Overload protection of motors

Before you knowing about overload protection check out the post about contactors link given below

what is contractors

Overload protection prevents an electric motor fromdrawing too much current, overheating, and literally burning out

How Motors Work?

A motor goes through three stages during normal operation: resting, starting,

A motor at rest does not require current because the circuit is open. But once the circuit is closed, the motor starts generating a tremendous input current; up to 6-8 times its operating current. Here’s the problem: this large input current can cause the immediate trip of the circuit breaker. A fuse or circuit breaker sized to handle the normal operating load of the motor will open the circuit during start-up. You might think that sizing the fuse or circuit breaker for the peak in current consumption would solve the problem. But if you did this, once the engine was running, only the most extreme overload would open the circuit. Minor overloads would not trip the switches, and the motor would shut down

What Is An Overload?

The term literally means that too much load has been placed on the motor. A motor is designed to run at a certain speed, called its synchronous speed. If the load on the motor increases, the motor draws more current to continue running at its synchronous speed. It is quite possible to put so much load on a motor that it will draw more and more current without being able to reach synchronous speed. If this happens for a long enough period of time, the motor can melt its insulation and burn out. This condition is called an overload.

Because of the way a motor works, an overload protection device is required that does not open the circuit while the motor is starting, but opens the circuit if the motor gets overloaded and the fuses do not blow.

Overload relay

The overload relay is the device used in starters for motor overload protection. It limits the amount of current drawn to protect the motor from overheating.

An overload relay consists of:

• A current sensing unit (connected in the line to the motor).
• A mechanism to break the circuit, either directly or indirectly.

To meet motor protection needs, overload relays have a time delay to allow harmless temporary overloads without breaking the circuit. They also have a trip capability to open the control circuit if mildly dangerous currents (that could result in motor damage) continue over a period of time. All overload relays also have some means of resetting the circuit once the overload is removed

Overload relay types:

• Eutectic (melting alloy)
• Bimetallic
• Solid State

1.The Eutectic Overload Relay

The fusion alloy (or eutectic) overload relay consists of a heating coil, a eutectic alloy, and a mechanical mechanism to activate a triggering device when an overload it happens The relay measures the motor temperature by controlling the amount of current that is drawn. This is done indirectly through a heater coil. There are many different types of heater coils available, but the operating principle is the same: a heater coil converts the excess current into heat that is used to determine if the engine is in danger. The magnitude of the current and the the time that is present determines the amount of heat registered in the heater coil.

Usually, a eutectic alloy tube is used in combination with a ratchet wheel to activate a tripping device when an overload occurs. A eutectic alloy is a metal that has a fixed temperature at which it changes directly from a solid to a liquid. When an overload occurs, the heater coil heats the eutectic alloy tube. The heat melts the alloy, freeing the ratchet wheel and allowing it to turn. This action opens the normally closed contacts in the overload relay.

2.The Bimetallic Overload Relay

A bimetallic device is made up of two strips of different metals. The dissimilar metals are permanently joined. Heating the bimetallic strip causes it to bend, because the dissimilar metals expand and contract at different rates. The bimetallic strip applies tension to a spring on a contact. If heat begins to rise, the strip bends, and the spring pulls the contacts apart, breaking the circuit,

Once the firing action has taken place, the bimetal strip cools and reshapes in itself, automatically restarting the circuit. The motor restarts even when the overload it has not been erased, and it will fire and restart again and again. (This assumes an automatic restart This type of relay can also be equipped with a manual reset).

The Solid State Overload Relay

Unlike the other two types of relays, the solid state overload relay does not generate heat to facilitate a trip. Instead, measure the current or a change in resistance. The advantage of this method is that the overload relay does not waste energy that generates heat and does not increase the cooling requirements of the panel.

The current can be measured through current transformers, then it becomes a voltage which is stored in the memory inside the overload relay. If the relay notices that the the current is higher than it should be for a too long period of time, it shoots up. Another type of solid state overload relay uses sensors to detect the heat generated in the engine When the sensor also detects heat above the preset value for a long period of time, it shoots up.

The solid state overload relay also provides some advanced functions.

1. It is possible to provide proactive functionality and improved protection against special conditions. For example, when the conditions of high ambient temperature exist, devices that use sensors can detect the effect of room temperature taking in the engine.
2. Some solid state overload relays offer programmable trip time. This could be Useful when a load takes longer to accelerate than traditional overload relays will allow, or when you want a travel time between traditional travel classes.
3. Some overload relays have a built-in emergency override to allow the motor to start even when you could damage the engine to do it. This could be useful in a situation where the process is more important than saving the engine.
4. Some solid state overload relays can detect the change in current when an engine suddenly it is downloaded. In such situation, the relay will trigger to notify the user that there is an application problem. Normally, this indicates a system problem instead of a motor problem.

