Electrical

What is MCC and what are the various components involved in it?

What is a motor control center?

A motor is a device that utilizes electrical energy to do mechanical operations such as rotating a pump, fan, blower, etc. Due to industrial development, the requirement for the automation and mechanization of the different industrial processes is increased. The mechanization of the different industrial processes requires many motors, so in order to control all these motors, MCC’s are required. In the below image you can see the MCC, it is a big enclosure that would house the major motor control equipment. The MCC has the ability to plug in the control components, these components can be plugged in or unplug and thus there is no need for wiring the devices. The MCC would be composed of a busbar and other control equipment which can be utilized to control the operation of the motor and also to place the components in an integrated panel. The MCC also has different motor starting methods such as the DOL and star-delta starter.

Representation of MCC
MCC operation

What are the major components in a motor control center?

  1. Bus bar
  2. Circuit breaker
  3. Magnetic contactor
  4. Contactor auxiliary contact
  5. Relay control
  6. Control transformer
  7. Cable installation control panel enclosure
  8. Limit switch

MCC can be used to control multiple motor operations, as we discussed above the MCC is composed of several components such as a motor starter, bus bar, control equipment, etc. All these components are used to control the electric motor and these components are placed in a panel which is a mixture of iron metal and carbon iron.

Bus bar

A bus bar is a bar that would be made up of brass, copper, etc, the major function of a bus bar is to provide or distribute the required power to the electrical equipment. So in an MCC, the bus bar would act as a power distributor. The major advantage of using a bus bar in an MCC is that we can eliminate the wiring of several components, so because of this, the size of the MCC would be reduced and also the number of parts in it. So we can easily connect the electrical equipment in an MCC with the help of a bus bar and this would also be useful to locate the error. We can easily troubleshoot the problems in an MCC with the help of a bus bar.

Overload relays

These relays would protect the motor from overheating, the overheating could be caused due to the overload of the driven machinery. It can also be caused due to the low voltage level or if there is an open phase in a three-phase system. So when the excessive current is drawn for a specified amount of time the overload relay would open and the motor will be disconnected from the power source. This relay would allow the temporary overload which could not cause any damage to the motor and block the over-load which could damage the motor.

Miniature circuit breaker

The purpose of the MCB is to disconnect a circuit in case if there is an excessive current flow in the circuit, or in case if there is an electrical load that exceeds capacity.

Control Transformers

Sometimes it would be necessary to operate the control circuit at low voltages than the power circuit. So to decrease the voltage level we can use a control transformer

Magnetic contactors

A magnetic contactor is used in an MCC to start or stop a motor. Mostly there would be a remote control device to start and stop the motor and for this purpose, we can use a contractor. The operation of the magnetic contactor would be based on the electromagnetic principle.

Time delay relay

The major purpose of a time delay relay is to provide the on delay and off delay timing. So this device can be used for the automatic time setting, this device can be combined with the other control equipment in an MCC, like a magnetic contactor.

Limit switch

This component is used to provide a signal in case if a specified limit is reached like if there is an excess overload. The limit could be the rotating speed of the machine, position of the machine part, etc. We can use this device as a replacement to the human operator and it can be used where it is not possible for a human operator to operate.

What is the major purpose of an MCC?

Motor starting

The motor starting is an important process, and the starting of the motor should be slow and gradual and this would be useful to protect the machine. Some industrial machines could be damaged if it is operated very fast initially instead of gradually increasing the speed of the machine.

Operational control

This device should maintain the required operational speeds and characteristics. So by maintaining or controlling the operational speed it can protect the motor, machine, materials, and operator. 

Stopping

The MCC must be capable to provide the fast stopping of the motor because in certain cases the machine must be stopped quickly and this should be achieved by the MCC. Emergency stops must be carried out for certain industrial machinery.

Reversing

The direction of the machine rotation would be required to change in an industrial process and this should be done easily. In certain cases, continuous changing of direction is required in an industrial process.

Damage prevention

The machine should be protected from any damages because it could cost a lot. An example of this would be preventing the pileups in a conveyor by a machine. So the machine must do the necessary actions such as reverse, stop, slow. The MCC must control the machine according to the process requirement.

Motor protection

The MCC must protect the motor from

  • Overcurrent
  • Overload
  • Low voltage
  • Phase failure
  • Phase reversal protection
  • Ground fault protection

What are the considerations that must be taken while installing a motor controller?

  • Electrical service – We must make sure whether the service is AC or DC, if it is AC then frequency and number of phases in addition to the voltage must be considered.
  • Motor- The motor must be connected to the electrical service and also make sure that it is correctly sized for the machine load in case of the HP ratings. We must also consider the speed and torque of the motor.
  • The controller should be capable to start, stop, reverse the motor operation according to the requirement
  • The controller must be placed where it won’t be affected by certain environmental conditions such as rain, snow, dirt, oils, or lubricants.

What are the different types of motor control centers?

Classification of the motor control center

We can classify the motor control center according to the type of supply voltage and also according to their operation.

Based on supply voltage

  • Low voltage MCC
  • Maximum voltage MCC

Low voltage MCC

Low voltage motor control centers are typically used for low current three-phase motors from 208 V to 600 V.

Maximum voltage MCC

The maximum voltage MCC is used for large motors that run from 2300V to 15000V. It uses a vacuum contactor to switch and also has separate compartments for switching and power control.

Based on their operation

  • Manual
  • Semiautomatic
  • Automatic

Manual control

So in this type of MCC operation, the operator would need to directly go to the controller location in case if any changes should be made. Basically, this is a simple device and it would directly connect the motor to the line. This type is used to control the motor which has a maximum of 10 horsepower. The manual operation of the starter is an ON-OFF switch, which can be manually operated and it would also act as a safety device too.

Semiautomatic control

This type of MCC operation is carried out by pushbuttons, limit switches, pressure switches, etc. This type of MCC also consists of certain sensing devices to control the operation of the magnetic contactor or the starter. The starter would connect the motor to the line and the pushbuttons would control the starter coil. Due to this, the control panel can be located away from the motor or the starter. The operator must do certain operations such as starting or stopping the motor but there is no need to go to the location of the motor or starter to do this process.

Automatic control

In this type of MCC operation, there would be a control circuit and this control circuit would be composed of certain sensing devices. This control circuit can start or stop the motor, it would stop the motor in case of any fault conditions such as overload of the motor excessive current.

How should we select a motor control center?

  • Ampacity – We should consider the ampacity of the MCC while selecting it, ampacity is the maximum amount of current that a horizontal bus can handle
  • We must also consider the available fault current level while selecting, basically, it is the amount of current which would flow in case of any short.
  • The bussing material which is used for the electrical bussing must be considered too.
  • Feeder cable is another important part of an MCC, we must consider how the feeder cable is connected to the MCC. The connection would be of two types one will be the overhead and the other would be the underground method. The power cables that would provide the power to the MCC would be large in size and numerous too. So if we know the details of the feeder cable then it could be useful to do the cable termination and also to avoid difficult wire bends.
  • We should also consider the operating conditions such as full load current, locked rotor current, noninductive current, number of poles, and also the total number of expected operations.

What are the advantages of motor control centers?

  • Easy installation
  • Floor space can be saved
  • Expandability is another feature of the MCC because we can add sections and units to it.
  • The safety level is increased
  • Faster operation
  • Centralized operation
  • Remote control with interlock
  • Downtime can be reduced because we can easily detect the problem by the MCC and we can unplug the faulty part

What are the applications of motor control centers?

  • Ships
  • Refineries
  • Hotels/shopping malls
  • Power plants
  • Steel plants

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