Choice of Correct Type of Stator

General

Throughout Australia today there are many three phase electric motors being started with every conceivable type of starting switchgear, ranging from the basic triple pole switch with no overload protection apart from line fuses, through a variety of manual devices , some with and some without overloads, to the most modern types of automatic starter with precision protection.

The form of protection ranges from simple thermal overload, GXrelays, with or without ambient temperature compensation and or differential tripping, through to built in thermostats and thermistors in the windings each with their own external control, to sophisticated relays which will protect against over-voltage, phase imbalance etc.

If a starter is to be purchased today it can be said that, with correct selection, the equipment will give maximum protection against all normal overload conditions and allows the unskilled operator to start without risk of damage to motor, starter or operator.

The following comments are based on the assumption that all starters used will be of the automatic type.

STARTING DUTY

The contactors used in modern automatic starters can be suitable, with correct selection, for over 1000 operations per hour. However the standard squirrel cage motor is not suitable for more than approximately 6-8 DOL starts per hour, equally spaced, with a run up time not exceeding 10 seconds.

If the motor is required for more frequent starting the motor manufacturer must be given full details of the drive, such as inertia of the load, number of starts required, absorbed power, etc to enable correct selection of the motor. If a motor is required for frequent starting please contact Western Electric with all available data and we will endeavour to make a selection to suit your application. For the types of starters discussed later in this paper we will assume that the motors are only required to start 6-8 times per hour.

DIRECT ON LINE STARTERS (DOL)

This type of starter is the simplest and therefore the cheapest and most reliable. It consists of a contactor, overload and push button. This is the obvious choice, where permissible under the supply authority's current limiting restrictions. We have discussed previously the starting characteristics of motors when started DOL They can be used for the majority of applications except where starting torque or current must be limited.

REDUCED CURRENT STARTERS

The next range of starters we will consider are reduced current type. These starters provide a means to start the motors without the large inrush current associated with a DOL starter. However they also have a marked effect on the starting torque available for the motor. The starting torque with a reduced current starter , in general, as a percentage of the full load torque, reduces with an increase in motor size. They are also more expensive then the equivalent DOL starter.

These are available in various types and we will discuss these and list some advantages and disadvantages of each.

STAR DELTA

This is probably the most commonly used reduced current starter as it is the most economical. It comprises a line contactor, a 'star' contactor, 'delta' contactor , timing relay, overload relay and push buttons. This type of starting requires that all six leads of the motor be brought out to the terminal box.

This method of starting avoids the initial high inrush current and also avoids the high torque at approx 80% speed, which in certain cases can be undesirable. When switched in 'star', a normally 'delta' connected motor has 57.7% of its nominal full 'delta' voltage impressed on the stator, and since a 'star' connected winding draws 57.7% of the current of a 'delta' connected winding, the nett result is a 33 1/3 % normal starting current. Unfortunately, this reduced current is accompanied by a proportional reduction in starting torque.

If, for instance, a motor had locked rotor figures of 625% full load current and 125% full load torque respectively, then with a star delta starter these would be reduced to approx 200% and 42% respectively. During acceleration the current falls, while torque increases to slightly higher than full load torque at approx 80% speed and then falls rapidly. When the motor reaches approximately 90% speed the timer operates to de-energise the "star" contactor and energise the "delta" contactor to enable the motor to reach full speed. At this point the current and torque will peak to the normal value on the DOL speed torque curve.

With this type of starter, it is important to match the speed/ torque curve of the load to the speed/torque curve of the motor under star/delta starting, to ensure that at no point does the load curve cross the motor curve. If it does the motor will stall or else will draw excessive currents until full speed is reached. This type of starter is generally used for centrifugal fans and pumps where the load varies as the square of the speed, and a low starting torque is therefore adequate.

AUTO TRANSFORMER

These starters are used where a higher starting torque is required than can be obtained from a star/delta starter and where current limitations rule out DOL starting. It is possible to reduce the current and torque to below that of a star delta starter but this has limited use.

