In hard-to-start electric drive systems based on asynchronous wound-rotor motors, during start-up, rotation speed is controlled by turning off following sections of starting resistor with accordance to increasing speed of the electric machine.
Along with Simpax (formery Simet) company, the leading Polish manufacturer of industrial resistors, we recommend starters of ARE series - the fully automatic device for starting and speed control of asynchronous wound-rotor motors. Switching of resistance sections takes place automatically depending on the actual speed measured on the basis of the frequency of the current induced in the rotor winding. ARE resistance starters are equipped with a microprocessor controller MCR which controls the start-up and the speed of the motor.
A LITTLE BIT OF THEORY: Start-up and properties of wound-rotor motors
A long time ago to switch sections of starting resistance, time-delay relays were used. This solution, however, has several disadvantages, such as non-optimal start-up and shorten the life of the motor, due to the large inrush currents which may occur during start-up. The use of the controller which switches sections of starting resistor with accordance to actual value of the frequency of the current induced in the rotor winding, directly translates to extend the life of the motor and the other devices coupled from it.
Wound-rotor motors at zero speed, provides torque up to three times the rated torque, which is twice the value for squirrel-cage motors. In addition, these motors during start-up charge over two times less current from the power supply network. The relatively low cost of electric drive systems with asynchronous wound-rotor motors causes that the devices of this type are still used in industrial plants.
Electric drive systems based on asynchronous wound-rotor motors are also used in machines that require speed control, eg. mining lifts, gantries and trolleys.
Depending on your needs, the power circuit of the resistance starter may be made on the basis of steel-grid fixed resistors, round edgewound resistors or oil-isulated resistors. The solution will be developed especifically for you by a team of experienced engineers, based on a nameplate of a motor and a destination of a machine.
The advantages of electric drive systems based on automatic resistance starters of ARE series:
- very high reliability (better than with any other solution),
- ability to work in very harsh industrial environment,
- lower price of the whole drive system than the solutions using the variable-frequency drives,
- easier handling and operation in relation to the variable-frequency drives,
- we offer the complete solution for controlling asynchronous wound-rotor motors.
THE DIGITAL CONTROLLER OF THE RESISTANCE STARTER: Microprocessor Frequency Controller
The automatic resistance starters are equipped with microprocessor controllers called MCR (Eng. MFC). Microprocessor Frequency Controller is a modern digital device which controls start-up and gradually adjusts the rotational speed of asynchronous wound-rotor motors. The main purpose of the device is to continuously measure the actual value of the frequency of the current induced in the rotor winding and switching the appropriate resistance sections, corresponding to the measured frequency. The device automatically provides a smooth and optimal start-up of the machine with the changing load, which is a big advantage compared to the formerly used solutions based on time-delay relays. It should be noted that when the operator presses the start button, then the start-up takes place fully automatically. The controller can handle up to eight sections of starting resistance - it has built-in eight fully configurable independent software relays, each of which can act as a frequency relay, time-delay relay or combined relay (frequency + time-delay). In addition, for individual relays you can set the lock triggered by an external signal or you can configure the special controller of relays which makes you able to activate individual relays by external signals. It is worth noting that mentioned controller of relays allows you to use the device to the progressive speed control of an asynchronous wound-rotor motor, which is used for example in mining lifts and gantries. The controller uses the transistor outputs, which translates into a lower failure of the device; electromechanical relays are installed directly in the switching housing, so it is easy to replace them if necessary. The MFC also offers many additional features that make it a complete solution for the optimal start-up, braking and stopping the asynchronous wound-rotor motor. The controller can be connected to a series of sensors and switches, including temperature sensors, emergency stop buttons, direction switching buttons and the stop button. The MFC allows you to control the cooling of resistors in time mode or temperature mode. In the case of oil-cooled resistors, you can use the function of the controller used to control the oil level and its temperature. The MFC has built-in anti-stalling protection, whose mission is to emergency stop control in a situation where, due to too heavy load, the motor cannot start. In addition, the controller has implemented a hysteresis function and two digital filter algorithms to eliminate possible interference in a harsh industrial environment. The controller has a graphic LCD display, which informs about the current state of the device. The operator can read the actual value of the frequency of the current induced in the rotor winding or check what starting sections are currently turned on. In addition, in the display is continuously plotted the chart showing the frequency as a function of time, so you can control the process of start-up the motor, without the need for additional measuring equipment. Any technological parameter settings is done via an intuitive menu that shows all messages in Polish or English. Access to the menu can be password-protected. The service staff can read the historical events that have occurred during start-up.
Detailed information about automatic resistance starters of ARE series can be found on the Simet company's website at: http://simpax.pl/pl/rodzaje-rezystorow/rezystory-rozruchowe/rozruszniki-automatyczne-rezystancyjne.
power resistors, industrial resistors, resistance starters, hard-to-start, hard start-up, asynchronous wound-rotor motors