Motor Starting - Electronic Soft Start
Breathing a sigh of relief, we are now onto the final motor starting system in the series.
Before talking about electronic soft starting, is is worth mentioning that the development of power electronics has and continues to revolutionize that way motors are used and applications to which they can be applied. You could write books (and some people have) on the application of power electronics to the running and control of motors. This is outside the scope of what we are looking at, and I will only be talking about the application to starting (and then in only a very general way).
While primarily discussing electronic soft starters, lets remember that more advanced devices (variable speed drives for example) have this functionality in-built.
Soft starters use a combination of power electronics and electronic control circuitry to slow increase the voltage on the motor during starting; ensuring a smooth acceleration.
Soft starters use thyristors (Silicon Controlled Rectifier) to control the energy delivered to the motor. A thyristor is a device which turns on when a pulse is applied to its gate and will continue to conduct until the current drops to zero (at which time it turns off). In an AC sine wave current goes to zero each half cycle, allowing the current to be turned off and making it possible to use use thyristors to implement soft starting.
Voltage Control by Thyristor Firing
If the thyristors are turned on at the start of each half cycle, the full voltage is applied to the motor. If the thyristors are never turned on, then no voltage is applied. If the thyristors are turned on part way through the half cycle, only a proportion of the voltage will be applied to the motor. By controlling the turning on (firing angle) of the thyristors, the amount of voltage on the motor can be controlled. Starting with a large firing delay, this is gradually reduced and the voltage on the motor will ramp-up during starting.
In addition to starting, soft start units can also be used for stopping the motor, by ramping the voltage down. This is particularly useful where sudden loss of driving torque would create mechanical shock on the load.
Electronic soft starters contain the thyristors (power side) and necessary electronics to control the firing (via user settings). Modern soft starters have a host of features; the most common being options to set varying start and stop ramps, setting of the initial starting voltage, current limiting control and thermal overload protection.
The easiest application of electronic soft starters is one unit per motor. To reduce cost, sometimes more than one motor is connected in parallel to a soft starter, or motors are started in sequence by switching the soft start unit between motors. While these methods can be used, care has to be taken to ensure that the units are adequately rated for the required duties.
As a cautionary note, firing of thyristors other than a zero voltage (current) will create a non-linear load characteristic, generating transients and harmonics. In general application (and given that the transients/harmonics are only present during starting), these are not a problem. However, there could be instances and special situations where these do have an adverse effect on the power system.
Power and Control Circuiting
The figure illustrates the simplest connection of an electronic soft starter. In this, the soft start unit is simply connected into the circuit and carries out the necessary functions.
Motor Soft Starting
While this is the simplest implementation; you will often find a bypass contactor (to switch-out the soft starter when the motor is up to speed) and a line contactor to switch full starting circuit in/out. Other variations of control circuit would include cascading start and control for both forward and reverse directions. Invariably there would also be communications to either control or monitor the functioning of the motor.
Features and Application
Soft starters have many features not found in traditional starting methods and which can be of benefit in some situations. These include:
- Adaptive Acceleration Control - soft starter learns motor’s performance during start and stop, then adjusts control to optimize performance
- Soft Stopping - slowly bring the motor to a stop
- DC Braking - injecting DC to reduce motor stopping time
- Soft Braking - changing contactors on starter input, reversing motor direction and applying braking
- Current Limiting - limit starting current to a pre-defined value (may lower torque to much and motor may not accelerate)
- Current Ramping - increasing the current over a given time to a specified limit
- Jog - allows the motors to be run at reduced speed for some time
- Kick Starting - using a brief high bust of current at the motor start to get it going, then reducing the current
- Thermal Modelling - motors the motors thermal characteristic, allow performance optimization
- Communication - most industrial protocols will be supported, enabling enhanced control/monitoring
Line contactors can be used (but not strictly necessary) to isolate the soft start when not in use (should be rated AC3). If required, by-pass contactors can by used to minimize heat build up due the to SCR's (can rated AC1 as they do not carry starting currents).
Sequence starting - can be used to start several motors in sequence (or parallel). Starters need to be rated for full start duty. Additional wiring, contactors and control relays may not make this an economic option.
Power factor correction if required should only be installed on the line side and switched in when the motor is at full speed (AC6 contactors). Capacitors installed on the motor side can cause resonance, increased voltages and equipment failure.
Inside delta connection is a technique which can be used with six winding motors. In this configuration, only have the delta is completed in that starter, thus reducing the current the starter is required to carry. Only a limited number of soft starters are able to be used in this type of application.
Electronic Soft Start Characteristics
Available starting current: 25 to 75%, adjustable
Peak starting current: 2 to 5 In, adjustable
Peak starting torque: 10 to 70%, adjustable
|Fully Adjustable Parameter Compact, Solid State Adaptable to Application
||More Expensive Can Inject Transients into Supply
That's it for all the starting methods. Now we have finished with each method in detail, there's just one more post to come. I will be writing a summary post to round off the series, pulling together what we’ve discussed and putting things into context.
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