The Star/Delta starter is probably the most commonly used reduced voltage starter, but in a large number of applications, the performance achieved is less than ideal, and in some cases, the damage and interference is much worse than that caused by a Direct On Line starter.
The Star/Delta starter requires a six terminal motor that is delta connected at the supply voltage. The Star Delta starter employs three contactors to initially start the motor in a star connection, then after a period of time, to reconnect the motor to the supply in a delta connection. While in the star connection, the voltage across each winding is reduced by a factor of the square root of 3. This results in a start current reduction to one third of the DOL start current and a start torque reduction to one third of the DOL start torque. If there is insufficient torque available while connected in star, the motor can only accelerate to partial speed. When the timer operates, the motor is disconnected from the supply and then reconnected in Delta resulting in full voltage start currents and torque.
The transition from star connection to Delta connection requires that the current flow through the motor is interrupted. This is termed "Open Transition Switching" and with an induction motor operating at partial speed (or Full load speed), there is a large current and torque transient produced at the point of reconnection. This transient is far worse than any produced by the DOL starter and causes sever damage to equipment and the supply.
If there is insufficient torque produced by the motor in star, there is no way to accelerate the load to full speed without switching to delta and causing severe current and torque transients.
Tuesday, April 28, 2009
Autotransformer starter
The autotransformer starter is an electromechanical means of reduced voltage starting an induction motor. Usually, there are three sets of output taps allowing connection on the 50% start voltage, 66% start voltage and 80% start voltage. The starter operates by connecting the motor to the reduced voltage tap for a period of time and then switching to full voltage. If there is sufficient torque to accelerate the motor to full speed at the reduced starting voltage, and the start timer is set long enough, there will be a useful reduction in starting current and starting torque. If the torque available at the reduced voltage is insufficient to accelerate the driven load to full speed, the starter will change to full voltage at less than full speed, resulting in a high start current and little or no advantage over DOL starting.
Where insufficient torque is available to accelerate the load to full speed, the starter can be set to a higher tap, increasing the start torque developed.
Where insufficient torque is available to accelerate the load to full speed, the starter can be set to a higher tap, increasing the start torque developed.
Minimum Start Current
The minimum start current is a function of the trhee major components in the system, the driven load, the motor and the starter.
The Driven Load requires a miimum amount of torque to start and accelerate it to full working speed. This is a function of the driven load and can not be changed by the motor or the starter. Changes to the driven load can alter the minimum starting torque. For example closing the valve on a centrugal pump, pressure equalising a screw compresor, lifting the valves on a reciprocating compresor.
The Motor converts amps into newton meters, or current into torque. Some motors are better than others in this function. In effect, the starting efficiency of motors varies tremendously from motor to motor. This is a function of the rotor design in the motor and is independant for the starter and the load provided that standard componentary is used. Starters that introduce high even harmonic contents or negative sequence currents will reduce the output torque from the motor for a given input current.
The motor has two characteristics that indicate it's ability to convert amps into newton meters. These are the Locked Rotor Current (LRC) and the Locked Rotor Torque (LRT). The ideal motor has a low LRC and a high LRT. Motor comparisons can be made by taking the LRT/LRC with both expressed as a percentage. A higher value equates to a better starting result.
The Starter controls the voltage applied to the motor and this in turn controls the start current and torque applied to the load. If the load requires a high torque, the starter can not cause the motor to develop that torque without providing the required current.
The load dictates the torque, the motor then dictates the current and the starter controls the voltage to deliver that current.
The Electrical Calculations software provides a means to estimate the minimum start current required based on load characteristics and motor characteristics.
Select your load, or enter an estimate of the value for the maximum load torque during start.
Enter your motor details, either by starting efficiency or Locked Rotor characteristics. This will give you an indicative start current requirement.
The Driven Load requires a miimum amount of torque to start and accelerate it to full working speed. This is a function of the driven load and can not be changed by the motor or the starter. Changes to the driven load can alter the minimum starting torque. For example closing the valve on a centrugal pump, pressure equalising a screw compresor, lifting the valves on a reciprocating compresor.
The Motor converts amps into newton meters, or current into torque. Some motors are better than others in this function. In effect, the starting efficiency of motors varies tremendously from motor to motor. This is a function of the rotor design in the motor and is independant for the starter and the load provided that standard componentary is used. Starters that introduce high even harmonic contents or negative sequence currents will reduce the output torque from the motor for a given input current.
