malfunction of engine, gearbox, and other mechanical accessories has improved 90 percent by using these drivers.