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AC Motor Control


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AC Motor Control

AC Motor Starting

The simplest way to start an AC motor is to apply full voltage directly to the motorís stator windings. This is the Direct-On-Line (DOL) method.

Fig. 43 Direct-On-Line starting scheme

The advantage of this technique is its simplicity.

When a motor is started DOL, a larger than normal current will flow. This large current can damage the motor windings and all motors are designed against this today. The sudden electrical shock affects the electrical supply system, and the mechanical shock can damage the load.

In the case where DOL starting is damaging, a form of soft starting is used. This involves limiting the voltage or current during motor startup. Soft starting techniques include Reduced Voltage and Reduced Current (Star-Delta) starting. Soft starts are typical when motors are coupled to gear boxes etc.

Reduced voltage - The stator winding of a squirrel cage induction motor may be energized through series resistors or reactors. As the rotor begins to turn, the impedance is switched out of the circuit. The switching may be done in steps, and may be manual or automatic. As the impedance is switched out of the circuit, the voltage across the stator winding increases and the motor is gradually brought up to full speed. Reduced voltage starting is also known as Auto Transformer start typical for high inertia loads.

Fig. 44 Reduced Voltage starting

Reduced Current or Star-Delta - In a three-phase squirrel cage motor, the stator consists of three windings. The connection of these windings can be changed using contactors.

Fig. 45 General arrangement of a star-delta circuit

On startup, the stator windings are connected in a star or wye configuration. This arrangement causes each coil to be energized with less than the supply voltage. The motor starts up gradually.

Fig. 46 In the wye or star configuration, contacts s4 and s5 are closed.

Once the rotor is turning, the contactors change over and connect the stator windings in a delta configuration. This arrangement causes each coil to be energized with the full supply voltage. The motor accelerates to full speed. Starting torque in star is 33% and may not be suitable for high inertia starts. Very good for fans/pumps. Voltage on start winding is equal to 240V or A415V winding and as torque is proportional to V2 i.e. (240/415)2 = 0.33

In the delta configuration, contacts s1, s2 and s3 are closed.

Fig. 47 In the delta configuration, contacts s1, s2 and s3 are closed.

The advantage of the star starting, delta running technique is that the motor and the supply system are subjected to reduced stresses.

Solid State starters - an alternative to electromechanical star-delta starters is the solid state starter. Semiconductor switches take the place of relays and contactors.

Soft starters - are a reduced voltage/current device which ramps up to the full line voltage. The acceleration is subject to load inertia. Starting torque is limited to 130% + current to

Fluid Couplings

Traction/Delayed Fill

A fluid coupling similar to torque conversion in a car uses hydraulic fluid as a soft start medium. The motor accelerates from rest as it is not fully connected to the load and hence the high startup current is minimised. As the impellor of the motor side spins the hydraulic/centronic effect creates a transmission of torque from one impellor to the other connected to the load. The load is then accelerated more gently and with line starting torque, i.e. adjustable from 140-175%. This device also offers overload protection and when used correctly can provide stable speed control with varying loads.

Variable Fill Fluid Couplings

Like the traction type the torque is transmitted via fluid flow from one impellor to another. The starting can be controlled much closer by metering into the hydraulic circuit a precise amount of oil at a given time hence how the coupling slips at an ever decreasing rate as the amount of oil in the circuit is increased.

During acceleration thin oil feed can be controlled to limit starting currents to the starting torque, or by using different control systems and synchronising or even load sharing start up.

Once operational at full speed, the load can be varied and the required torque varied and so the coupling will respond accordingly in transmitting the required torque from the motor to the load.

This method can also be used as a very cost effective and efficient method of variable speed when the speed is varied from full speed on occasions such as maintenance inspections or surge control. Where constant speed reductions are required a VVVF drive may be more efficient.

AC Motor Direction Control

To reverse the direction of a single phase AC motor, the motor must be of the split-phase type. The motor direction is reversed by swapping the polarity of the supply voltage (i.e. swapping the active and neutral conductors).

Fig. 48 Reversing a split-phase motor

The direction of a three-phase motor can be reversed by swapping two of the three motor supply connections. This swapping is done using contactors.

Fig. 49 Reversing a three phase motor

The contactors are mechanically interlocked to ensure that the motor supply connections are switched correctly. Without interlocking there is a chance that mechanical failure in the contactor could cause two of the motor supply connections to short together.

AC Motor Speed Control

AC motors are widely used for constant speed requirements. The speed of the motor is governed by the speed of the rotating stator field, and this relates directly to the frequency of the electrical supply. Ordinarily, the supply frequency is a constant which is not easily changed. Variable Speed Drives are electronic circuits for changing the supply frequency and voltage to achieve speed control of AC motors.

Variable Speed Drives

Variable speed drives (VSDs) are electronic devices used to control the speed of induction motors. VSDs are also known as converters or inverters.

VSDs control speed by changing the frequency and voltage of the motor supply current.   In order to maintain the correct magnetizing conditions within the motor, the voltage is also changed.

The circuitry in the VSD changes the AC supply voltage into a DC voltage, then modifies the DC to form an AC voltage with a new frequency and magnitude. The new AC voltage is slightly distorted, and this distortion can lead to unwanted heating in the motor. This heating can be a problem. According to motor manufacturers, the lifetime of a motor is halved if it is subjected to a continuous overtemperature of 10 percent. More modern inverters have much improved outputs hence less of a problem except where load is close to motor rating.

VSDs are chosen to meet the requirements of the motor and its load. The most important factors to consider when choosing a VSD are the speed range of the motor, the torque required by the load, the inertia of the motor and its load, and the effect the VSD will have on the motor.

The advantages of VSDs are controlled starting currents, speeds, direction and stopping. The disadvantage is the expense of the drive and the care needed to choose the correct drive for an application.

AC Motor Stopping

The simplest way to stop an AC motor is to remove power and let it coast to a stop. When this method is unsuitable, mechanical braking is be used.

Mechanical Braking - a mechanical brake can be fitted around the shaft of the motor. This brake is actuated by an electromagnet. The brake may be made to be electrically held open or closed against the force of a return spring. For safety reasons , most brakes are spring applied and air release.

Motor Selection

The following factors need to be considered when selecting a motor:

Speed Range - the base speed of the motor is determined by the minimum and maximum required operating speeds.

Allowable Speed Variation - Constant speed at all torque value applications should use a shunt-wound motor. If the accuracy of speed must be less than a few percent then a closed loop control system using a tachometer will be required.

Torque Requirements - Starting torque and running torque may need to be considered separately. The speed-torque relationship of the application determines the most economical motor to use. Many applications require constant torque, such as conveyors. Others, such as machine tools, pumps and fans require decreasing torque as speed increases. The peak torque available may be limited by the construction of the motor or the electrical capability of the power supply. The amount of torque available will determine how quickly a load may be accelerated.

Duty cycle - The load may be steady or may vary repetitively or randomly. The amount of time which a motor remains idle may influence motor choice.

Environment - The motor enclosure is selected after considering such factors as ambient temperature, cooling requirements, dust, dirt, hazardous gases etc.

Troubleshooting and Maintenance

Preventive maintenance is probably the best way to avoid problems with electric motors. Regular maintenance can eliminate problems before they start. The manufacturer will supply recommended maintenance intervals and procedures. If the motor does break down, the following list of common problems may provide a useful reference.

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