HowTo: How an Inverter Drive Works and Controls the Speed of an AC Induction Motor

An Inverter Drive (VFD) works by taking AC mains (single or three phase) and first rectifying it into DC, the DC is usually smoothed with Capacitors and often a DC choke before it is connected to a network of Power Transistors to turn it into three phases for the motor.

The network of Power Transistors of a small Inverter drive is actually one ‘Intelligent Power Module’ (known as an IPM) and includes its own protection and basic control circuits. The IPM inverts the DC into AC - hence the term ‘Inverter’.

The control method is known as ‘PWM’ for 'Pulse Width Modulation'. This means the DC is switched on and off very quickly (chopped) by the Transistor switches. A sine wave of motor current is made by a series of DC pulses where the first has a very short ‘on’ period, followed by a longer on period, then longer until the widest pulse appears in the centre of the positive sine wave, then smaller until the DC is inverted and the same pattern of pulses generate the negative part of the sine wave.

Since the Transistors can be controlled to any time base the other phases are controlled by more Transistors, displaced by the time necessary to equally space the phases at 120 degrees. The frequency of the pulses being turned on is known as the ‘Switching Frequency’.

Switching frequency is usually around 3kHz to 4kHz, so the pulses it makes for 50Hz will be 3000/50 or 60 pulses per full sine wave or each phase. When the fixed Voltage pulses are presented to the inductance of the motor, the result is control of both Voltage (by width of the fixed Voltage pulses) and Frequency (by spreading the progression and regression of the pulse widths over more of the switching frequency base pulses).

From the above you can see the IPM in the Inverter drive will control Voltage and Frequency over virtually any range the parameter settings in the VFD tells it to. This means when setting up an Inverter drive we can choose to run a small ‘Delta’ connected 230V motor from a 230V single phase supply with a base frequency set at 50Hz, a 400V Star Connected small motor from a 400V three phase supply or any other arrangement of Voltage and frequency we choose that will correctly flux the motor.

The motor will be correctly fluxed when its Voltage curve rises from around zero x 0Hz to its base frequency x normal Voltage. Base frequency and Voltage being what is shown on the motor nameplate.

This also means we can correctly flux other motors such as a 400V x 50Hz motor from a 230V supply at 230V three phase by setting the base frequency to 29Hz (at reduced speed) or run a 230V connected motor from 400V by setting the base frequency at 87Hz (at increased speed and power).

Electrical Braking is applied to the motor shaft, via the Inverter Drive when the product installed has this provision and a braking resistor (DBR) is present.  The input stage of the Inverter Drive is a one way power device, while the output stage allows power to flow in both directions.  It follows that inertia of a load will return its stored energy to the Inverter Drive when an attempt is made to slow its speed at a greater rate than it would achieve for natural deceleration or coast down.

In this case the Bus Voltage will rise unless there is provision to hold it down.  The smoothing capacitors will be charged during an increase in Bus Voltage and this will apply a small amount of braking to the motor shaft.  Usually this is around 10%, but is dependent on the smoothing capacitor size.

A Brake Switch or ‘Chopper’ needs to be present to divert the braking energy into a braking resistor.  The resistor is usually external and is sized to pass enough current to match the Current rating of the brake switch, not so high as to be ineffective and of physical size (Watts) such that it is not overheated. 
The ‘Which Resistor’ button at the bottom right of your screen has instructions for sizing the resistors at the very bottom of the calculated resistor combination table.

Note: Generally motors accept switching frequencies up to 5kHz before needing special insulation or de-rating for use with Inverters.