What is a transformer and what are its uses? How does a transformer work

What is a Transformer

A transformer is a static equipment, with two or more windings linked to a common magnetic field. By using electromagnetic induction this device will transfer electrical energy from one circuit to another without any frequency change.

• Real and ideal transformer 34
• Toroidal transformer 4
• Transformer losses and how to reduce it, Efficiency of a transformer 9
• How transformer is constructed 15
• Instrument and audio transformer 4
• Distribution transformer 5
• Control transformer 4
• Transformer faults and how to protect the transformer from these faults 4
• Current transformer and its types 2
• Voltage transformer 3
• Auto transformer 3
• Power transformer 1
• Isolation transformer 6
• Transformer cooling 1
• Three phase transformer 3
• How to test a transformer 4
• Grounding transformer 3
• Pulse transformer 3
• Furnace transformer
• Transformer protective devices 4
• Auxilary current transformers 4
• Instrument and audio transformer 3

What are the functions of a Transformer

• It changes the voltage level

• It changes the current level

• It changes the impedance

• It can provide isolation

• Filters DC from a waveform that is a mixture of AC and DC

How does a transformer work

A transformer has two electric circuits called primary and secondary, a magnetic circuit provides the link between the primary and the secondary. Alternating current is supplied to one of the winding and when the current will reach the electric coil there will be an alternating flux surrounded to that coil so if we bring another coil near to the first one then there will be an alternating flux linkage with that second coil and so there will be rate of change in flux with respect to time in the second coil and thus EMF will be induced in it.

What is the use of a transformer

Transformer plays a major role in power transmission, transformers are used to increase the voltage before transmitting electrical energy over long distance through wires. Step-up transformer will decrease the current to keep the power into the device equal to the power out of it. In power stations the power is generated at the voltage of 12KV to 25KV, transformers are used to step up this voltage between 110KV to 1000KV for transmission over long distance with a small loss. Step down transformer is used to decrease the voltage for local distributions such as in homes, factories, and offices as low as 120V.

Types of service

Power transformer

• These transformers are used in generating stations and these are above 500 KVA

• They have maximum efficiency near full load

• Iron losses are 5-6 times when compared to distribution transformer

Distribution transformer

• It is used in the distribution section and they are up to 500 KVA

• They are operated for 24 hours

• It has maximum efficiency near 50% of full load

What are the major parts of a transformer

• Core

• Windings

• Transformer tank

• Conservator

• Breather

• Cooling arrangements

• Tap Changer

• Bushings

• Accessories

Core

The cores of the transformers are made from a special type of iron. Iron cores are used to reduce the reluctance of flux path, and because of that only a small amount of current is required to induce flux. The cores won’t be made up of a solid block of iron. Because the solid core would act as a short-circuited turn. High circulating currents will be formed in the core because of the solid core, and that would result in high losses. These circulating currents are called eddy current loss, and it can be reduced by laminating the transformer cores next to each other and each lamination will be insulated from others. Core construction is of two types they are core form type and shell form type. In the core form type, the windings are constructed around the laminated cores. In case of the shell form type, the laminated core is constructed around the windings

Parts of a core

The part which carries transformer windings is called legs and the part which connects the legs, which also closes the magnetic circuit is called yokes.

Windings

The winding is the electrical circuit of the transformer, it is made up of highly conducting copper with high dielectric strength.

These winding needs to be strong mechanically and electrically to withstand both over-voltage and mechanical stress.

Dielectric circuit

It consists of insulation used at different places in a transformer all the magnetic and conducting part of the transformer must be insulated well.

Major insulation

This insulation is between the core and low voltage winding and similar insulation is used for low and high voltage winding.

Minor insulation

It is done between the elements of a given winding, insulating turns, layers, coil, conductor.

Transformer tank

It provides a protective cover to the core, it provides an external surface for heat dissipation, it is filled with insulating oil for placing core and coil assembly. It does the protection of the windings and other internal parts.

Conservator

It is a cylindrical metal drum, the main function of the conservator is to keep the main tank of the transformer completely filled with oil in any circumstances. It takes up the contraction and the expansion of transformer oil and keeps the main tank full of oil and it reduces the rate of oxidation of the oil because less, oil surface is exposed to the air and thus the sludge formation can be reduced mostly the capacity of the conservator is nearly 10 to 12 percent of the volume of oil of the main tank.

Installation procedure for Motor Control Center (MCC)

Here we discuss the installation procedure for MCC panel.

Using the crane/fork lift, MCC panels 21 must be carefully removed from the vehicle and moved to the correct storage location. Items found inappropriate for the project shall be immediately removed from the site.

MCC panels shall be stored separately in a protected position as specified by the manufacturer for the ambient condition. Factory packing of MCC panels shall be removed only at the place of installation to prevent damage.

