The issue of reduced-voltage starting techniques comes up frequently in connection with medium-voltage motors and their starters. A reduced-voltage motor starter offers many advantages, and a few drawbacks as well.
The reduced-voltage motor starter reduces the inrush current required of the primary system, and hence, the voltage drop on the system during starting. The serving utility often has limitations on the allowable inrush current, and may impose severe economic penalties if violated. In addition, excessive voltage drop on the system during starting may have unacceptable consequences for the rest of the system.
Once the decision is made to use reduced-voltage starting, the next step is to determine which type is best for your application: a traditional autotransformer starter (commonly referred to as reduced-voltage autotransformer or RVAT starter) or a modern solid-state reduced voltage starter (SSRVS, often called a “soft starter”).
What are the characteristics of each type of reduced-voltage starter? These are summarized in a tabular form.
In an autotransformer (RVAT) starter, the motor is connected to the secondary of an autotransformer during the early stage of the starting process, and then the autotransformer is disconnected when the machine is transferred to full voltage to complete the starting process and subsequent normal operation.
The basic schematic for an autotransformer starter is as shown in Figure 1. The sequence of operation is:
- Close isolation switch.
- Close shorting contactor.
- Close start (main) contactor – this connects motor at the particular voltage tap of the autotransformer for which the starter is configured (typical taps are 50 percent, 65 percent, and 80 percent). The motor begins to accelerate.
- After a preset time delay (timer in
control circuit or setting in
protective relay), the shorting
contactor opens. In lieu of a timer, it
is also possible to initiate the
transfer to full voltage as a function
of the line current, using modern
microprocessor protective relays. At
least in theory, the transfer from
reduced voltage to full voltage is
made when the motor torque equals
the torque required by the motor /
load combination, at which point,
acceleration ceases. In practice, the
transfer is seldom so precisely
- After a very short time delay, the
run contactor closes, connecting the
motor to full voltage.
- The autotransformer remains
connected to the system, but with
no voltage across the
- Once the run contactor closes, the
motor completes acceleration to full
speed and is then in normal
The starting torque and the starting current of the starter at reduced voltage is a function of the selected tap on the autotransformer. Refer to table three.
The tap connection on the autotransformer must be selected so that the motor produces sufficient torque to accelerate, while reducing the impact on system voltage during starting and controlling starting current.
The autotransformer starter uses three contactors for starting and running operation of the machine, making the power circuit connections somewhat complex.
The autotransformer scheme has been in use for many decades and is well proven in experience.
The autotransformer imposes two inrush transients on the power system, the first when the starting sequence is initiated, and the second when the machine is transferred from reduced voltage to full voltage. When the start sequence is initiated, the machine produces the reduced level of torque indicated in the table, and when the machine is transferred to full voltage, the full torque of the machine is available to complete the acceleration to full speed. To reduce the impact of these starting transient currents, it is desirable not to use the 80-percent tap connection. On the other hand, the 50-percent tap connection often does not provide sufficient torque to accelerate the machine. For this reason, the 65-percent tap is the most frequently used connection.
There are some reports in the literature of damage to autotransformers resulting from the voltage transients associated with the transfer from reduced voltage to full voltage, but these seem to be related to use of a two-coil autotransformer design rather than the three-coil autotransformer configuration shown in Figure 1. Siemens has not experienced such problems in the past.
The autotransformer is a custom component, whose characteristics must be tailored to those of the machine that the unit controls. As a result, in the event of a failure of an autotransformer, the time required to obtain a duplicate unit can be excessive.
Solid-state, reduced-voltage starter (SSRVS) (soft starter)
A solid-state, reduced-voltage starter uses electronic control of the voltage applied to the machine during starting.
This allows control of the starting current throughout the accelerating process, reducing the impact of starting transients on the system. This facet is why such controllers are often referred to as “soft starters”.
SSRVS controllers have been available for several decades but adoption was initially slow due to significantly higher purchase cost of the soft-starter unit compared to an autotransformer starter. More recently, the purchase cost differential has declined as the usage of SSRVS starters has become dominant in the market, but there is still a premium for a soft starter over the purchase cost of an autotransformer starter.
The normal operation of a soft starter imposes a preselected initial voltage on the machine (referred to as voltage ramp), and the voltage is gradually ramped up until full voltage is reached, at which point the soft starter electronics are disconnected (or bypassed) and the machine is operated at full voltage. The starting time is typically selected by the user and is of the order of up to 30 seconds. An alternative approach is to use the soft starter to limit starting current (referred to as current ramp) to some value, typically 300 percent - 600 percent of full-load current, although limiting to 600 percent would essentially be full-voltage starting and hence not an attractive setting.
The current-limit value during starting must be sufficient to allow the machine to accelerate the machine and the connected load.
A soft starter allows for fine tuning of the allowed current during starting. In an autotransformer starter, the starting current can only be adjusted in fixed intervals, from 300 percent of full-load current (at 50-percent tap) to 390 percent (at 65-percent tap) to 480 percent (at 80-percent tap), based on an assumed locked rotor current of 600 percent of full-load current. In contrast, the user can adjust the current limit with a soft starter to any intermediate values, for example, 280 percent or 320 percent, to fine tune the actual start conditions.
SSRVS starters also allow more sophisticated motor-control profiles, including pump-control profiles, soft stop, and the like. A pump-control profile recognizes the load imposed by a typical pump application, adjusting the voltage imposed on the motor at the end of the acceleration period so as to minimize the chances of water hammer (pressure surges) in pumping applications.
SSRVS units can also be configured for pulse-starting profile, for loads that have a high starting friction and thus require a higher torque at the beginning of the start process, with the voltage reduced after the relatively short pulse start time.
SSRVS units have complex electronics but the power circuit for an SSRVS starter is less complicated than with an autotransformer starter.
The electronics portion of the starter is quite sophisticated and typically requires that the manufacturer be involved in any commissioning and troubleshooting.
Modern SSRVS power stack assemblies usually have provisions for testing and troubleshooting using a low-voltage source and a small motor, simplifying initial configuration and subsequent maintenance activities.
Capacitors, whether power factor correction capacitors or surge capacitors, must not be connected the load side of an SSRVS soft starter. Capacitors, if used, must be connected on the line side of the controller, and arranged so that the capacitors are disconnected during starting and during stopping of the machine. For more information on this topic, consult TechTopics No. 125.
A growing application for SSRVS starters is for applications that have multiple motors of the same horsepower rating, where additional motors are brought on-line as the load demands dictate. For such applications, it is possible to arrange the equipment so that one soft-starter electronic module is shared among the motors, with the electronic module used repetitively to start each additional machine. While this complicates the arrangement of the power circuit of the motor control lineup, it has the advantage of needing only one expensive SCR power stack assembly. The SCR power stack assembly must, of course, be engineered to accommodate the increased heat associated with use for multiple starts over a short period of time.
Should you have any questions about this issue of TechTopics or any of our products, solutions, or services, please contact your local Siemens sales representative for more information.