Soft Starters for Induction Motors

Soft Starter

A soft starter is another form of reduced voltage starter for A.C. induction motors. The soft starter is similar to a primary resistance or primary reactance starter in that it is in series with the supply to the motor. (Three wire or standard connection) The current into the starter equals the current out. The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor. In theory, soft starters can be connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding. (Six wire or Inside Delta connection)

Voltage Control

 

Voltage control is achieved by means of solid state A.C. switches in series with one or more phases. These switches comprise either: 1 x Triac per phase

 

1 x SCR and 1 x Diode reverse parallel connected per phase.

 

2 x SCRs reverse parallel connected per phase.

Solid state switches

 

These Solid State Switches are phase controlled in a similar manner to a light dimmer, in that they are turned on for a part of each cycle. The average voltage is controlled by varying the conduction angle of the switches. Increasing the conduction angle will increase the average output voltage. Controlling the average output voltage by means of solid state switches has a number of advantages, one of the major advantages being the vast improvement in efficiency relative to the primary resistance starter, due to the low on state voltage of the solid state switches. Typically, the power dissipation in the starter, during start, will be less than 1% of the power dissipated in a primary resistance starter during start. Another major advantage of the solid state starter is that the average voltage can be easily altered to suit the required starting conditions. By variation of the conduction angle, the output voltage can be increased or reduced, and this can be achieved automatically by the control electronics. The control electronics can be preprogrammed to provide a particular output voltage contour based on a timed sequence (open loop), or can dynamically control the output voltage to achieve an output profile based on measurements made of such characteristics as current and speed (closed loop).

Switching Elements.

 

The switching elements must be able to control the current applied to the motor at line voltage. In order to maintain a high level of reliability on a real industrial type supply, the switching elements need to be rated at least 3 times the line voltage. On a 400 volt supply, this means that the requirement is for 1200 Volt devices, and 600 Volt devices on a 200 volt supply. It is also important that the switching elements have a good transient current overload capacity.

 

1200 Volt triacs with good current transient overload characteristics are not readily available, and so the choice is really between the SCR-Diode and SCR-SCR. There are some triacs which are suitable for this operation, but they are not easily attainable.

 

The major differences between the SCR-SCR and the SCR-Diode options are price, and the harmonic content of the output voltage. The SCR-SCR method provides a symmetrical output which is technically desirable from the point of supply disturbances and harmonics, while the SCR-Diode method is inferior technically, it is commercially more effective and easier to implement.

 

Harmonics awareness and paranoia has drastically reduced the number of SCR-Diode type soft starters on today's market, but they do still exist. The technology is not always easily recognizable as such with terms such as three pulse technology being used to describe SCR-Diode systems as opposed to six pulse technology describing SCR-SCR systems.

 

The soft starter can be designed to control one phase, reducing the torque but not the current on two phases, (SCR/Diode can not be used in this connection)

 

or two phases reducing the torque but the current will not be optimally reduced or balanced, there will be negative sequence currents heating the rotor and reducing the torque per unit start current, (SCR/Diode can not be used in this connection)

 

or three phases, reducing current and torque, providing the optimum results for torque generated per unit of start current.

 

Open Loop Control.

 

Open Loop soft starters are soft starters producing a start voltage profile which is independent of the current drawn, or the speed of the motor. The start voltage profileis programmed to follow a predetermined contour against time. A very basic Timed Voltage Ramp (TVR) system operates by applying an initial voltage to the motor, and causing this voltage to slowly ramp up to full voltage. On basic systems, the initial start voltage is not adjustable, but the ramp time is. Commonly the voltage ramps time is referred to as the acceleration ramp time and is calibrated in seconds. This is not an accurate description as it does not directly control the acceleration of the motor. A lightly loaded motor can accelerate to full speed even with a sixty second ramp selected. More correctly this should be referred to as the voltage ramp time. On more comprehensive units, the start voltage is pre-setable, typically from 10% to 70% of full line voltage. This should be set to achieve at least breakaway torque for the motor at start. There is little advantage in the motor sitting, staining to start due to insufficient torque. this will only increase the heat dissipated in the motor. The start voltage setting is often referred to as the start torque setting and calibrated in percent. This is a nonsense, as although increasing the start voltage is going to increase the starting torque of the connected motor, the actual starting torque is a function of both the start voltage and the motor design. The starter does not know anything about the connected motor, and so is not able to deliver a prescribed amount of torque under open loop conditions. The actual start torque produced is initially equal to the LRT multiplied by the square of: (the start voltage divided by the line voltage). The LRT of the motor could vary from as low as 60% FLT to as high as 350% FLT which is a range of almost 6 to 1.

