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Powerboss - Technically Speaking
Starting Induction Motors - The Powerboss Solution
In common with all soft start devices Powerboss uses Thyristors to accurately control the voltage applied at the motor terminals.
A characteristic of the Thyristor to switch rapidly from "OFF" to "ON" when pulsed, and to remain "ON" until the current through the device falls to zero at the end of each half-cycle in the AC supply, is called self commutation.
By controlling the switch-on point relative to the voltage zero crossing in each half cycle of the supply, it is possible to regulate the current flowing through the Thyristor. The closer the turn-on point is to the end of the cycle the smaller the value of current that will be allowed to flow. Conversely, the closer the turn-on point is to the beginning of the cycle the higher the value of current will be. Using this principle and by connecting two Thyristors in anti-parallel to each of the phase connections to a motor Powerboss can continuously adjust the voltage to the motor terminals by precisely controlling the Thyristors turn-on points. This provides just sufficient voltage for the motor to accelerate the load. See Figure 1.
So, for instance, by starting with a large delay to the turn on point in each half cycle, and progressively reducing it over a selected time period, the voltage applied to the motor starts from a relatively low value and increases to full voltage. Due to the motor torque being proportional to the square of the applied voltage, the starting torque increases in a step less manner ensuring a soft start for both the motor and the driven load.
Figure 1
Motor Efficiency – The Powerboss Solution
When working at or near full load, the typical 3-phase induction motor is relatively efficient, achieving efficiencies of between 80% to 92%. However, as shown in Figure 2 below, motor efficiency falls dramatically when the load falls to less than 50% of rated output. In fact, very few motors actually experience consistent fully rated operation, the vast majority operate at much lower loads due to either over-sizing (a very frequent situation), or natural load variations.
In applications where motor speeds do not need to be varied, the optimization software in the Powerboss will produce energy savings in lightly loaded motors.
Less sophisticated soft-starters remain at full conduction and the motor then behaves as if it were connected directly to the mains supply. However, at light loads at full mains voltages, induction motors always have excess magnetizing current (see Figure 3). By detecting the load at any instant, and adjusting the motor terminal voltage accordingly, Powerboss is able to save some of the excitation energy and load loss, and improve motor Power Factor when the motor is running inefficiently at light loads.
Figure 2
Induction Motor Efficiency
Figure 3
Induction Motor Losses
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