what is really the reason why motors and other inductive loads, cause spikes when turned on and turned off??? please explain...
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The switching on and switching off spikes are two different things.
When a motor is switched on its coils have a very low inductance, so they act as very low value resistors. This means that you get a surge of current when a motor is first switched on. This can cause a voltage spike on the supply as the supply tries to cope with the high current being drawn. The surge in current can also inductively couple onto parallel wires or PCB tracks, causing voltage spikes on them.
Switch-off spikes are different. You know how capacitance is the tendency of a component to store electric charge? Well, you can think of inductance as the tendency of a component to maintain an electric current.
Suppose you have an inductor with one end connected to ground and the other connected to a power supply which is putting 1A through it. What happens when you cut off the supply is that the inductor still "wants" to have 1A running through it. The voltage at the top end of the inductor will go negative as it tries to draw current out of whatever it is connected to. For a perfect inductor with its terminals open-circuit, the voltage it will generate is theoretically infinite. In practice of course inductors are never perfect, but you can still get many thousands of volts out of an inductor just by putting a current through it then shutting off the current.
The spikes of current an inductor can create can easily destroy semiconductors. that's why, when you see an inductive load like a relay coil, there is nearly always a diode wired in reverse polarity across it. The diode acts as a path for the current when the load is switched off, thus preventing the voltage spike.
When a motor is switched on its coils have a very low inductance, so they act as very low value resistors. This means that you get a surge of current when a motor is first switched on. This can cause a voltage spike on the supply as the supply tries to cope with the high current being drawn. The surge in current can also inductively couple onto parallel wires or PCB tracks, causing voltage spikes on them.
Switch-off spikes are different. You know how capacitance is the tendency of a component to store electric charge? Well, you can think of inductance as the tendency of a component to maintain an electric current.
Suppose you have an inductor with one end connected to ground and the other connected to a power supply which is putting 1A through it. What happens when you cut off the supply is that the inductor still "wants" to have 1A running through it. The voltage at the top end of the inductor will go negative as it tries to draw current out of whatever it is connected to. For a perfect inductor with its terminals open-circuit, the voltage it will generate is theoretically infinite. In practice of course inductors are never perfect, but you can still get many thousands of volts out of an inductor just by putting a current through it then shutting off the current.
The spikes of current an inductor can create can easily destroy semiconductors. that's why, when you see an inductive load like a relay coil, there is nearly always a diode wired in reverse polarity across it. The diode acts as a path for the current when the load is switched off, thus preventing the voltage spike.
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inductive loads - coils and motors - store energy. When they are turned on they have very low resistance and the power slamming into them and then setting up an electric field produces a voltage spike on rebound.
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because a magnetic field is collapsing