 

• The start voltage profile

The Start Profile can be a simple single slope from zero voltage to full voltage, or it can be a complex shape to more closely emulate a controlled current start.


Like electromechanical starters, open loop soft starters cause the start voltage applied to the motor, to change with time irrespective of the motor and load conditions, eventually getting to full voltage, and under jammed load conditions, developing LRC and LRT until something trips or breaks.

Closed Loop Control.



Closed Loop starters monitor an output characteristic or effect from the starting action and dynamically modify the start voltage profile to cause the desired response. The most common closed loop soft starter is the controlled current soft starter where the current drawn by the motor during start is monitored and controlled to give either a constant current, or a current ramp soft start. A much rarer closed loop format is the constant acceleration soft start where the motor speed is monitored by a tachogenerator or shaft encoder and the voltage is controlled to maintain a constant rate of acceleration or a linear increase in motor speed.


The controlled current soft starters are available with varying levels of sophistication. In the most basic systems, the soft starter is essentially a standard TVR soft starter with a ramp freeze option where the current on one phase is monitored and compared to a set point. If the current exceeds the set point, the ramp is frozen until the current drops below that set point. At the other end of the scale, a comprehensive closed loop soft starter will monitor the current on all three phases and dynamically change the output voltage to correct the start current to the required profile. This system is able to both increase and reduce the start voltage to suit the application.


A constant current starter will start initially at zero volts and rapidly increase the output voltage until the required current is delivered to the motor, and then adjust the output voltage while the motor is starting until either full voltage is reached, or the motor overload protection operates. Constant current starters are ideal for high inertia loads, or loads where the starting torque requirements do not alter.


The current ramp soft starter operates in the same manner as the constant current soft starter except that the current is ramped from an initial start current to a current limit setting over a period of time. The initial start current, current limit, and the ramp time are all user adjustable settings and should be customize to suit the application. The current ramp soft starter can be used for a number of advantages over constant current in some applications. Machines which have a varying start torque requirement, such as on load conveyers, or applications requiring a reduced initial torque such as pumping applications, or genset applications where the relatively slow application of current load will allow the genset to track the load are examples of situation where the current ramp soft start can be used to advantage.


Another form of closed loop starter is the torque control starter where the starter models the motor under high slip and low slip conditions and uses this mathematical model to calculate the shaft torque being produced by the motor. This is then used as a feed back source with linear and square law start torque curves being used to control the start voltage applied to the motor. The true torque control starter is able to give much better control of the acceleration of the motor being started.




Starting Torque.



To start a machine, the motor must develop sufficient torque over the entire speed range to exceed the work and loss torque of the driven load, and provide a surplus torque for accelerating the machine to full speed. The starting torque delivered by the motor at any speed, is equal to the full voltage starting torque at that speed, multiplied by the current or voltage reduction squared. Provided the full voltage speed/torque curves and the full voltage speed/current curves are available, the reduced voltage (or current) speed/torque curves can be calculated. This curve can be superimposed onto the load speed torque curve, and provided the torque developed at all speeds exceeds the load torque, the motor will accelerate to full speed. If the curves cross, the start current (or voltage) will need to be increased to increase the start torque developed by the motor. The difference between the torque developed and the load torque is essentially the acceleration torque that will accelerate the machine to full speed. A high acceleration torque may be desirable for a high inertia machine in order to minimize the starting time.