This starter comprises a line contactor, a "transformer" contactor, "delta" contactor , timing relay, overload relay, transformer and push buttons. This method of starting inserts the transformer in the line when the "transformer" contactor is energised. Also, when the "transformer" contactor is de-energised, a portion of the transformer is left in circuit to prevent high transient currents occurring when the "delta" contactor is closed, which then shorts out the transformer and places the motor on line.

The current and torque values with this method of starting vary with the tapping on the transformer. The tapping values are generally 50% 65% and 80% of line voltage. The torque available at the motor varies as the square of the applied voltage, Thus the available starting torque will be 25%, 42% and 64% of the D.O.L. torque respectively. The motor current varies directly as the voltage, and as the line current is the motor current multiplied by the transformation ratio, the line current varies as the square of the tapping ratio used. It is possible to have further tapping's on the transformer and also to have two stage starting. However these are special case considerations and are only used for special applications.

With this type of starting only three leads are required from the motor. They would be used where a higher starting torque is required than that available with a Star/delta starter such as conveyors, compressors etc. Again, the speed torque curve of the driven equipment must be matched to the speed/torque curve of the motor under auto-transformer starting, to ensure that at no point does the load curve cross the motor curve.

PRIMARY RESISTOR AND PRIMARY REACTOR

This type of starting is in very limited use now, but will be mentioned as some may exist in rural areas. It consists of a line contactor, running contactor, timing relay, overload relay, push buttons and a resistor or reactor which acts as the current reducing device. For a given current from the line much less starting torque is available with these starters than either an auto-transformer or star/delta. These starters were generally used where the starting current was limited to 50% of DOL current. This means that the starting torque was reduced to approx 25% of the DOL torque. With the primary resistor starter the torque increased with speed until, at approximately 90% speed, full load torque was reached. With the primary reactor starter, due to the more favourable relationship between the voltage drop across the reactor and the motor, the torque reaches a peak of approx 150% of full load torque at 90% speed.

They were mainly used in rural areas for milking machines etc but are no longer manufactured as far as the writer is aware.

SECONDARY RESISTOR

These starters are for use with slip ring motors and are usually designed to give equal current peaks in each step. They are generally custom designed to suit a particular application Their main use is for cranes and large ball mill drives etc where high starting torque with low starting currents are required.

All of the above starters are of the electro-mechanical type. We will now consider electronic reduced current starters.

SOLID STATE STARTERS

With the availability of transistors etc and the advent of high power devices, solid state starters have come into their own, where reduced currents can be made available with higher starting torques than are available with either star/delta or auto-transformer starters. These types of starters provide soft starting or stepless reduced voltage starting of three phase motors. The same principles of current and torque apply to both electromechanical and solid state controllers. However solid state starters offer the choice of three starting modes: soft start, current limit, or full voltage in the one unit.

In addition to selecting the modes, the solid state controller allows adjustment of the time for the soft start ramp or current limit maximum value, which enables selection of the starting characteristics to meet the specification. The most widely used version is the soft start.

With a solid state starter the starting current and starting torque is fully adjustable up to the DOL values. The major advantages of solid state controllers are the elimination of the current transition point and the time to full voltage can be adjusted in the range of approximately 2 seconds to 30 seconds. The result is no large current surge when the solid state controller is correctly matched to the load. Current limit can be used where power line limitations or restrictions require a specific current load. This may be 200%, 300% 400% etc of full load current.

Current limit is also used where higher starting torque is required compared to soft start, which typically starts at less than 300% current. An example of where current limit is used is on high inertia loads such as ball mills.

As solid state controllers use semiconductors in the power circuit, even when the motor is up to speed, energy saving is achieved when motors are run unloaded or lightly loaded for a considerable time. Intelligence within the controller determines when the motor is lightly loaded. The voltage to the motor can then be reduced by controlling the semiconductors until the motor is operating at an optimum point. This same intelligence can detect when the load is reapplied and increase the voltage to prevent stalling. They can also provide very sophisticated protection such as phase loss etc.

A version of the solid state controller is the variable voltage variable frequency (VVVF) drive which incorporates a soft starter and has the ability to vary the speed of the motor.