Electric actuators allow valves to be remotely operated and, by use of a motor, allow rapid valve operation that otherwise would not be feasible.
This section deals with the control circuit and operation of the actuator in the motor mode. Although there are several power sources used in actuators, this section will be directed strictly to the use of electric motors.
When the close direction switch (close switch) is closed, the closing coil of the Reversing Starter is energized. This will close the main line (motor leads) "CL" contacts to start the motor in the close direction, close the "CL" contact around the close switch (seal-in contact) and open the "CL" contact (electrical interlock contact) in series with the open coil. The actuator will continue to position the valve in the close direction until the torque and/or limit switch detect binding or full stem travel, the respective contact will then open deenergizing the close coil which results in the main line contacts opening, the seal-in contact opening and the electrical interlock contact shutting.
The actuator can then be operated in the open direction in the same manner as described above. In the mid-position, the actuator can be operated in either direction.
Power Supply:
The function of the power supply is to supply the energy required to operate the valve to which the actuator is attached. Although there are many sources of power, the assumption is made that 240 Volt Alternating Current (VAC), which is a typical selection, is the power source.
Motor:
The function of the motor is to convert electrical power to mechanical power. Reversing any two of the three leads to the motor will result in a change in the direction of rotation. The typical motor on Limitorque actuators is limited to a 15 minute duty cycle which must be considered when performing maintenance.
Overload Heater Coils:
Overload heater coils (thermal overload relays) are a form of protection in the event of excessive motor current. Care must be exercised when sizing the heaters due to their time delay.
Reversing Starter:
Reversing starters have two separate functions: 1) to interchange power leads which change the direction of rotation, and 2) to provide mechanical and electrical safety interlocks that prevent the contacts for both directions being closed at the same time which would cause a direct short between phases. The operation of the reversing starter is based on using a small control current to control the larger motor current through electromagnetic switching. The coils shown in Figure 5-1 operate the main contacts of the starter when an open or close pushbutton is pushed. In addition, the seal-in contacts and contacts labeled "CL" and "O" are operated by the same coils in the reversing starter.
Control Transformer:
The function of the control transformer is to reduce the control voltage to a lower and safer level. Normally, the primary windings of the transformer is connected to two phases of the motor power. The secondary windings provide the control voltage as single phase, normally 115 VAC. The primary side may have two fuses for protection, while the secondary side normally has one.
Stop Pushbutton(s):
The stop pushbutton(s) are always functional and are wired in series in the control circuit so that an operation by any one of them will open the circuit, which causes starter drop out and halts actuator operation. They are a normally closed momentary open contact which de-energizes part of the circuit when pushed. Stop pushbuttons are usually located at each of the operating stations and locally at the actuator.
Open and Close Pushbuttons:
The function of the open and close pushbuttons is to initiate operation of the control circuit, which will result in energizing the actuator motor. In typical applications, there are two sets installed - one at the valve (local) and one in the control room (remote). In some systems, only one set of open and close pushbuttons is energized at a time. See REMOTE/LOCAL switch.
Contactors Auxiliary Contacts:
The function of seal-in contacts is to allow the person operating the actuator to release the open and close pushbuttons without having the actuator stop. This allows electric controls to stop the operation automatically at a pre-set condition without operator intervention. As an example, once energized, the actuator can be stopped by either the torque switch or limit switch depending on set-up of the valve-actuator. The seal-in contacts are labeled "O" and "CL" in Figure 10-1 and are in parallel with either the open or closed switch.
Remote/Local Swith:
This is a selector switch which determines the location of control for the actuator. If the remote location is selected, the local control pushbuttons will not work.
Overload Contacts:
The function of the overload contacts is to protect the circuit from an overload condition by interrupting the control circuit. The contacts are an integral part of the sensing heaters, normally 1 per phase. Great care must be exercised when working on equipment which is protected with heaters which reset automatically when they cool. Some nuclear plants do not have protective heaters and some have them only when the actuator operates in a certain direction.
Electrical Interlock Contacts:
The electrical interlock contacts, (contactor auxiliary contacts), prevent both the open and close contactors from operating at the same time. If the relay is protected by mechanical means, the electrical interlocks operate as a backup. The contacts are normally CLOSED contacts that open when the associated contactor operates. The open auxiliary contact is wired into the close circuit, and the close auxiliary contact is wired into the open circuit. They are labeled "O" and "CL" in Figure 10-1 and are in series with either the open or closed coils of the reversing starter.
Lights:
The lights' functions are to give approximate valve position information and as a useful tool for troubleshooting the actuator. The lights indicate the point where a particular rotor operates when activated by the limit switch, and is the same point where associated actions, if any, should be activated by contacts on the same rotor. Normal operation has the open rotor turning off the red closed light and the close rotor turning off the green open light, with both lights on between the open and closed position. The contacts are normally aligned with the motor contacts on the rotor and are 90 degrees off from the spare and torque switch bypass contacts. In actuators where the functions are divided by the use of additional rotors, (4 train limit switches) the lights may not function at the same time as the rest of the contacts. The lights may be driven by relays or actuated by external switches on the valve. There are many different control circuit arrangements.
Limit Switches:
There exists two (2) types of limit switches in electric actuators, 1. Position Limit Switches, 2. Torque Limit Switches.
1. Position Limit Switches:
The function of the geared limit switch is to count turns of the drive sleeve in order to keep track of valve position, to shut off power to the actuator motor at the proper stroke position, to turn indicating lights on and off at the proper positions, and provide interlocks, etc, as required. The limit switch is a relative mechanism and proper operating points must be set to match any desired valve positions.
The rotors are a form of drum switch with 4 contacts each. Normally 2 make and 2 break, although other Vendors will provide other configurations. There is usually one rotor for the open position which trips at the full open position, and one rotor for the close position which trips at the full close position. The finger base is where the wiring is connected to the limit switch.