With a controlled current soft starter, the voltage reduction reduces as the motor impedance accelerates due to the rising motor impedance. As the motor approaches full speed, the voltage rises quickly (against speed) to full voltage. When the torque curve for a motor started by a constant current starter is compared with that of a constant voltage starter such as an auto transformer starter, it can be seen that there is an increase in the torque as the motor accelerates with a constant current start. This is ideal because as the motor and machine increase in speed, the actual load on the motor shaft will increase also. This characteristic will often enable a load to be started with a lower current on a soft starter than traditional starter methods.






Soft Stop.



Soft starters can have soft stop included for no extra cost.


Soft stop is the oposite to soft start. The voltage is gradually reduced, reducing the torque capacity of the motor. The reduction of available torque causes the motor to begin to stall when the shaft torque of the motor is less than the torque that is required by the load. As the torque is reduced, the speed of the load will reduce to the point where the load torque equals the shaft torque.


Typically, the soft stop used is an open loop voltage ramp, but there are some torque control soft stop systems that use torque feedback to provide better control over the deceleration of the motor.


Open loop soft stop performance is very dependent on the characterisitcs of the motor and driven load. On larger machines this can be very non linear and provide poor performance.


Soft stope effectively adds inertia to the load and extends the braking time. It should only be applied to installations where the stopping time is too short and needs to be extended. Soft stop does not provide any measure of braking.


DC Brake.



DC Braking can be added to soft starters, but the effectiveness is not as good as the braking that can be achieved with a specialist DC brake circuit. DC braking is achieved by turning ON a positive SCR on one phase and a negative SCR on a second phase for a small angle of each cycle. This causes a high pulse of DC current to flow through the motor windings and creates a stationary torque field in the stator. The stationary torque field causes the motor to slow down. The short pulses at line frequency also produce a synchronous component in the torque field that can limit the effectiveness at close to synchronous speed. In some cases, a shorting contactor is connected across a motor winding to prolong the period of current flow and reduce the line frequency component.


DC braking is used to apply a braking torque to the motor and load and to make it stop quicker. During DC braking, the energy of the driven load is dissipated in the rotor of the motor.


Slip Ring Motors.



Soft starters can be applied to many slip ring motors, however there are some where the application of a soft starter will not give satisfactory results.


Slip ring motors are often employed for their ability to produce a very high torque across the entire speed range. The slip ring motor is able to do this at a very low start current. Another reason for the application of a slip ring motor is that it is able to offer a high degree of control.


If the slip ring motor is employed to give a very high start torque across the entire speed range, then the soft starter is not going to provide a satisfactory solution. This is because the application of a soft starter or any other primary starter, is going to reduce the torque available. Where the requirement is for a gentle start at reduced torque, the soft starter is of benefit.


A common misconception is that the slip ring starter can be converted to a cage type motor by shorting the slip rings and starting by the normal methods. If the secondary winding is shorted, the slip ring motor will exhibit a very high LRC (typically >1000%) and a very low LRT (typically < 100%). If a reduced voltage starter is applied under these conditions, the start torque will be very low and will not start a machine. To apply a reduced voltage starter to a slip ring motor, first ascertain that a reduced torque is going to start the machine, then fit resistors to the rotor circuit which will give curves similar to a high start torque cage motor. These resistors must then be bridged once the machine has reached full speed. The value of the resistance is dependent on the motor and the curve required, however the resistors must absorb a lot of energy, dependent on the inertia of the load. It is common to use the final stage resistance of the existing starter when available.
























 

 

 

 

 

Single-Phase Electric Motor Characteristics & Applications

Where three-phase power is unavailable or impractical, it's single-phase motors to the rescue. Though they lack the higher efficiencies of their three-phase siblings, single-phase motors, correctly sized and rated, can last a lifetime with little maintenance. Occasionally a manufacturing defect can result in early motor failure. However, most failures result from inappropriate application. Pay careful attention to the application requirements before choosing a motor for replacement of a failed one or for a new design application. Not choosing the correct motor type and horsepower can cause repeated motor failure and equipment downtime. Obviously, you don't want to specify a motor too small for the application, thus resulting in electrical stresses that cause premature motor failure. But neither should you specify a motor too powerful, either because of its power or its inherent design characteristics. It can also have serious effects. For example, a LEESON electric motor with high locked-rotor and breakdown torques can damage the equipment it drives. Also, running a motor at less than full rated load is inefficient, costing you money for power wasted.