The motor has two characteristics that indicate it's ability to convert amps into newton meters. These are the Locked Rotor Current (LRC) and the Locked Rotor Torque (LRT). The ideal motor has a low LRC and a high LRT. Motor comparisons can be made by taking the LRT/LRC with both expressed as a percentage. A higher value equates to a better starting result.
The Starter controls the voltage applied to the motor and this in turn controls the start current and torque applied to the load. If the load requires a high torque, the starter can not cause the motor to develop that torque without providing the required current.
The load dictates the torque, the motor then dictates the current and the starter controls the voltage to deliver that current.
The Electrical Calculations software provides a means to estimate the minimum start current required based on load characteristics and motor characteristics.
Select your load, or enter an estimate of the value for the maximum load torque during start.
Enter your motor details, either by starting efficiency or Locked Rotor characteristics. This will give you an indicative start current requirement.
Load Characteristics
The induction motor is used to convert electrical energy into mechanical energy. The driven load presents a mechanical load to the shaft of the motor. As the motor is started, it accelerates the driven load from zero speed to the rated full load speed of the motor. As the load accelerates, the torque presented to the motor shaft will vary depending on the design of the machine.
Generally, the load torque is expected to be higher at full speed than at lower speeds. Some applications such as loaded conveyors may require a high breakaway torque to get the load to begin to move from zero speed.
In order to correctly design a motor starting system, it is important to know the load torque curve. The load (machine) design determines the required starting torque. The motor design then determines how much current is required to develop that torque. If the torque developed by the motor is insufficient, the motor can not accelerate the load to full speed.
The load torque can be expressed in Newton Meters, Pound Foot, or as a percentage of the motor full load torque.
For a specific machine, it is best to always work in absolute units such as Newton meters or Pound Foot. This way, if the motor size is changed, the starting characteristics and curves will be changed automatically. When relative units (%) are used, all the values need to be altered to reflect the change in motor Full Load Torque.
Generic or indicative curves can be saved in relative units (%) to enable approximations to be made where absolute details for a specific load are not available.
Generally, the load torque is expected to be higher at full speed than at lower speeds. Some applications such as loaded conveyors may require a high breakaway torque to get the load to begin to move from zero speed.
In order to correctly design a motor starting system, it is important to know the load torque curve. The load (machine) design determines the required starting torque. The motor design then determines how much current is required to develop that torque. If the torque developed by the motor is insufficient, the motor can not accelerate the load to full speed.
The load torque can be expressed in Newton Meters, Pound Foot, or as a percentage of the motor full load torque.
For a specific machine, it is best to always work in absolute units such as Newton meters or Pound Foot. This way, if the motor size is changed, the starting characteristics and curves will be changed automatically. When relative units (%) are used, all the values need to be altered to reflect the change in motor Full Load Torque.
Generic or indicative curves can be saved in relative units (%) to enable approximations to be made where absolute details for a specific load are not available.
Induction Motor Characteristics
The induction motor has two major components: The Rotor and The Stator. In most motors, the Stator is in the outer part of the motor and comprises a stack of steel laminations and two or more windings. The inner part of the stator is hollow, and the windings are distributed around the inner surface of the stator imbedded in a number of slots. The windings are organized to form two or more electromagnetic poles.
The Rotor is a solid cylindrical stack of laminations with a series of conducting bars imbedded near the surface. The ends of these bares are shorted together by shorting rings.
When the supply is connected to the stator windings, a magnetic field is created which is rotating at the supply frequency. The field in a two pole machine will do one complete revolution per cycle of the supply. A Four pole machine requires two cycles for a complete revolution and a Six pole machine requires three cycles for a complete revolution.
The rotating magnetic field developed by the stator, causes a current to flow in the short circuited rotor winding in the same manner as the secondary current is caused to flow in a transformer. - infact the motor emulates a transformer with a short circuited secondary.
The rotor current in turn develops a rotating magnetic field which interacts with the stator field to develop a rotating torque field in the direction of the stator field rotation. The strength of the torque field is dependent on the interaction of the two magnetic fields, and is therefore dependent on the magnitude of the fields and their relative phase angle.
The full voltage start current and start torque curves vary tremendously between different motor designs due to the variations in rotor designs.
In designing a motor starting system, it is important to base the design on the actual motor being used. A design based on "typical" curves can yield very erroneous results.
The Rotor is a solid cylindrical stack of laminations with a series of conducting bars imbedded near the surface. The ends of these bares are shorted together by shorting rings.
