Whether it is a continuous process plant or a water treatment system, an induction motor is invariably a part of it. And the plant´s efficiency depends a lot on the performance of this critical component. Any unplanned stoppage of any of these motors can lead to multiple faults in the process sequence. Ramprasad Satyam elaborates on how to select the right motor manager and avoid huge losses in production due to such breakdowns.
As more and more companies are getting focused on efficient performance of complete systems, the demand for intelligent motor managers is on the rise too. The following parameters must be considered while selecting a motor manager:
Versatile motor control and protection options
Flexible communication and redundancy option
Motor modelling and diagnostics features, and
Devices like circuit breakers, soft starters with flexible communication options for complete integration.
Motor Control and Protection
Selection of the right motor manager is very important, especially if intelligent motor control center is planned for. The most basic requirement is that the electrical wiring within the MCC unit is kept to the bare minimum. More the wiring, lesser is the reliability of starter module. When motor managers are equipped with various control functions, e.g., star-delta starting function, the total wiring and the number of components per starter are automatically reduced. This improves the reliability of starters.
One must ensure that the motor manager has the following functions built-in and that they are configurable:
Starter options like: Direct online, star-delta starter, pole changing, actuator module and reverse starting.
Application specific, easily programmable logic with function blocks. These blocks should be configurable via LCD, without the need to connect with a PC or a laptop.
Protection functions include over/under load protection, over/under current protection, locked rotor protection, phase failure, phase imbalance, phase sequence and thermistor motor protection.
Protection trip class setting must be in accordance to IEC 60947-4-1 from Class 5 to Class 40.
Earth fault protection with the option of inbuilt or with external sensor must be available.
The minimum and maximum trip time, based on the trip class selected, can be referred to in the IEC 60947-4-1 document, which defines the trip class criteria. For example, the standard specifies that an overload relay with trip class 20 should switch off the motor within 6 to 20 seconds, when current is 7.2 times the rated current for the motor. The motor manager, to the least, has to meet the clauses mentioned in the standard. If the motor manager has a function that allows for changing the trip class, users will be able to change it at the site as well and reduce the maintenance inventory by getting rid of multiple relays of different trip class from the system.
Another important aspect, which the user must not forget, is to use external protection class CTs along with the motor manager. Some manufacturers insist on using their own CTs. That makes the system a bit costlier. However, there are others who offer standalone motor managers, while the external CTs can be sourced locally.
Communication capability is an important selection criteria considered while choosing the type of motor manager. Various manufacturers are offering systems that are restricted to a single communication protocol like Ethernet TCP, Modbus, Profibus, DeviceNet, etc. This restricts the user planning to expand the control system to only such devices that are capable of communicating via the existing protocol of the motor manager.
With developments in communication protocol and the introduction of Fieldbus Plug concept, electrical equipment like circuit breakers, soft starters, and motor managers can now communicate using multiple protocols.
Now the end user has the flexibility to change the communication protocol even after the MCCs are installed in place. Within an MCC, one can achieve various network topologies like daisy chain or star network as shown.
Some applications make it necessary to have redundancy in communication as well as in processors.
With the following configuration, one can achieve communication redundancy with single communication port on motor manager or circuit breakers (ACBs/MCCBs) or on soft starters.
With this configuration, one can achieve high availability or ´self healing ring´ characteristic, as it is known popularly in the market. With this configuration, the customer can achieve both communication redundancies in the processors as well as in the communication network. The configuration offers a very cost effective and reliable solution.
Motor modelling and diagnostics feature
Motor managers are usually equipped with various logarithmic functions, which develop thermal models based on both iron and copper parts of the motor. These models provide the best possible protection criteria for the motor. The motor manager continuously monitors the thermal capacity of the motor and gives a warning to the user in case of motor overloading. Based on such inputs, the user can take corrective actions in time and ensure that the motor does not trip and affect the production. Even if in any instance the motor trips, the motor manager indicates the cooling time required before the motor can be restarted.
Apart from this, the motor manager provides a variety of diagnostic information like operating hours, number of starts and trips, motor status, warnings and fault history, etc. The motor manager also monitors and records parameters like current, voltage, power factor, active/apparent power, energy and THD (total harmonic distortion). The THD parameter is very important since it helps users in knowing the exact level of harmonics in the network and can provide the right level of compensation to mitigate the harmonics.
For feeder protection, it is advisable to use circuit breakers with extensive protection features, making the overall system reliable and cost effective. The latest circuit breakers have the ability to adapt to various communication protocols with the Fieldbus Plug technology.
Offerings from ABB
ABB has expertise in both power and motor control. It offers complete solutions in line with customers´ requirements for motor and feeder protection in low voltage system with various communication protocols like Ethernet TCP, Modbus, Profibus, DeviceNet and CANopen. These solutions are offered both as product and as system depending on the needs of the customer.