RESISTANCE TEMPERATURE DETECTORS(RTDs)

The RTD (Resistance Temperature Detector) , also called resistance thermometer, is a temperature sensor that exploits the predictable change in electrical resistance of some materials with changing temperature. Resistance is measured by applying a constant current and measuring the voltage drop across the resistor.

Ohm’s Law defines the proportionality between the resistance and the voltage. Therefore the voltage is a direct measure for the resistance and thereby the temperature. That means, when using resistors (RTD's) for temperature measurements, the electrical resistance of a sensor subjected to the temperature is the variable utilized.

By far the most common RTD's used in industry have a nominal resistance of 100 ohms at 0 °C, and are called Pt-100 sensors.

• Circuit Configurations

In practice three different circuit configurations are distinguished and described in the following.

In a two-wire circuit a current is applied to the temperature dependent resistor RT from a constant current source. The voltage drop across RT is measured by the temperature transmitter and converted. The resultant value, however, is incorrect because of the series resistances of the measurement leads (RL1 + RL2) and the contact resistances at the terminals (RK1 + RK2). Accordingly the two-wirde circuit is not suitable for exact temperature measurements. 

In a three-wire circuit two constant current sources are used, in order to compensate for the disadvantages described above for the two-wire circuits. Similar to the two-wire circuit the current source IK2 is used to measure the temperature dependent resistance RT including the lead and terminal contact resistances. The additional current source IK1 together with a third lead is used to separately compensate the lead and terminal contact resistances. Assuming the exact same lead and terminal contact resistances for all three leads, the effect on the accuracy of the temperature measurements can be eliminated. But practice has shown that it is not always possible to assure that the terminal contact resistances are always identical.

The four-wire circuit eliminates all the previously described disadvantages. In this configuration a constant current source is used to apply a current to the temperature dependent resistance RT. The voltage drop across resistance RT used for the temperature measurement is measured by two high resistance leads. In this way the voltage drop due to current flowing during the measurement is negligible and the lead and terminal contact resistances RL1, RK1, RL2, RK2 do not impact the measurement result. The four-wire circuit is therefore always used when highly accurate temperature measurements are required.

The thermal voltage resulting from the Seebeck-Effect is utilized in a thermocouple (TC) as the measuring principle. You will find additional information on thermocouples

WINDING THERMOSTATES

Winding thermostats are snap-action, bi-metallic, temperature actuated switches normally installed in the connection end turns of the motor winding. Their purpose is to activate a warning device, or shut down the motor upon excessive winding temperatures.

Thermostats are made with contacts that are either normally closed (N.C. - open at high temperatures) or normally open (N.O. - closed at high temperatures). The thermostat temperature switch point is pre-calibrated by the manufacturer and is not adjustable. Reset is automatic after a decrease in temperature. This feature allows the motor to cool before restarting. No control unit is required for use with thermostats.

The normal procedure is to wire three thermostats together as a set, with one thermostat embedded in each phase. Normally open thermostats are wired in parallel internal to the motor. Generally normally open thermostats are used to activate a warning alarm (bell, light, etc.)

Methods of Motor Winding Temperature Protection

Winding protection is providing protection against excessive temperature. Excessive winding temperature can permanently damage the winding, greatly reducing winding life and can cause complete winding insulation breakdown and failure. Down time in many motor applications are prohibitive and a good monitoring system is essential to minimize costs which can be incurred. A complete protection system for a winding is designed to remove the motor from the line when any of the following conditions occur:

1. Locked Rotor - a mechanical failure of some type which locks the rotor and prevents it from turning when power is applied to the motor.

2. Starting Overload - an excessively high overload due to increased friction or inertia on the drive system can cause excessive temperature on the winding and permanent damage.

3. Running Overload - an abnormal condition which overloads the motor when it is running causing it to draw higher current then designed for. This will permanently damage the motor winding.

4. Abnormally High Temperatures - an environmental condition where the motor is exposed to abnormally high temperatures can cause the total winding temperature to reach a point where permanent damage can occur.

