reduced voltage starters

Starting Squirrel Cage Motors With Reduced Voltage.

A big misconception surrounding reduced voltage starters is their job is to protect the motor. This is not usually the case, and most motors are quite comfortable with full-voltage starting. Full voltage starting typically allows the motor speed to get from zero to full speed within a half-second or less. What happens is that the motor current goes from zero to locked rotor amps, then drops to running amps at full speed.
Squirrel cage motors draw a high inrush starting current and produce high starting torque when started at full voltage, typically about 600% of the motors nameplate F.L.A.(full load amperage). The starting torque produced will be about 150% of full load torque at full voltage.

This High inrush current and high starting torque can create other problems to both electrical and mechanical components, and could also be undesireable if it upsets material being transported due to abrupt high touque starts. Though it doesn't hurt the motor, it may dim lights or cause voltage dips to other electrical equipment.

Starting a motor using reduced voltage causes the motor current at the motor to be reduced in direct proportion to the line voltage reduction. The torque however is reduced by the square of the voltage reduction. For Example: When you start a motor at 50% of the line voltage, the starting current would be 50% of the full voltage value (.50 x 600% = 300% FLA). The torque would then be (0.50)squared or 25% of the normal starting torque (0.25 x 150% = 37.5% Full Load Torque). You can see that reduced voltage starting is a good way to reduce both inrush current and starting torque.

You have to be aware that If the motor has a heavy starting load, lowering the starting torque too much could cause the overloads to trip before the motor gets up to speed.

Types of Reduced Voltage Starters

Autotransformer Starter:

With an auto transformer, a standard 3-lead motor is used. Autotransformers provide reduced voltage to the motor terminals during starting through the use of a tapped, autotransformer. Typical auto transformers deliver 50%, 65%, or 80% of full line voltage during start-up. After a preset period the transformer is switched out of the circuit and the motor is connected directly to full line voltage supplying 50%, 65% or 80% of the current inrush seen during a full voltage start. The resulting starting torque will be 25%, 42% or 64% of full voltage values, as will be the current draw on the line. Autotransformers provides maximum torque with minimum line current. Autotransformer starters provide the highest starting torque per ampere of line current, and is an effective means of motor starting for applications where the inrush current must be reduced with a minimum sacrifice of starting torque.

Wye-Delta (Often called Star- Delta )

Wye-delta starters and a special 6-lead motor that allows for the motor winding to be connected in either a wye or delta configuration are needed for this. During the start, the windings are connected in the wye resulting in 58% of line voltage applied across any two windings. This reduces both inrush and starting torque to 33% of the delta connected values. After a preset time delay, the motor leads are switched to the delta connection.
Wye-Delta starters come in 2 variations being open and closed transition. Open - transition type momentarily interrupt power during the starting cycle, and Closed transition starters are supplied with an additional contactor and resistor bank used to keep the motor windings energized for a few cycles until the transition from wye to delta is complete.

Part Winding Starters: (This starter is not really a true reduced voltage starter).

Part winding starters can only be used with part winding motors which have two separate sets of windings. The starter energizes half of the motor's windings with full voltage during starting and then the other half of the windings for running. During a part winding start, only one winding is energized, which reduces the inrush current to about 60-70% and starting torque to 50% of normal starting values with both windings energized. Most dual voltage 230/460 volt motors are suitable for part winding starting at 230 volts. The motor must have two separate windings, which is standard on dual voltage (230/460V) motors, but these motors can only be started part winding on the lower voltage (230V). Only loads which have low starting torque requirement such as low inertia fans and blowers can be started using part-winding starters. They usually meet the Utility Company's requirement that all motors over a certain hp must be started by reduced voltage.

Primary resistance starting:

Standard 3-lead motors and a control panel are used. When the motor is started, a resistor is is put in series with each winding. The voltage drop across the resistors lowers the voltage to the motor. After a preset time, usually a few seconds, the resistors are shorted out causing full voltage to be applied to the motor. The amount of voltage drop (Reduced voltage) is determined by the values of the resistors. These are not very common any more.

Secondary resistance starting:

These require a special motor called a "Wound Rotor Motor". The motor has a set of slip rings that bring out the ends of the rotor windings. While starting, resistors are placed in series with the rotor windings and when the motor is at a preset speed some or all of the resistance is shorted out. Once all of the resistors are shorted out the rotor essentially becomes the same as a squirrel cage and acts the same as any other squirrel cage motor.

Solid State "Soft Start" starters

The Most efficient method is a solid-state, reduced-voltage starter. This is considered the most effective way to regulate the in-rush current to the equipment motor.

Essentially, the solid-state starter regulates voltage to the motor in a series of small bursts of power turned on and off until the motor reaches full power. A reduced-voltage starter uses a series of switches (silicon-controlled rectifiers). The switches turn voltage on and off approximately 60 times per second. Rather than being clicked on instantly, the power to the motor is delivered smoothly, much like a dimmer switch gradually increases room lighting.

In Applications greater than 75 hp, the cost of a solid-state, reduced-voltage starter is about the same as an across-line starter. Applications greater than 50 hp should be reviewed for possible soft starting.