Utilizing Autotransformers for Load Currents and Voltage Regulators
Autotransformers are used in many applications. A very common application is the use of autotransformer in reduced-voltage motor starting. Other applications include the use of autotransformers as voltage regulators and as variable transformers.
Reduced-Voltage Motor Starting
Reduced-voltage motor starting is a technique for reducing the current through the motor windings and reducing the load on the power distribution circuit during motor starting. Autotransformer starting uses a tapped 3-phase autotransformer to provide a reduced-voltage starting. Autotransformer starting is one of the most efficient techniques of reduced-voltage starting.
Induction motors and large synchronous motors use autotransformers to reduce the stator currents to a level that does not overtax the distribution systems that feed them. Synchronous motors have a two-part rotor. The rotor has a standard induction motor rotor section and a wound-rotor section. A synchronous motor is started as an induction motor and is accelerated to near synchronous speed using the induction part of the rotor. When the rotor approaches the synchronous speed, power is applied to the wound-rotor section, and the motor pulls into synchronous speed (see Figure 1).
Figure 1. Synchronous motors are started as induction motors to bring the rotor to near synchronous speed.
Autotransformer starting is preferred over primary resistor starting when the starting current is drawn from the line must be held below a maximum allowed value, yet the maximum starting torque per line ampere is required. Autotransformer reduced-voltage starting is used for starting blower, compressor, conveyor, and pump motors over about 10 HP.
Autotransformer reduced-voltage starting provides the highest possible starting torque per ampere of line current. However, because autotransformers are required, the installation cost is higher than with other reduced-voltage starting methods. Autotransformer reduced-voltage starting can be used with any 3-phase motor.
In autotransformer starting, the motor terminal voltage is independent of the load current. The current to the motor may change because of the motor's changing characteristics, but the voltage to the motor remains relatively constant.
Autotransformer starting may use the turns ratio advantage to provide more current on the load side of the transformer than on the line side. In autotransformer starting, transformer motor current and line current are not equal as they are in primary resistor starting.
For example, a motor may have a full-voltage starting torque of 120% and a full-voltage starting current of 600%. The electric utility commonly sets a limit of 400% current draw from the power line. This limitation is for the line side of the transformer. Because the transformer has a step-down ratio, the motor current on the transformer secondary is larger than the line current, even though the primary current of the transformer does not exceed 400%.
In this example, 80% voltage can be applied to the motor, generating 80% motor current. The motor draws only 64% of line current (80% of 80% = 64%) due to the 1:0.8 turns ratio of the transformer (see Figure 2).
Figure 2. Autotransformers are used in reduced-voltage motor starting to reduce the current drawn from the power line.
A large electric motor may not be started very often. Therefore, the autotransformer can be overloaded during starting and then allowed to cool while the motor is running. Since autotransformers are only in the starting circuit for a short time, an autotransformer can be made smaller than a general-purpose transformer. It does not have to survive a temperature rise caused by high sustained currents. This makes it essential for the technician to know how often a motor can be started in a certain time period. It is often possible that a motor cannot be restarted for a required length of time to allow cooling. A typical duty cycle is 10 secs ON and 10 min OFF.
Autotransformer reduced-voltage motor starting reduces the applied motor voltage to 50%, 65%, or 80% of the line voltage when starting. This is accomplished by placing a transformer coil in series with the motor for a given time period. When this time period elapses, the motor is connected to full line voltage. The appropriate windings of the transformer are connected to the motor circuit to provide the required reduced voltage when starting. The reduced voltage results in reduced current and torque.
Starting autotransformers are generally connected in a wye or open delta configuration. The open delta configuration is used for small motors, and the wye configuration is used for large machines. The open delta configuration provides a simple means of voltage control, especially when more than one starting voltage is required. The wye configuration gives a better voltage balance than the open delta configuration. Transformers connected in an open delta configuration require that the taps be changed on two coils only. Transformers connected in a wye configuration require taps changing on three coils.
The different taps make an autotransformer very versatile by providing the option of different voltages for reducing the current in the stators of large motors (see Figure 3). Low stator current in a motor does not induce a strong pole in the rotor. If the rotor does not start, the next higher voltage can be selected for starting.
Figure 3. Autotransformer taps allow different voltages to be available to a motor depending on the load.
The control circuit consists of an ON-delay timer, TR1, and contactor coils C1, C2, and C3. Pressing the start push-button PB2 energizes the timer, causing instantaneous contacts TR1 in lines 2 and 3 of the line diagram to close. Closing the normally open (NO) timer contacts in line 2 provides memory for timer TR1, while closing NO timer contacts in line 3 completes an electrical path through line 4, energizing contactor coil C2. Energizing coil C2 causes NO contacts C2 in line 5 to close, energizing contactor coil C3. The normally closed (NC) contacts in line 3 also provide electrical interlocking for coil C1 so that they cannot be energized together. The NO contacts of contactor C2 close, connecting the ends of the autotransformers together when coil C2 energizes. When coil C3 energizes, the NO contacts of contactor C3 close and connect the motor through the transformer taps to the power line, starting the motor at reduced inrush current and starting torque. Memory is also provided to coil C3 by contacts C3 in line 6.
After a predetermined time, the ON-delay timer times out, and the NC timer contacts TR1 open in line 4, de-energizing contactor coil C2, and NO timer contacts TR1 close in line 3, energizing coil C1. In addition, NC contacts C1 provide electrical interlock in line 4, and NC contacts C2 in line 3 return to their NC position. The net result of de-energizing C2 and energizing C1 is that the motor is connected to full line voltage.
An autotransformer can be used to provide a slight boost (step-up) or a slight buck (step down) to the line voltage to correct for small overvoltage or under-voltage conditions. The taps may be manually set, or automatic switchgear may be provided to adjust the taps in order to regulate the voltage at the desired value. This use allows changes in the voltage without interruption of the power to the loads and allows for small changes in the voltage to the end-user.
Figure 4. Autotransformers are used with automatic switchgear to make small adjustments to the output of a two-winding transformer.
An autotransformer can be installed in combination with the tap changer to split the percentage of change as the changer is operated (see Figure 4). The position of the tap changer determines how much of the high-voltage winding is in the circuit. As the switches in the tap changer close, varying amounts of the high-voltage winding provide flux for the secondary. The output can be adjusted to compensate for heavy loads. One end of a portion of the high-voltage winding can be connected to the middle of an autotransformer. This method is used to take a 5% tap change and reduce it to a 2.5% change. This provides for closer control of the voltage in the circuit.
Autotransformers are commonly used as variable transformers (see Figure 5). A variable transformer is a continuously adjustable autotransformer consisting of a single layer of wire wound on a toroidal core and a carbon brush that traverses this winding.
Figure 5. Variable autotransformers can be used whenever a variable voltage source is needed.
The brush track is made by removing a portion of the insulation from each turn of the winding, forming a series of commutator elements. The basic principle is that of a tap-changing transformer. The brush is always in contact with one or more wires and continuously taps off any desired fraction of the winding voltage. It is possible to remove the contact under load without interrupting the circuit.