AC motors provide the majority of electrically powered motion in the world today, accounting for around 70% of global power consumption. AC motors have almost entirely replaced DC motors, due to their lower cost thanks to simpler construction and the fact that they can be connected directly to the AC mains supply for fixed speed applications.
The speed at which an AC motor's shaft rotates is proportional to the frequency of the AC supply fed to it (either 50 or 60Hz for mains supplies around the world), and the number of poles with which the motor is wound. For example, a 4 pole motor, operating on a 50Hz supply has a base speed of approximately 1500 Rpm. In practice, the speed is slightly lower due to the 'slip' of the motor - the shaft does not exactly follow the rotating field within the motor, and torque is generated due to this slip.
This means that for applications where variable speed is required, we have to provide a means to vary the frequency of the supply fed to the motor, and hence the AC Drive was born.
AC variable speed drives have been around now for more than 30 years, and whilst the design has been significantly refined over the years and components improved, the basic power stage circuit design used remains the same. Their purpose is to provide a means to control the speed, and in some cases also torque, or power, of AC motors.
The power circuit consists of the following basic parts
* Rectifier
* Pre-Charge Circuit
* DC Link
* Output Stage
The mains power connects to the input of the rectifier(s). The rectifiers are typically diodes on smaller drives (often referred to as an uncontrolled rectifier). On higher power rating drives, it's more common to use Thyristors, which allow some control over the input. The purpose of the rectifier is to convert the incoming AC supply into a DC supply.
The DC Supply is then fed to the DC Link. This consists of one or more large capacitors, which are used as an energy storage. As the incoming AC supply is a sine wave, the voltage is continuously rising and falling, and so the purpose of the DC link is to provide a stabilised voltage supply, which is required for the output stage to work correctly.
When the drive is first powered on from the mains supply, the capacitor energy store is empty, and so a large current would flow. The purpose of the Pre-Charge or Inrush circuit is to control this current flow at power on, and limit it to a safe value. The pre-charge circuit only operates for a few seconds following power on to limit this charging current and prevent tripping of circuit breakers or blowing fuses. All Invertek drives are designed to limit this charging current to less than the full load current of the drive.
On higher power rated drives, the DC link may also include a choke. Again, the purpose of this is to stabilise and smooth the DC voltage, whilst also limiting the peak current drawn. An alternative to this is to use an AC Line choke at the input of the drive, before the rectifier, which fulfils the same purpose, but is typically a little larger.
Finally, after a suitab
le power supply has been created within the drive, we now need to create our variable frequency output. This is achieved using a technique called Pulse Width Modulation - PWM for short.
The AC supply fed from the mains to the inverter input is a sinusoidal AC voltage, and if this is connected directly to the motor, the motor current is also sinusoidal. The output voltage of the inverter however, is actually a high frequency square wave!
The frequency of this waveform remains the same at all times, selected by the Effective Switching frequency parameter within the drive, and can sometimes be audibly heard by the motor as a high pitch 'ringing' sound.
However, the width of the On / Off phases of the pulses is varied, and when this is fed to the motor, the inductance of the motor creates a sinusoidal current, and so the motor behaves as though it were connected to a sinusoidal voltage supply. However, we now have the ability to control the frequency by adjusting the output pulses, and hence the motor speed can be controlled.
Visit our dedicated case studies page to find out how Invertek's variable speed drives are used in a huge range of applications around the World.
