How the power circuit and ladder diagam work together
Single-phase and 3-phase AC squirrel cage induction motors need some type of control circuit to initiate a start or stop function. The two types of ladder control circuits commonly used are the 2-wire control circuit and the 3-wire control circuit. The 2-wire control circuit uses maintained contact devices to control the magnetic motor starter. The 3-wire control circuit uses momentary contact devices to control the magnetic motor starter.
A typical 2-wire control circuit is shown in Fig. 1 (click here to see Fig. 1). It consists of a normally open maintained contact device that, when closed, energizes the coil of a magnetic motor starter, which, in turn, energizes the connected motor load. The 2-wire control circuit provides what is known as “low-voltage release.” In the event of a power failure, the magnetic motor starter will drop out. Once power is restored, the magnetic motor starter will automatically re-energize, provided that none of the maintained contact devices have changed state. This can be very advantageous in applications such as refrigeration or air conditioning where you do not need someone to restart the equipment after a power failure. However, it can be extremely dangerous in applications where equipment starts automatically, placing the operator in danger.
A typical 3-wire control circuit is shown in Fig. 2 (click here to see Fig. 2). It consists of a normally closed stop button (STOP), a normally open start button (START), a sealing contact (M), and the coil of a magnetic motor starter. When the normally open start button is pressed, the coil of the magnetic motor starter is energized. An auxiliary contact seals around the start button to provide a latched circuit. Pressing the normally closed stop button disrupts the circuit. The 3-wire control circuit provides what is known as “low-voltage protection.” In the event of a power failure, the magnetic motor starter will drop out. In this case, however, once power is restored the magnetic motor starter will not automatically re-energize. The operator must press the start button to initiate the sequence of operations once again.
Compared to the 2-wire control circuit, the 3-wire control circuit provides much more safety to the operator because machinery will not automatically start once power has been restored. Figure 3 (click here to see Fig. 3) illustrates a control circuit with multiple start and stop push buttons. In this circuit, multiple normally closed stop buttons are placed in series, and multiple normally open start buttons are placed in parallel to operate a magnetic motor starter. This is a common application of a 3-wire control circuit in which you need to start and stop the same motor from multiple locations within the facility. The 3-wire control circuit can be used in a variety of ways to meet specific circuit application.
Typically, single-phase motors can be started with full voltage across the line. Smaller horsepower 3-phase motors can also be started with full voltage across the line. However, larger horsepower 3-phase motors require you to use reduced voltage starting techniques.
The power circuit used in full voltage across-the-line starting consists of the overcurrent protective device (OCPD); the line conductors that terminate on the L1, L2, and L3 terminals; the magnetic motor starter or solid-state device; and the load conductors that terminate on the T1, T2, and T3 terminals. The power circuit is sized according to the voltage rating of the motor load (i.e., 115V, 200V, 230V, 460V, or 575V). The control circuit can operate at the same voltage as the power circuit as well as at lower voltages by using a machine tool transformer to step down the voltage to lower levels.
The electric utility typically has rules in place for how large a motor you can start across the line. Once the horsepower of a motor exceeds that rating, reduced voltage starting techniques must be used. Motors are inductive loads; therefore, they have very high starting currents in the range of 2.5 to 10 times the full load running current of the motor. This excessive inrush current (also called locked rotor current) causes voltage fluctuations on the power lines. You probably have observed the effect of inrush current whenever the lights in a building dip when an HVAC piece of equipment comes online. When this excessive inrush current is drawn from the voltage source for a few seconds, it causes a voltage drop. This voltage drop means a lower voltage is available to equipment, causing lighting fixtures, in particular, to reduce their light output.
There are primarily six styles of reduced voltage starters: primary resistor, reactor, autotransformer, part winding, wye-delta, and solid-state.
Primary resistor starters use resistors in series with the motor leads during the start function. Because this is now a series circuit, the applied voltage between the series resistor and the motor winding drops, causing a lower starting current. A timing relay operates a control relay whose contacts short the series resistors once startup is achieved.
Reactor starters operate in the same manner except reactors are used instead of resistors. The use of reactor-type starters are far less common today than in the past.
Autotransformer starters rely on tapped autotransformers for operation. The taps are typically set at 50%, 65%, or 80% of the available line voltage. Relying on the concept of “turns ratio” in a transformer, this type of starter allows for smaller currents on the line side as seen by the electric utility and larger currents on the load side as seen by the motor during startup. An autotransformer is different from a 2-winding transformer in that it does not provide electrical isolation between the primary and secondary windings. A step-up autotransformer is known as a “boosting” transformer, and a step-down autotransformer is referred to as a “bucking” transformer.
Do you recall the concept of turns ratio for a transformer? When looking at voltage, you rely on the following formula:
(Vprimary ÷ Vsecondary) = (Nprimary ÷ Nsecondary).
When looking at current, you rely on the following formula:
(Iprimary ÷ Isecondary) = (Nsecondary ÷ Nprimary).
Let's review a simple example for illustration of this concept. A 1kVA transformer has a 240V primary and a 120V secondary voltage rating. The primary current is 4.17A at 240V, and the secondary current is 8.33A at 120V. By simply plugging in the values to the aforementioned equation, you can easily see the transformer has a 2:1 turns ratio. What does this mean? It simply means the voltage is stepped down by a factor of two while the current is stepped up by a factor of two. This principle allows the autotransformer-type starter to operate.
The part-winding starter is designed to work with a part-winding motor, which features two identical sets of windings. You can use 230V/460V dual-voltage motors, but you must do so with extreme caution. The concept is that a 230V/460V motor operated at 230V does so with its windings in parallel. Therefore, one half of the motor windings are in the circuit during startup. Then, a few seconds later, the other half of the motor windings are brought into the circuit. Serious problems can develop if the timing circuit does not connect the other half of the motor windings immediately after startup. For example, if the control circuit does not connect the delta windings of the motor back together after startup, the motor will fail.
A wye-delta starter operates by allowing the motor to be started in a wye configuration and run in a delta configuration. Using this design configuration allows the inrush current to be lower during startup while still maintaining a starting torque of approximately 33%, a percent rating of rated torque of the motor during startup. Open transition is an important concept to keep in mind with wye-delta starters because there will be a period of time between the wye configuration for start and the delta configuration for run when the motor windings will be disconnected. Closed transition starters overcome this disadvantage but at a much higher cost.
Solid-state starters are often called “soft start” starters because they rely on silicon-controlled rectifiers (SCRs) to accomplish the starting task. The SCR has three elements: anode, cathode, and gate. By applying a signal to the gate element at precisely the right time, you can control how much current the SCR will either pass or block during a cycle. This is known as phase control. The ability of this device to allow either partial conduction or full conduction during a cycle offers much flexibility to the design engineer. This capability allows for precise control of current to a motor during startup. Solid-state reduced voltage starters are in common use today because they interface well with variable-frequency drives (VFDs) and programmable logic controllers (PLCs).
Vidal is president of Joseph J. Vidal & Sons, Inc., Throop, Pa.