5. Voltage Unbalance - since the winding temperatures increases by a percentage equal to 2 times the square of the voltage unbalance, this can easily cause excessively high winding temperatures resulting in permanent winding damage.

6. High or Low Voltage - depending on winding design, high or low voltage can cause excessive currents and permanent winding damage.

7. Ventilation Failure - blocked air openings, broken fans, or anything which may disrupt the normal ventilation system on a motor to open when the motor is running will cause abnormally high winding temperatures resulting in failure.

There are three basic methods of protecting a motor from winding failure.

1. Current Sensitive Devices - these include circuit breakers, fuses, motor starter heaters of all types and instantaneous current relays. These are usually always external to the motor and are used in accordance with "National Electrical Code" which sets installation of motor branch circuits.

2. Combination Current & Temperature Sensitive Devices - these consist of a thermal disc and heater in on e unit are line break devices. These devices must be properly sized for each rating and application. They are often used in high volume U/L listed applications such as air conditioning.

3. Temperature Sensitive Devices - since temperature is the limiting factor in protecting a motor, a direct sensing of motor winding temperature is the logical approach. There are a variety of winding temperature devices and systems all being designed to open the pilot circuit to the motor starter. These winding temperature protection systems can be classified according to their mode of operation. The basic detector systems in use today are:

A. Temperature Switches

B. Resistance Temperature Detectors (RTDs)

C. Thermistors

D. Thermasentry

E. Thermocouples

SYNCHRONOUS MOTOR

Synchronous Motor Working Principle

Electrical Motor in general is an electromechanical device that converts energy from electrical domain to mechanical domain. Based on the type of input we have classified it into single phase and 3 phase motors. Among 3 phase motors Induction and synchronous motors are more widely used.

When a 3 phase electric conductors are placed in a certain geometrical positions (In certain angle from one another) there is an electrical field generate. Now the rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with same speed of rotating field. Synchronous motors is called so because the speed of the rotor of this motor is same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words it’s in synchronism with the supply frequency. Synchronous speed is given by

Construction of synchronous motor

Normally it's construction is almost similar to that of a 3 phase induction motor, except the fact that the rotor is given dc supply, the reason of which is explained later. Now, let us first go through the basic construction of this type of motor

Main features of synchronous motors are

• Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.

• The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

• This motor has the unique characteristics of operating under any power factor. This makes it being used in power factor improvement

Principle of Operation Synchronous Motor

Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it. It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply. The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux. The rotor carrying DC supply also produces a constant flux. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolution in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of same polarity (N-N or S-S) causing repulsive force on rotor and the very next second it will be N-S causing attractive force. But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Hence it is not self starting.

To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed. After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.

Methods of starting of Synchronous Motor

• Synchronous motors are mechanically coupled with another motor. It could be either 3 phase induction motor or DC shunt motor. DC excitation is not fed initially. It is rotated at speed very close to its synchronous speed and after that DC excitation is given. After some time when magnetic locking takes place supply to the external motor is cut off.

• Damper winding : In case, synchronous motor is of salient pole type, additional winding is placed in rotor pole face. Initially when rotor is standstill, relative speed between damper winding and rotating air gap flux in large and an emf is induced in it which produces the required starting torque. As speed approaches synchronous speed , emf and torque is reduced and finally when magnetic locking takes place, torque also reduces to zero. Hence in this case synchronous is first run as induction motor using additional winding and finally it is synchronized with the frequency.

Application of Synchronous Motor

• Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any power factor, it is used in power system in situations where static capacitors are expensive.

• Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500KW, the size, weight and cost of the corresponding induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc

TYPES OF INDUCTION MOTOR

Types Induction Motor

SINGLE PHASE INDUCTION MOTOR

• Split phase Induction motor

Capacitor start Induction motor

• Capacitor start capacitor run Induction motor

 

Shaded Pole Induction Motor

THREE PHASE INDUCTION MOTOR

• Squirrel cage Induction motor
• Slip ring Induction motor

We had mentioned above that single phase Induction motor is not a self starting and three phase Induction motor is self starting. So what is self starting? When the machine starts running automatically without any external force to the machine, then it is called as self starting. For example we see that when we press the key the fan starts to rotate automatically, so it is self starting. Point to be note that fan used in home appliances is single phase induction motor but it is self starting. How? We will discuss it how.

Why is three phase induction motor self starting?

In three phase system, there are three single phase line with 120° phase difference. So the rotating magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a, b and c, when phase a is magnetized, the rotor will move towards the phase a winding, in the next moment phase b will get magnetized and it will attract the rotor and than phase c. So the rotor will continue to rotate.

Why single phase induction motor is not self starting?

But what about single phase. It will be having only one phase still it makes the rotor to rotate, so it is quite interesting. Before that we need to know why single phase induction motor is not a self starting motor and how the problem is overcome. We know that the ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly distributed stator winding. Since pulsating magnetic field can be assumed as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting and due to this the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by external force, then the motor will start to run. This problem has been solved by making the stator winding into two winding, one is main winding and another is auxiliary winding and a capacitor is fixed in series with the auxiliary winding. This will make a phase difference when current will flow through the both coils. When there will be phase difference, the rotor will generate a starting torque and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor is disconnected from the motor but if we rotate with hand it will start to rotate. So this is the reason of using capacitor in the single phase motor. There are several advantages of induction motor which makes this motor to have wider application. It is having good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is an disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the sequence of three phase supply, i.e. if RYB is in forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.

INDUCTION MOTOR

Induction Motor | Working Principle | Types of Induction Motor

One of the most common electrical motor used in most applications which is known as Induction Motor. This motor is also called as asynchronous motor because it runs at a speed less than synchronous speed. In this, we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it depends upon the frequency and number poles of the machine. An induction motor always runs at a speed less than synchronous speed because the rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed. There are basically two types of induction motor that depend upon the input supply - Single Phase Induction Motor and Three phase Induction motor. Single phase induction motor is not a self starting motor which we will discuss later and three phase Induction motor is a self-starting motor. Now in general we need to give two supply i.e. double excitation to make a machine to rotate. For example if we consider a DC motor, we will give one supply to the stator and another to the rotor through brush arrangement.

Working Principle of Induction Motor

An electrical motor is such an electromechanical device which converts electrical energy into a mechanical energy. In case of three phase AC operation, most widely used motor is Three phase induction motor as this type of motor does not require any starting device or we can say they are self starting induction motor.

For better understanding the principle of three phase induction motor, the basic constructional feature of this motor must be known to us. This Motor consists of two major parts:

Stator: Stator of three phase induction motor is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase windings are arranged in such a manner in the slots that they produce a rotating magnetic field after AC is given to them.

Rotor: Rotor of three phase induction motor consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motor.

Working of Three Phase Induction Motor

Production of Rotating Magnetic field

The stator of the motor consists of overlapping windings offset by an electrical angle of 120°. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Secrets behind the rotation:

According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor windings in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.

Here the relative velocity between the rotating flux and static rotor conductor is the cause of electric current generation; hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.

Thus from the working principle of three phase induction motor it may observed that the rotor speed should not reach the synchronous speed produced by the stator. If the speeds equals, there would be no such relative velocity, so no emf induction in the rotor, & no current would be flowing, and therefore no torque would be generated. Consequently the rotor can not reach at the synchronous speed. The difference between the stator (synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor has the advantage that no electrical connections need to be made to the rotor.

Thus the Three Phase Induction Motor is:

• Self-starting.
• Less armature reaction and brush sparking because of the absence of commutators and brushes that may cause sparks.
• Robust in construction.
• Economical.
• Easier to maintain.

But in induction motor we give only one supply, so it is really interesting to know that how it works. It is very simple, from the name itself we can understand that there is induction process occurred. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction, electric current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type.