When the supply is connected to the stator windings, a magnetic field is created which is rotating at the supply frequency. The field in a two pole machine will do one complete revolution per cycle of the supply. A Four pole machine requires two cycles for a complete revolution and a Six pole machine requires three cycles for a complete revolution.
The rotating magnetic field developed by the stator, causes a current to flow in the short circuited rotor winding in the same manner as the secondary current is caused to flow in a transformer. - infact the motor emulates a transformer with a short circuited secondary.
The rotor current in turn develops a rotating magnetic field which interacts with the stator field to develop a rotating torque field in the direction of the stator field rotation. The strength of the torque field is dependent on the interaction of the two magnetic fields, and is therefore dependent on the magnitude of the fields and their relative phase angle.
The full voltage start current and start torque curves vary tremendously between different motor designs due to the variations in rotor designs.
In designing a motor starting system, it is important to base the design on the actual motor being used. A design based on "typical" curves can yield very erroneous results.
Introduction to Induction Motor Starting
An induction motor is part of a system comprising the driven load, the induction motor, the starter and the supply. The best starting conditions can only be met if all components of the system are correctly engineered as a group. The driven load requires torque to accelerate to full speed. If insufficient torque is applied to the driven load, it can not reach full speed. The Induction motor converts current into torque to accelerate the motor. If there is insufficient start current available, the motor can not develop enough torque, and the load can not reach full speed.
To engineer the system, it is important to firstly establish the starting torque requirements of the driven load. Next the starting characteristics of the induction motor should be analyzed in order to establish the start current required by the motor to develop the required starting torque. A starter can now be designed/selected to meet the start current requirement, and an appropriate supply connected.
Induction motors exhibit a very low impedance at speeds less than their rated speed. This results in a very high start current when Direct On Line started. The Direct On Line starting current is independent of the motor load and is dependent only on the motor design, rotor speed and the applied voltage. Variations in motor loading will affect the start duration only. Typically, the Direct On Line starting current falls somewhere between 550% Full Load Current and 900% Full Load Current. The actual start current of a given design is determined primarily by the design of the rotor. Shallow bar rotor designs are generally referred to as Design 'A' rotors and are characterized by a high start current (650% - 900%) and a low starting torque (60% - 150%). Design 'B' rotors are deeper bar rotors and typically exhibit a starting current of (550% - 650%) and a starting torque of (150% - 300%).
In many installations, the maximum starting torque is not required, and the very high starting current places stress on the supply causes voltage disturbances and interference to other users on the supply. Reduced voltage starting is a means of reducing the start current, however a reduction in the start voltage will also reduce the starting torque.
In order to achieve a useful start at a reduced starting current, it is important that the motor is able to develop sufficient torque at all speeds up to full speed to exceed the load torque at those speeds. If the reduced torque developed by the motor is less than the load torque at any speed, the motor will not accelerate to full speed. Stepping the starter to full voltage at less than full speed will result in a high current and little if any advantage over using a Direct On Line starter. The selection of a start voltage that is too low will result in an inferior start characteristic.
Star/Delta (Wye/Delta) starters are open transition. When the transition is made from the reduced voltage to full voltage, there is a period of time when the motor is effectively open circuited from the supply. During this period, the motor is effectively acting as a generator at a frequency proportional to it's actual shaft speed. When the starter reconnects the motor to the supply in Delta, there is a very high transient current and resulting transient torque which is much more severe and damaging than the Direct On Line starting conditions.
Other reduced voltage starters commonly employed are the Autotransformer Starter and the Solid State Soft Starter.
To engineer the system, it is important to firstly establish the starting torque requirements of the driven load. Next the starting characteristics of the induction motor should be analyzed in order to establish the start current required by the motor to develop the required starting torque. A starter can now be designed/selected to meet the start current requirement, and an appropriate supply connected.
Induction motors exhibit a very low impedance at speeds less than their rated speed. This results in a very high start current when Direct On Line started. The Direct On Line starting current is independent of the motor load and is dependent only on the motor design, rotor speed and the applied voltage. Variations in motor loading will affect the start duration only. Typically, the Direct On Line starting current falls somewhere between 550% Full Load Current and 900% Full Load Current. The actual start current of a given design is determined primarily by the design of the rotor. Shallow bar rotor designs are generally referred to as Design 'A' rotors and are characterized by a high start current (650% - 900%) and a low starting torque (60% - 150%). Design 'B' rotors are deeper bar rotors and typically exhibit a starting current of (550% - 650%) and a starting torque of (150% - 300%).
