Lesson 6 - Electricity
In this lesson we take a closer look at electricity from a utilities perspective. We will review the basics of electricity and consider the different forms (direct current and alternating current, single phase, and three phase) and voltages. The basic reticulation systems within the brewery will also be covered
Variable Speed Drives and Soft Starters
Variable speed drives and soft starters play an important role in reducing maximum demand, as well as protecting motors and associated equipment from damage.
When electrical motors are started, the initial current drawn can be large. This is due to the inertia of the equipment and the fact that it is being accelerated until it reaches its operating speed. You can relate this to getting your car moving from stationary. You normally have to depress the fuel pedal quite hard to get the car moving, and as you get to the desired speed you can decrease the pressure a bit. Speaking in electrical terms, full voltage is applied, which results in an ‘inrush’ current that can be as high as six to seven times that of normal running. The torque on the equipment will also be very high.
Soft starters
The most commonly used soft starters are called solid state soft starters. They use semiconductor devices to reduce the voltage to the motor for two or three phases. This in turn can reduce the inrush current and the torque whilst the motor is started up gradually. This is shown in the figure below.
Figure: Circuit diagram for a soft starter
Soft starters can provide soft starting and soft stopping. They are cheaper than VSDs (see below), and provide a feasible solution where the equipment can be run at a constant speed during normal operation.
Variable speed drives (VSDs)
VSDs are also known as variable frequency drives (VFDs). They take the set frequency and voltage supply and convert it to an adjustable output which allows for the control of the frequency and voltage to a motor. The frequency results in a lower motor speed.
VSDs can also be set to ramp up and down slowly at equipment start-up and shut down to reduce the maximum demand.
So, which one do we choose?
Soft starter or VSD?
Soft starters are cheaper than VSDs, but can only be used to reduce start-up load. The motor will still go to full speed.
VSDs are more expensive. They allow for control of the output, and thus can provide energy savings across the run, in addition to providing a way to reduce maximum demand at start-up.