PWM in HVAC Chiller Drives - Pulse Width Modulation

PWM has done a lot to move the HVAC industry forward with our ability to control power to our loads. 

This gets used with magnetic bearings, modulating electric heat, motor soft starters, and VFD. 

I have covered PWM from a controls perspective previously, go check it out. 

PWM = Pulse Width Modulation 

With a power perspective, we are controlling power flow through OFF to ON pulses.

The more current we want to allow through, the longer ON pulse will be output.

An SCR is one of the devices we use to modulate power output. 

When applying this to a motor for soft start, the SCR passes power to the motor gradually to start rotation. 

The pulse time applied through the SCR increases as our motor increases speed. 

This is not variable speed control as a SCR is only pulsing at the initial start of the motor. 

Once the motor is at speed a bypass contactor will pull in or the SCR will apply full line power depending on the arrangement. 

Magnetic bearings use PWM to control the bearing coils fields.

As the field needs to be strengthened or weakened to keep the compressor motor shaft in position the output pulses to the coils will be increased for a stronger field or decreased to weaken. 

Now a VFD is where it gets more complex. 

VFD have a negative and positive DC bus, but we are controlling an AC induction motor. 

AC by definition has a positive and negative swing in its sine wave.

By using IGBT (inverter) we can synchronize positive DC pulses then negative DC pulses to create an AC sine wave. 

Then by speeding this pulse cycle up or down we can adjust our output hertz to the motor adjusting our speed. 

Please keep in mind these are very basic explanations of these processes so I encourage studying deeper into each of these applications further. 

MTT

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