Power control circuit pdf

M The electricity flows from L1, through the Stop NC , through the path completed when contact M closed, to the coil, through the overload relay control circuit, to L2 Figure 9. Figure L1 L2 There is sometimes confusion with overload relays that have an Automatic Reset capability. Electrical Path is through Contact M is still open. This type of cir- mally Closed NC contact to open the circuit, de-energize cuitry must be used only on applications where personnel the coil, and drop out contact M.

Releasing the STOP button safety is not an issue. If there is a power outage or loss, the coil is de-energized, contact M opens, and the starter drops the load off line. Magnetic field from primary winding induces voltage in the secondary winding. The motor branch circuit is usually a segment of a larger electrical distribution network in an industrial plant. The Applying AC voltage to the primary winding of the trans- motor circuit supplies the required power to the various con- former causes alternating current to flow in the winding.

In some cases, the This produces a magnetic field that extends outside the various control devices are operated at the same voltage as winding, in the shape of concentric loops as shown in the motor. Sometimes, the voltage required to operate the Figure The magnetic field fluctuates as the AC changes motor is too high to safely operate the control circuit, partic- direction. These magnetic lines cut across the conductors of ularly in regards to personnel safety.

As a means of reducing the secondary winding and induce a voltage. A typical control transformer is shown in Figure 11 below. It consists of two separate coils of wire windings placed adja- cent to each other on a common iron core. Note that the pri- mary winding is connected to the power source. The secondary winding is connected to the control circuit. The purpose of the transformer is to transfer electric power from AC Power Source Load the primary circuit to the secondary circuit.

The transformer either reduces steps down or increases steps up the volt- age to match the requirements of the control circuit. Voltage Induced in Secondary Winding Figure AC Flow in Windings Voltages based on number of turns on both windings.

The relationship of the voltage across the primary to the voltage across the secondary is in direct proportion to the number of turns on both windings. For example, turns Primary Winding Secondary Winding on the primary, and 10 turns on the secondary, is a 10 to 1 Connected to Connected to Load ratio.

If the primary is volts, we will get 50 volts at the AC Power Source secondary. This is referred to as a Step-Down transformer. They are most commonly used in control circuits where the motor voltage is V, V, or higher. The step-down con- trol transformer would reduce the voltage to pushbuttons or PLCs to V or even 24V. Iron Core A transformer with the reverse proportion of more turns on Figure Typical Control Transformer the secondary winding than on the primary is called a Step- Up transformer.

It will increase the voltage according to the ratio of turns. These represent X1 X2 the primary and the secondary windings. The secondary winding is con- G X2 X1 2 nected to the control circuit. When you are installing a Control Power Transformer into a starter, you must be sure that the magnet coil is rated for the same voltage as the secondary of the transformer.

Note: Connections for Dual In addition, any pilot lights in this circuit must have the same Voltage Rated Transformer- voltage as the secondary. CPT Dual Voltage units shipped with connections made for the higher voltage. When you are using a Control Power Transformer with a dual voltage primary, check the transformer connections to To Terminal L1 To Terminal L2 be sure that they match the voltage of your power source.

L1 X2 X1 L2 on the Starter. Remote Pilot Device. CPT Wiring Diagram must remove it. This will convert the starter from Common Control to Separate Control. The leads from the primary of the transformer are connected to L1 and L2 on the starter. When using a reversing magnetic starter, this task is Mechanically Interlocked accomplished using two contactors, one overload relay, and F OL a mechanical interlock.

The control circuit is designed to pro- vide an electrical interlock as well. R Both the mechanical and electrical interlocking prevent the T1 F OL T2 simultaneous operation of the forward and reverse contactors.

Figure 15 shows the wiring diagram for a full-voltage mag- R T3 netic reversing starter. Control Lines to the No. All of the contacts return to their normal state. Reversing Starter Wiring Diagram R to complete the circuit to the motor. Power can flow through NC contact F, since coil F is not energized.

When coil Mechanical Interlock prevents simultaneous operation. R is energized, NO contact R acts as the seal-in contact. To keep the forward and reverse contacts from operating In this control circuit, the STOP button must be operated simultaneously, a mechanical device is used. This device mounts in between the two contactors and phys- The power circuit connections reverse the rotation of the ically prevents the power contacts from operating at that motor.