In many installations, the maximum starting torque is not required, and the very high starting current places stress on the supply causes voltage disturbances and interference to other users on the supply. Reduced voltage starting is a means of reducing the start current, however a reduction in the start voltage will also reduce the starting torque.
In order to achieve a useful start at a reduced starting current, it is important that the motor is able to develop sufficient torque at all speeds up to full speed to exceed the load torque at those speeds. If the reduced torque developed by the motor is less than the load torque at any speed, the motor will not accelerate to full speed. Stepping the starter to full voltage at less than full speed will result in a high current and little if any advantage over using a Direct On Line starter. The selection of a start voltage that is too low will result in an inferior start characteristic.
Star/Delta (Wye/Delta) starters are open transition. When the transition is made from the reduced voltage to full voltage, there is a period of time when the motor is effectively open circuited from the supply. During this period, the motor is effectively acting as a generator at a frequency proportional to it's actual shaft speed. When the starter reconnects the motor to the supply in Delta, there is a very high transient current and resulting transient torque which is much more severe and damaging than the Direct On Line starting conditions.
Other reduced voltage starters commonly employed are the Autotransformer Starter and the Solid State Soft Starter.
Power dissipated in Enclosure
The total Power dissipated in the enclosure is the sum of all power dissipated by all components mounted within the enclosure.
In order to calculate/approximate the ventilation required for an enclosure, the power dissipated within the enclosure must be known.
Here are some guidelines for calculating the total power dissipated in the enclosure.
Soft Starter. Allow 4.5 Watts per amp. i.e. MSX 0175 operating at 150 amps, allow 150 x 4.5 = 675 Watts.
Speed Drive. Allow 20 Watts per amp.
Contactor AC3. If dissipation not known, allow 0.15 Watts per Amp contact dissipation, plus 0.1 Watts per amp coil dissipation.
Contactor AC1. If dissipation not known, allow 0.6 Watts per Amp contact dissipation, plus 0.1 Watts per amp coil dissipation.
Thermal overload. If not known, allow:
10 watts if Ie < 32A
18 watts if 32A < Ie < 70A
22 watts if 70A < Ie < 500A
50 watts if Ie > 500A.
Fans. (Not part of starter. i.e. cabinet ventilation fans)
Allow 10 watts for 25mm x 120 mm.
Allow 18 watts for 38mm x 120 mm.
Allow 30 watts for 51mm x 172 mm.
Lamps. Allow 2.5 watts
Fuses.
Type F06. Allow 2 watts per fuse.
Semiconductor Fuses.
Fuse
Soft Starter
Speed Drive
40AFE
2W
10W
80AFE
3W
20W
200FM
7W
40W
350FM
10W
55W
700FMM
22W
120W
Isolators. If not known, allow 0.6 watts per line amp.
Power factor correction capacitors. Refer to data sheet and/or supplier.
In order to calculate/approximate the ventilation required for an enclosure, the power dissipated within the enclosure must be known.
Here are some guidelines for calculating the total power dissipated in the enclosure.
Soft Starter. Allow 4.5 Watts per amp. i.e. MSX 0175 operating at 150 amps, allow 150 x 4.5 = 675 Watts.
Speed Drive. Allow 20 Watts per amp.
Contactor AC3. If dissipation not known, allow 0.15 Watts per Amp contact dissipation, plus 0.1 Watts per amp coil dissipation.
Contactor AC1. If dissipation not known, allow 0.6 Watts per Amp contact dissipation, plus 0.1 Watts per amp coil dissipation.
Thermal overload. If not known, allow:
10 watts if Ie < 32A
18 watts if 32A < Ie < 70A
22 watts if 70A < Ie < 500A
50 watts if Ie > 500A.
Fans. (Not part of starter. i.e. cabinet ventilation fans)
Allow 10 watts for 25mm x 120 mm.
Allow 18 watts for 38mm x 120 mm.
Allow 30 watts for 51mm x 172 mm.
Lamps. Allow 2.5 watts
Fuses.
Type F06. Allow 2 watts per fuse.
Semiconductor Fuses.
Fuse
Soft Starter
Speed Drive
40AFE
2W
10W
80AFE
3W
20W
200FM
7W
40W
350FM
10W
55W
700FMM
22W
120W
Isolators. If not known, allow 0.6 watts per line amp.
Power factor correction capacitors. Refer to data sheet and/or supplier.
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