An inverter is a key component of an electric vehicle, providing the variable frequency 3-phase alternating current required to drive an AC motor. An internal combustion engine vehicle (ICE) does not require an inverter — rather, it uses DC supplied by a battery and the current does not change direction or ‘alternate.’

Electric motors use the force between magnets to turn the rotor, but if the magnets maintained the same polarity, a motor could only rotate by a maximum of 180°. All electric motors therefore require an alternating current that reverses direction and changes the polarity of the magnets at a frequency corresponding to the rotational speed of the motor. This even applies to so-called DC motors, which use mechanical switching within the motor to alternate the current through the electromagnets. So in a sense ICE vehicles do require a type of alternator — if you count the brush and commutator arrangement within the DC starter motor.

EVs use more efficient 3-phase AC motors, which have three magnets, each driven by a separate sinusoidal current with a 120 degree phase shift between them. EV inverters use microprocessor controlled switch-mode power supplies to generate the 3-phase AC supply and vary its frequency to control the speed of the motor.

The alternator in an ICE vehicle works like the motor in an EV only in reverse, it produces alternating current which is then rectified into DC to charge the battery.

In spark ignition ICE vehicles, very high voltages are required to produce the arcs in the spark plugs. This means stepping up the 12V supplied by the battery, to the many thousands of volts required by the spark plug. In most electrical systems, the normal way of changing voltage is by using a transformer. Transformers use the principle of inductance, whereby a change in electrical current produces a resisting magnetic force. If two electrical circuits have coils which wrap around the same magnetic core, then a changing current in one circuit will include a changing current in the other circuit. The relative voltages of these currents will depend on the relative number of coils. However, this only works if the current is constantly changing, so transformers only work with AC electricity, a major reason why electricity is transmitted and distributed as AC.

In a spark ignition system, a distributor sends a short pulse of current to each spark plug in turn. Because this pulse represents a change in the current, an ignition coil, which is essentially a transformer, can increase its voltage. So in the case of a distributor and ignition coil, there is no need to reverse the direction of the current and an alternator is not required.

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Filed Under: Inverter

 

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