The key: First, size the motor to the application but, just as importantly, understand the characteristics of the major types of single-phase motors characteristics that go right to the heart of matching a motor to an application. In general, an ac polyphase squirrel-cage motor connected to a polyphase line will develop starting torque. A squirrel-cage motor connected to a single-phase line develops no starting torque, but having been started by some external means, it runs approximately like a polyphase motor. The many types of single-phase motors are distinguished by the means by which they are started.


The Split Phase Motor, also called an induction-start/induction-run motors, is probably the simplest single-phase motor made for industrial use, though somewhat limited. It has two windings : a start and a main winding, Figure 1. The start winding is made with smaller gage wire and fewer turns relative to the main winding to create more resistance, thus putting the start winding's field at a different angle than that of the main winding, and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time. A split-phase motor uses a switching mechanism that disconnects the start winding from the main winding when the motor comes up to about 75% of rated speed. In most cases, it is a centrifugal switch on the motor shaft.







The split-phase motor's simple design makes it typically less expensive than other single-phase motor types made for industrial use. However, it also limits performance. Starting torques are low, typically 100% to 175% of rated load. Also, the motor develops high starting currents, approximately 700 to 1,000% of rated. Consequently, prolonged starting times cause the start winding to overheat and fail; so don't use this motor if you need high starting torques. Other split-phase motor characteristics: Maximum running torques range from 250 to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with trip time fast enough to prevent start-winding burnout. And, these motors usually are designed for single voltage, limiting application flexibility. Good applications for split-phase motors include small grinders, small fans and blowers, and other low starting torque applications with power needs from 1/20 to 1/3 HP. Avoid any applications requiring high cycle rates or high torques.

Capacitor Start/Induction Run



Here is a true wide-application, industrial-duty motors. Think of it as a split-phase motor, but with a beefed-up start winding that includes a Capacitor In The Circuit to provide a start "boost", Figure 2. Like the split-phase motor, the capacitor start motor also has a starting mechanism, either a mechanical or solid state electronic switch. This disconnects not only the start winding, but also the capacitor when the motor reaches about 75% of rated speed. Capacitor start/induction run motors have several advantages over split-phase motors. Since the capacitor is in series with the start circuit, it creates more starting torque, typically 200 to 400% of rated load. And the starting current, usually 450 to 575% of rated current, is much lower than the split-phase due to the larger wire in the start circuit. This allows higher cycle rates and reliable thermal protection. The cap-start/induction-run motor is more expensive than a comparable split phase design because of the additional cost of the start capacitor. But the application range is much wider because of higher starting torque and lower starting current. Use them on a wide range of belt-drive applications like small conveyors, large blowers and pumps, as well as many direct-drive or geared applications. These are the "workhorses" of general-purpose single-phase industrial motors.

A Permanent Split Capacitor (PSC) motors, Figure 3, has neither a starting switch, nor a capacitor strictly for starting. Instead, it has a run-type capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor reaches running speed. Because the run capacitor must be designed for continuous use, it cannot provide the starting boost of a starting capacitor. Typical starting torques of PSC motors are low, from 30 to 150% of rated load, so these motors are not for hard-to-start applications. However, unlike split-phase motors, PSC motors have low starting currents, usually less than 200% of rated load current, making them excellent for applications with high cycle rates. Breakdown torque varies depending on the design type and application, though it is typically somewhat lower than with a cap start motors.







