Wednesday, December 7, 2011

Inductive Loads

Inductive parts make a circuit complicated, but it can be said like “inductive parts make the circuit interesting” too. What is the special with these parts? These inductive parts come with some interesting characteristics like; back EMF, magnetic filed, Heat generation, coil in shape, changing resistance. When we are working with any type of coils, motors, heaters we are working with these inductors so it is good to understand them.
Any inductive part can be represented by a back EMF (an opposite voltage that is produced due to electromagnetic act in the coil) and a load. This back EMF limits the current through the circuit; with the current get higher back EMF become higher.


Effect of the back EMF can be clearly seen when the switch is turned ON or OFF


When working with inductors we have to be careful about lot of factors mainly,
1. Back EMF can damage the components
2. Generated noise (by magnetic filed) can lead the system to produce uneven and un predictable outputs.

Electromagnetic relays

This allows us to isolate the switching part from high current circuit area, For the high load part (relay output) there is a rated current and a voltage. As there is a small difference between two output terminals, we cannot have a large voltage between them. And large current can lead to heating and bending of contacts.

Relay Types

Single pole, single terminal
The simplest relay is a single pole, single throw (SPST) design. This designation refers to the switching part of the relay where when it is activated; one wire (a "single pole") can be connected only one-way (a "single throw"). Just like an on/off switch, when you power up the relay's coil, the connection is made; when you un-power the coil, the connection is broken.
Single pole, double terminal
Here one contact is opened and the other one (the Normally Open contact) is closed. We still have only a single pole to be switched, but now it can be connected two ways - a double throw design. As you can see, it has both Normally Open (NO) and Normally Closed (NC) contacts. (Some people call this a changeover relay.)
Double pole, double terminal
A Double Pole, Double Throw relay allows you to switch two different circuits simultaneously. The 'Double Pole' bit just means that it has two separate inputs that can be switched - and we now know the 'double throw' means that one contact is opened as the other is closed. With this type of relay, you can:
· turn on two completely independent circuits
· turn one off and one on
· turn off two completely independent circuits

In this dual pole relays we may have a minor time difference between two gates and we can have a higher bouncing effect.

Relay drivers

These are circuits that allow us to use the relay, now we can find ICs that do the same thing (which can drive more than one relay). Nevertheless, when we are using a IC for this purpose there is a limitation in current that can go through it. If we need to use a larger current we have to use a external circuit.
EX: relay driver IC


Driving a motor

Motor is one of the most popular inductive loads. This also have those characteristics that a inductive loads normally have. This asks for a large current to startup the operation but after starting it need much less than that. If motor does not start that large current flows through it for long time, which will end in burning of the motor.
There is two, highly used techniques to overcome this.
1. Thermal fuses
2. Stopping flow of current after some time.
After the start, back EMF of the motor depends on two reasons.
1. Rotation speed
2. Load of the motor
Controlling the direction of the rotation

As you can see when we close S1, S4 set or S3, S2 set voltage across the motor changes direction. Note that we cannot close S1 with S2 (it will create a direct path and ask for large current). With below circuit that problem can be solved.

Controlling the speed of the rotation
How can we change or control the speed of the rotation? This can be done by changing the voltage cross the motor. To do that we can have a simple circuit with a variable resistance and our motor connected serially to a power source. If we can maintain same current, we can change the resistance of the variable resistor resistance and control voltage across the motor.
This is not possible, as back EMF depends on the speed of rotation, any change to voltage will change the speed and speed will again affect the voltage. This goes in a never-ending loop and finally we cannot control speed using this technique. According to above problem it is clear that we need a system that is dynamically check the speed and change according to that and we cannot use variable voltage.

We will not change the voltage but will stop and resume the power supply at a specific frequency that is not to high (it should be long enough to rotate the motor) not to low (we should not see motor stopping and starting). Duty cycle will decide the speed.


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