Refer back to the wiring diagram Figure Trace the Control circuitry provides an electrical interlock to prevent power circuit lines connected to Coil R. A connection at L1 is traced through the power contacts, and Additionally, the control circuits are wired in such a way as connected to T3, through the overload relay and to the motor. This is done with the use of Normally Closed NC auxiliary contacts. The connection at L3 is traced through to be connected at T1.

As the coil of one direction contactor is energized, its associ- L2 is connected to T2 as it would be in a non-reversing starter. Figure 16 shows this circuitry. When the motor is off, the light is on — when the motor is We will be reviewing many basic control circuits. We will on, the light is off. When wiring this circuit on enclosed con- look at both the wiring diagram and the line diagram, and trollers, remember that a NC auxiliary contact is needed in discuss their operation.

In some cases, alternative circuits addition to the standard NO auxiliary contact. Pilot Many of the circuits have their basis in the Three-Wire 1 Light 2 circuit. We saw that using a maintained contact pilot device Rear gave us automatic control.

Then we looked at a three-wire Start circuit using momentary contacts and an auxiliary contact to OL hold the circuit closed. X2 One very common variation to this circuit is the addition of a T1 T2 T3 pilot light. In Figure 17 the pilot light will be on when the motor is running. Power is also supplied to the Pilot light. When the coil is energized, holding contact M 1 2 M 3 Pilot Light closes, and allows power to continue to pass to the indicat- M ing light.

As long as the motor is on, the pilot light will be G 4 energized. It can be accomplished using either a three- position selector switch, or three maintained pushbuttons. Figure 18 shows this circuit.

An additional auxiliary contact on the starter is what 2 3 95 96 operates the pilot light. Notice that it is an NC contact.

When the coil is energized, this contact will open. At that point, the Figure When the starter is not energized, this TDE For more information visit: www. If the contacts on the remote device are closed, the Auto 2 Light position allows the coil to energize.

L1 L2 L3 ever the remote device is closed. In Figure 20 we show this same circuit, but with the addition of two pilots to indicate the direction in which the REV. The 95 Motor 8 electrical interlock of the circuit. Additionally, the power is 8 also passed through the circuit to Pilot Light FOR indicating that the motor is in the forward mode of operation. Diagrams, publication number TDE. This publication is available to you at no charge from your local sales office.

Also known as Full-Voltage. Normally Open: The position of a set of contacts that are Auxiliary Contact: Contact of a switching device within the open separated when the device has no electricity applied control circuit, and operated when the magnet coil is to it.

Open Circuit: A circuit without a complete path for the elec- Bimetal: Two different metals bonded together to provide tricity to flow through. Used in thermal overload relays. Overload Relay: A device designed to protect a motor by sensing the heat from excessive amperage draw of a Common Control: Electricity to both Power circuit and Con- jammed or overworked motor, and dropping the devices off- trol circuit are supplied from the same voltage source.

See line open circuit before damage occurs. Often uses either a Page 4. Contactor: A device to repeatedly establish or interrupt an electric power circuit. Parallel Circuit: An electrical circuit that has more than one path for current flow. Connected rungs on a ladder Control Circuit: A circuit that controls the magnet coil of a diagram. See Page 4. Plugging: Braking of a motor by reverse line voltage or phase sequence. Motor develops retarding force.

The electri- Control Power Transformer: A device that increases or cal path that passes electricity to the motor enabling it to decreases voltage to a desired level to operate the control run. See Page 8.

Reduced Voltage Starter: Applies a reduced voltage or cur- De-energize: To disconnect a component or circuit from the rent supply to the motor during starting to avoid excessive power source. In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself.

Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor. A sharp and fast rise in current over a short period of time fractions of a second where the value of current is far greater than the nominal line current.

A set of contacts that has been designed to make or break the flow of current to a motor and can handle loads up to a specified amount of horsepower at a specific voltage. The size of contacts are determined by the size of the motor they are controlling. An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.

A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection. An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.

In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load.

The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation. With respect to magnetic contactors, the armature or plunger is the movable part of the magnetic circuit. A heater element paired with normally-closed contacts that open once the heater gets too hot.

Two types of relays are the bimetallic strip and the melting solder pot. A contact that under normal conditions has continuity through it. When the contact changes its state it interrupts the flow of current by opening its contacts.

Can be associated with pushbuttons, pilot devices or magnetic contactors. Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage. Examples include the microwave or power tools.