PSC motors have several advantages. They need no starting mechanism and so can be reversed easily. Designs can be easily altered for use with Speed Controllers. They can also be designed for optimum efficiency and high power factor at rated load. And they're considered to be the most reliable of the single phase motors, mostly because no starting switch is needed. Permanent split capacitor motors have a wide variety of applications depending on the design. These include fans, blowers with low starting torque needs, and intermittent cycling uses such as adjusting mechanisms, gate operators and garage door openers, many of which also need instant reversing.


Capacitor Start/Capacitor Run



This type, Figure 4, combines the best of the capacitor-start/induction-run motor and the permanent split capacitor motor. It has a start-type capacitor in series with the auxiliary winding like the capacitor-start motor for high starting torque. And, like a PSC motor, it also has a run-type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit. This allows high breakdown or overload torque.






Another advantage of the capacitor-start/capacitor-run type motor: It can be designed for lower full-load currents and higher efficiency. Among other things, this means it operates at lower temperature than other single-phase motor types of comparable horsepower. The only disadvantage to a cap-start/cap-run motor is its higher price, mostly the result of more capacitors, plus a Starting Switch. But it's a real powerhouse, able to handle applications too demanding for any other kind of single-phase motor. These include woodworking machinery, air compressors, high-pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp.


Shaded-Pole



Unlike all the previous types of single-phase motors discussed, shaded-pole motors have only one main winding and no start winding, Figure 5. Starting is by means of a design that rings a continuous copper loop around a small portion of each motor pole. This "shades" that portion of the pole, causing the magnetic field in the ringed area to lag the field in the unringed portion. The reaction of the two fields gets the shaft rotating. Because the shaded pole motor lacks a start winding, starting switch or capacitor, it is electrically simple and inexpensive. Plus, speed can be controlled merely by varying voltage, or through a multi-tap winding. Mechanically, shade-pole motor construction allows high-volume production. In fact, these are usually considered "disposable" motors, meaning they are much cheaper to replace than to repair.






The shaded pole motors have many positive features, but it also has several disadvantages. Its low starting torque is typically 25 to 75% of full load torque. It is a high slip motor with running speed 7 to 10% below synchronous speed. Also, it is very inefficient, usually below 20%. Low initial cost suits shaded pole motors to low-horsepower or light-duty applications. Perhaps their largest use is in multi-speed fans for household use. But low torque, low efficiency, and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial uses where higher cycle rates or continuous duty are the norm. The preceding information establishes guidelines for determining the proper motor type for your application. However, there are always special cases and applications in which it is acceptable to vary from these guidelines. Make it a point to check with your motor manufacturer for technical support in these areas.


Capacitor Details



Start Capacitor: The electrolytic start capacitor helps motors achieve the most beneficial phase angles between start and main windings for the most locked-rotor torque per locked-rotor ampere. It is disconnected from the start circuit when the motor reaches about 75% of full-load speed. The start capacitor is designed for short-time duty. Extended application of voltage to the capacitor will cause permature failure, if not immediate destruction. Typical ratings for motor start capacitors range from 100 to 1,000 microfarad (uF) capacitance and 115 to 125 volts AC. However, special applications require 165 to 250-Vac capacitors, which are physically larger than capacitors of lower voltage rating for the same capacitance. Capacitance is a measure of how much charge a capacitor can store relative to the voltage applied.






Run Capacitor: These are constructed similarly to start capacitors, except for the electrolyte. They are designed to serve continuously in the run circuit of a capacitor-start /capacitor-run motors. They withstand higher voltages, in the range of 250 to 370 Vac. They also have lower capacitance, usually less than 65 uF.

Continuous Torque Density

The continuous torque density of conventional electric machines is determined by the size of the air-gap area and the back-iron depth, which are determined by the power rating of the armature winding set, the speed of the machine, and the achievable air-gap flux density before core saturation. Despite the high coercivity of neodymium or samarium-cobalt permanent magnets, continuous torque density is virtually the same amongst electric machines with optimally designed armature winding sets. Continuous torque density should never be confused with peak torque density, which comes with the manufacturer's chosen method of cooling, which is available to all, or period of operation before destruction by overheating of windings or even permanent magnet damage.

