Five Things to Know About Reducing Energy Costs - Part 4 of 5

Install Soft Starters

Many electrical devices will draw a large initial current when they are first started, and motors are no different.  This initial power draw, called an inrush current, can be up to ten times the normal operating current of the motor.  This is due to the motor’s windings possessing a much lower impedance when cold than when hot.  The inrush current not only increases energy costs, but can also damage the motor windings over time.  Several options for soft-starting are available, including:

A. TRIAC controls and alternistors
A triode for alternating current (TRIAC) is the most popular soft start option available for electric motors.  A TRIAC control ramps up the motor’s duty cycle gradually, thereby reducing the effects of inrush current.  An alternistor is a specialized type of TRIAC designed specifically for applications with a high inductive load – such as motors – which may cause trouble with traditional TRIACs due to the high voltage and current angles of a three-phase motor.

B. Y-Delta start
This option starts the windings in series, and then switches to parallel once a certain partial speed has been reached.  However, this requires a motor to be designed to start in this fashion, and as such is not a viable option for all motor types.

C. Inrush current limiter
An inrush limiter is a device consisting of a negative temperature coefficient thermistor or a fixed resistor; inrush limiters are placed inline with the motor whose inrush current is to be limited.  Fixed-resistance inrush limiters are less efficient, since the resistance never falls from a preset value; as such, they are not widely used save for low-power circuitry.  A thermistor is a device whose initial resistance is very high; as current is drawn by the motor, it heats the thermistor, which gradually reduces the thermistor’s resistance.  Note that when using a thermistor-based inrush limiter, it is impossible for the full line amperage to reach the motor, as the thermistor must constantly draw power in order to remain hot and at low impedance.

D. Line reactor
A line reactor is a set of phase-dependent coils (i.e. three coils for three-phase power) connected in series with the power supply.  The impedance of line reactors limits inrush current by opposing rapid changes in current; thus, they hold down current spikes and limit any peak currents.  Note that this impedance is directly proportional to current frequency: the higher the current frequency, the greater the line reactor’s impedance.  Line reactors sized by percent impedance, operating voltage, and motor horsepower.

E. Variable frequency drives
Variable frequency drives (VFDs) are seeing increasing use as soft-start devices.  Most drives give a current limiting ramp to start and stop a motor, thus fulfilling the traditional roles of a soft-starter.  VFDs also have several speed control advantages that can vastly improve an application’s efficiency, such as regenerative braking, freewheeling shutdown, and software- or timer-controlled ramping times.  VFDs are also phase-insensitive, and the direction of the motor can be adjusted via the drive rather than the wiring.  Additionally, many VFDs – including servo and stepper VFDs – feature kill circuits to prevent a motor from single-phasing during a brownout, saving additional maintenance and repair costs.

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