Installation procedure for MCC panel:

• The exact location of the panel and the fixing holes to be located on the concrete plinth given for the installation by the main contractor
• Remove the packing and ensure that the panel is free from transportation damages
• In the case of wall-mounted panels, the exact location of the panel and the fixing holes to be identified on the basis of approved shop drawings shall be installed in an approved manner.
• Provide fixing arrangement in an approved manner in the marked location.
• Place the panel on the plinth, correctly align
• Gland the cables on the panel’s gland plate (power / earthing) and finish properly.
• All the connections should be tightened
• Entry around the panel to be reviewed in compliance with the regulations for future maintenance.
• Ensure that the services contain water away from the panel or are adequately protected from accidental leakage.
• Panel identification and outgoing breaker identification shall be contrary to approved shop drawings.
• Incoming and outgoing cables are marked / identified according to the authorised drawings of the store.
• All panel components such as MCCB, MCB, Relays, Fuses, Meters, CTs, Contractors, Terminals, etc. shall be tested for correct rating and size against the approved panel drawings.
• The supplier of the specific switchgear must perform any internal connections / modifications.
• All breakers (incoming / outgoing) must be in the “OFF” role and must be locked in order to prevent mishandling.
• Check and ensure adequate space is available for maintenance.
• To prevent dust and contamination, the panel must be properly cleaned and protected after installation.
• A request for inspection shall be submitted for verification of QA / QC 10. Work inspection request shall be submitted for inspection and sign-off by the consultant.

How to set parameters for ABB ACS880 drive

This drive has a new panel while compared to the previous version of ABB ACS800, this device also has a bigger LCD display. It also has a USB port so we can connect a PC to this.

First press the local remote key to, do the local operation in the below image we can see a button on the top right side it can be used to select the menu. By using the same button we can also choose the parameter too and it is shown in the below image. In order to set the parameters, we must enter the motor data so select the parameter group 99 and we can see many sub-parameters and we can enter the motor data to it. Set the parameter 9903 to the asynchronous motor and this is for induction motors. Next chose the parameter 9904 which is the motor control mode and this must be set as DTC the DTC stands for direct torque control. Select the parameter 9906 to set the motor nominal current so check the motor nameplate that is chosen to connect to the drive and after setting the parameter we must save it. Then select the parameter 9907 for motor nominal voltage and we must enter the valve that is shown in the motor nameplate.

After that select the parameter 9908 to set the motor nominal frequency and set the required frequency. Then select the parameter 9909 to set the motor nominal speed so enter the value shown in the motor nameplate and this will be in hertz. Select the parameter 9910 to set the nominal power in the motor nameplate this will be displayed in horsepower or in kilowatts. Select the parameter group 23 in order to change the acceleration and deceleration time.

Press the top left key which is seen in the panel. Select the parameter 23.12 to set the acceleration time after adding the value save it and then go to parameter 23.13 to set the deceleration time. To go back to the home menu press the top left key more than once. We can also change the speed reference by using the up and down buttons. We can also change the speed during the operation according to our need, in order to change the direction of the motor select the options in the menu.

MCC panel & smart MCCs

MCC panel:

MCC stands for Motor Control Center which is a floor mounted assembly for motor control. The MCC contains all the motor control units, feeders for the motors 15 and blowers. They are specially designed for different motor ratings and they are available in automatic and manual operation.

An MCC is a steel structure that contains the combination of engine control units, raceways, internal wiring and busbars. A vertical section can be independent as a complete motor control centre, or several sections can be screwed and transported together.

The MCC is connected with PLC 25/DCS 12 control panels, for that large quantity of cabling and connections was necessary in conventional MCC. But it is less needed with latest smart MCCs. It receives a digital signal from the control panel and also can be operated by the field operators remote mode. The MCC produce the control action for the motor in response to the digital input.

The MCC links the digital controller with the analog field actuators. The MCC contains timers, counters, for the necessary operations.

What is the difference between Control panel and MCC?

A control panel usually consists of a controller that helps to give digital signals to the MCC panel to start the engine. They are specially designed to protect electrical equipment from heavy fluctuations in loads. These include PLC, VFD, fuses, switches; transformers and many other necessary components that are necessary to control the voltage.

While MCC is designed especially for motor operations. The MCC receives a signal from the control panel and which is used to control motor. The MCC is directly connected to the analoge motor. Thus both are incomplete without the other.

What is a Smart MCC?

An smart MCC is an intelligent device used to control motors and monitor their operation; to monitor the energy consumption, the quality of the energy and the functioning of the system; and to communicate quickly with a PLC or process control system through a data network.

Advantages of Smart MCCs:

• Low system installation and commissioning costs
• Low maintenance cost
• Low energy costs
• Reduced downtime
• Increased system efficiency
• Increased system information