Torque capability of motor types

When optimally designed within a given core saturation constraint and for a given active current (i.e., torque current), voltage, pole-pair number, excitation frequency (i.e., synchronous speed), and air-gap flux density, all categories of electric motors or generators will exhibit virtually the same maximum continuous shaft torque (i.e., operating torque) within a given air-gap area with winding slots and back-iron depth, which determines the physical size of electromagnetic core. Some applications require bursts of torque beyond the maximum operating torque, such as short bursts of torque to accelerate an electric vehicle from standstill. Always limited by magnetic core saturation or safe operating temperature rise and voltage, the capacity for torque bursts beyond the maximum operating torque differs significantly between categories of electric motors or generators.


Capacity for bursts of torque should not be confused with field weakening capability inherent in fully electromagnetic electric machines (Permanent Magnet (PM) electric machine are excluded). Field weakening, which is not available with PM electric machines, allows an electric machine to operate beyond the designed frequency of excitation.






Electric machines without a transformer circuit topology, such as Field-Wound (i.e., electromagnet) or Permanent Magnet (PM) Synchronous electric machines cannot realize bursts of torque higher than the maximum designed torque without saturating the magnetic core and rendering any increase in current as useless. Furthermore, the permanent magnet assembly of PM synchronous electric machines can be irreparably damaged, if bursts of torque exceeding the maximum operating torque rating are attempted.

Electric machines with a transformer circuit topology, such as Induction (i.e., asynchronous) electric machines, Induction Doubly-Fed electric machines, and Induction or Synchronous Wound-Rotor Doubly-Fed (WRDF) electric machines, exhibit very high bursts of torque because the active current (i.e., Magneto-Motive-Force or the product of current and winding-turns) induced on either side of the transformer oppose each other and as a result, the active current contributes nothing to the transformer coupled magnetic core flux density, which would otherwise lead to core saturation.


Electric machines that rely on Induction or Asynchronous principles short-circuit one port of the transformer circuit and as a result, the reactive impedance of the transformer circuit becomes dominant as slip increases, which limits the magnitude of active (i.e., real) current. Still, bursts of torque that are two to three times higher than the maximum design torque are realizable.




The Synchronous WRDF electric machine is the only electric machine with a truly dual ported transformer circuit topology (i.e., both ports independently excited with no short-circuited port). The dual ported transformer circuit topology is known to be unstable and requires a multiphase slip-ring-brush assembly to propagate limited power to the rotor winding set. If a precision means were available to instantaneously control torque angle and slip for synchronous operation during motoring or generating while simultaneously providing brushless power to the rotor winding set (see Brushless wound-rotor doubly-fed electric machine), the active current of the Synchronous WRDF electric machine would be independent of the reactive impedance of the transformer circuit and bursts of torque significantly higher than the maximum operating torque and far beyond the practical capability of any other type of electric machine would be realizable. Torque bursts greater than eight times operating torque have been calculated.

Nanotube nanomotor

Researchers at University of California, Berkeley, recently developed rotational bearings based upon multiwall carbon nanotubes. By attaching a gold plate (with dimensions of the order of 100 nm) to the outer shell of a suspended multiwall carbon nanotube (like nested carbon cylinders), they are able to electrostatically rotate the outer shell relative to the inner core. These bearings are very robust; devices have been oscillated thousands of times with no indication of wear. These nanoelectromechanical systems (NEMS) are the next step in miniaturization and may find their way into commercial applications in the future.

See also:


Molecular motors
Electrostatic motor

Linear motors

A linear motor is essentially an electric motor that has been "unrolled" so that, instead of producing a torque (rotation), it produces a straight-line force along its length by setting up a traveling electromagnetic field.

Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground, and in many roller-coasters where the rapid motion of the motorless railcar is controlled by the rail. On a smaller scale, at least one letter-size (8.5" x 11") computer graphics X-Y pen plotter made by Hewlett-Packard (in the late 1970s to mid 1980's) used two linear stepper motors to move the pen along the two orthogonal axes.

Introduction to Machinery Principles