The indication contacts operate opposite from their expected manner. The indication contact on the close rotor turns off the open light when the valve is closed, and the indication contact on the open rotor turns off the close light when the valve is open. Both lights are on during in between open and closed travel.
The interlock, or spare, contact is located above the indication contact and is 90 degrees out of phase with the motor and light contact. When the motor and light contacts are CLOSED, the spare contact is OPEN.
The purpose of the torque switch bypass contact is to act as a bypass around the torque switch FOR THE OPPOSITE DIRECTION to allow for high starting torque situations. The open rotor has the close torque switch bypass contact and the close rotor has the open torque switch bypass contact. These contacts allow a valve to be opened or closed when there is a high pressure across the valve, thermal expansion of the stem, or some other requirement.
The torque switch bypass is normally set by percentage of valve stroke. For example, if the drive sleeve of a valve turns 100 turns from full open to full closed, then 10% bypass will be ten drive sleeve turns from the open or closed position. The bypass is normally percentage-based for the shut seat only, because of pressure differentials across the valve adding to the torque load for opening the valve. The problem with adding any percentage bypass when using a two train limit switch is you have to trip the associated rotor early. Example: the torque switch is bypassed for 10 percent of valve stroke when opening a valve, the close rotor has to trip at 90 percent of the closing stroke. This requires the motor contact on the close rotor to be jumpered and have the torque switch shut off the actuator, plus you get a closed light indication at 90 percent closed instead of 100 percent. For this reason, many plants have gone to 4-train limit switches where the light indication, or the bypass, is placed on a different rotor.
2. Torque Limit Switch:
The torque switch used has two possible functions. The first, on torque closed valves, is to ensure that the valve has sufficient and accurate thrust on the valve stem to guarantee seating. The second, on limit controlled valve operations, is to ensure that the actuator and valve are protected from possible excessive thrust. The double-contact torque switch, with one set of contacts being for the open direction and the other being for the close direction, is normally used.
The torque switch is operated by the axial motion of the worm in both directions. The contacts on the torque switch are double break contacts and are not self-wiping, which can lead to continuity problems in some actuators because of an oil film forming on the contacts.
Torque switches are provided with limiter plates which limit settings to a safe value, and prevents overtorquing a valve in case the setting screws come loose. There is no overtorque protection from incorrectly set or installed torque and limit switches.
The torque switch performs as though it senses torque, although it is simply sensing how far the worm shifts on the wormshaft, which depends on the resistance of the drive sleeve to turning. When the torque or thrust reaches a preset amount, the torque switch will open. The torque switch doesn't care how the torque and thrust forces are applied and used. If packing is too tight, if valve components are blocking the stem during the valve stroke, or if the stem threads are damaged, the torque switch will still operate at the set amount of torque.
Testing after Maintenance:
One of the most critical steps after performing maintenance is the initial operation and follow-up testing. Many newly rebuilt actuators, and/or associated valves have been damaged the first time they are run after having been repaired because of mistakes in the testing process.
An actuator should never be run until the limit switches and direction of rotation of the motor have been checked. The limit switch functions should be checked in manual by positioning the valve at the desired positions and observing that the switches operate. Something as simple as leaving the set rod screwed in at the wrong time can cause major damage, and the only way to verify this is by manually checking.
The most important operational check is the direction of rotation test. If the motor runs the actuator in the wrong direction for the full stroke, the only protection will be the motor overloads. None of the other designed-in protection will protect the actuator due to the operating protection being on the wrong side of the control circuit.
The most effective steps for testing after maintenance are:
1. Manually open the valve and verify the open limit switches.
2. Manually close the valve and check the close limit switches. Observe any binding or resistance to operation.
3. (The above two can be reversed depending on starting position.)
4. Manually mid-position the valve for the first electrical operation. If there are problems with the settings, the valve will not immediately be in a position which could cause damage.
5. Energize the actuator and verify the indication is correct for mid-position. This will indicate that the limit switches are in an expected position.
6. Prepare for the first electrical run by placing your hand physically on the STOP control, so that the actuator can be stopped immediately if a problem develops.
7. Check the direction of rotation. Press the CLOSE button and verify that the actuator rotates in the correct direction, then stop the actuator. The easiest way to verify the direction of rotation is to compare drive sleeve rotation by motor to drive sleeve rotation in manual after checking the arrow on the handwheel for proper rotation.
8. If the direction of rotation is incorrect, stop the actuator IMMEDIATELY and reverse two of the motor leads for opposite rotation. If the actuator is allowed to run to the point where the limit or torque switch should stop the operation, the actuator will continue to run because the protective and controlling features are on the wrong side of the control circuit.
9. If the direction of rotation is correct, start the actuator in the close direction. Be ready to stop the actuator if the motor begins to sound like the load is increasing beyond an acceptable point, otherwise, let the motor run until the limit or torque switch stops the motor.
10. Verify that the shut indication is correct. If not, the limit switch is out of adjustment.
11. Verify that the valve is fully closed by placing it in manual and turning the handwheel in the close direction. If it is not, the limit or torque switch will need to be adjusted.
12. Operate the actuator in the open direction. Be ready to stop the actuator if the motor begins to sound like the load is increasing beyond an acceptable point, otherwise, let the motor run until the limit or torque switch stops the motor.
13. Verify that the open indication is correct.
14. Verify that the valve is in the correct open position by placing it in manual and checking in the open direction.
15. Operate the actuator from open to closed as necessary to verify that everything is functioning.
16. Place the actuator in manual. Note any problems in going from electric operation to manual. Operate electrically. Verify the operation of the declutch components in going from manual to electric. This will insure that the actuator is ready to be released for unrestricted use.
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