Reduced Voltage Control Systems

Reduced Voltage Controls...

As we begin this topic, we're offering you a couple of articles that are appropriate for our discussion. They are both "white papers" on Reduced Voltage Starters from two major manufacturers. One is from Rockwell International (Allen Bradley) and the second is from Eaton Corporation (Cutler-Hammer). Both are excellent articles and while rather technical, they may help you understand and properly apply an electro-mechanical or solid-state reduced voltage starter in your project. You can click on the appropriate link to open and read the articles.

The title Reduced Voltage Controller is self-explanatory. The device lowers (reduces) the voltage being sent to the motor at startup.

There are several reasons that one might want to lower the starting voltage. One critical issue is to lower the shock loading to which the motor and it's driven equipment is subjected upon starting. Sometimes this value can be very high and damaging to both the motor and the driven equipment. Another reason is to reduce the "inrush current" that occurs during startup.

Electric motors require a significant amount of electrical current to get them going. Just the motor itself requires a lot of power, but when we add the load of the driven equipment, say a heavy flywheel, the inrush current can go "through the roof" and exist for a significant amount of time. But by using "reduced" voltage, the inrush current is much lower and we can adjust the amount of time of starting to allow the motor to come to "near operating speed." An additional benefit is that our electric bill is reduced since the "demand", measured by the utility company, remains lower. For large motors, reduced voltage starting has enormous benefits.

Typical Starting Methods

The most common methods of Reduced Voltage Starting for three-phase motors include:

• Primary-resistance starting, where a resistance unit is connected in series with the stator to reduce the starting current.

• Impedance starting is similar to the primary-resistance type of starting except it uses reactors in series with the motor windings rather than resistors.

• Autotransformer or compensator starting uses extra contactors and a special transformer with multiple taps to allow a selection of starting voltages. Manual or automatic switching between the taps of the autotransformer gives reduced voltage starting.

• Star-delta starting is a very common method of reduced voltage starting. The stator of the motor is star connected for starting and delta connected for running. This requirement means the motor must have specific winding leads brought to the terminal box. A 3-lead motor (for single voltage connection) cannot be used on this type of starting.

• Part-winding starting takes advantage of the fact that the stator windings of the motor are made up of two or more circuits; these individual circuits are connected to the line in series for starting and in parallel for normal operation. This is an additional starting method that will NOT work on 3-lead motors.

• Solid-state motor starters work on nearly ALL three-phase electric motors. The starting currents are adjustable and contain adjustable current sensors for overload protection. This provides for reduced voltage and gradual starting of motors.

Details of Reduced Voltage Controllers

Reduced-voltage, star-delta, and soft starters connect the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.

Common features and reasons for using Reduced Voltage Starters apply to all of the various types, so we'll put that information here, in a general information section.

Reduced Voltage Starters are used to start squirrel cage induction motors in situations where limited torque is required to prevent damage to driven machinery. These starters are also used to limit current inrush to prevent excessive power line disturbances.

It is desirable to limit the starting current to an electric motor in certain cases, such as when the power system does not have the capacity for full-voltage starting. Another case is when full-voltage starting may cause serious line disturbances, such as in lighting circuits, or electronic circuits with the simultaneous starting of many motors, or when the motor is distant from the incoming power supply.

 Special starters are required for very high inertia loads with long acceleration periods or where power companies require that current surges be limited to specific increments at stated intervals. In these situations, reduced voltage starters may be recommended for motors with ratings as small as 5 horsepower.

Reduced voltage starting must be used for driving machinery in which a sudden high starting torque or the shock of sudden acceleration would be detrimental to the load being driven. Among typical applications are those where belt drives may slip or where large gears, fan blades, or couplings may be damaged by sudden starts. An additional case to note is a conveyor, where such initial quick starting would cause the conveyed product to "spill" or cause containers on the line to fall over.

Primary-Resistance Starters

A simple and common method of starting a motor at reduced voltage is used in primary resistor-type starters. In this method, a resistor is connected in series in the lines to the motor. As current flows through the resistors, a voltage drop occurs, and the voltage at the motor terminals is reduced by that amount. Reduced motor starting speed and current are the results. As the motor accelerates, the current through the resistor decreases, reducing the voltage drop and increasing the voltage across the motor terminals. Smooth acceleration is obtained with gradually increasing torque and voltage. In some designs the line resistors are "tapped", allowing the starting voltage to be "adjusted" by the user.

The resistance is disconnected when the motor reaches a certain speed. The motor is then connected to run on full-line voltage. The introduction and removal of resistance in the motor starting circuit may be accomplished manually or automatically.

Automatic primary resistor starters may use one or more than one step of acceleration, depending on the size of the motor being controlled. These starters provide smooth acceleration without the line current surges normally experienced when switching autotransformer types of reduced voltage starters.

Primary resistor starters provide closed transition starting. This means the motor is never disconnected from the line from the moment it's first connected until the motor is operating at full line voltage. This feature may be important in wiring systems sensitive to voltage changes or surges.

Primary resistor starters do consume energy, with the energy being dissipated as heat. Due to this generated heat, the resistors are usually located "outside" the starter cabinet.

Primary-Reactor Starters

The Primary-Reactor Starter is a duplicate of the Primary-Resistor Starter except the resistors are replaced with inductive reactors. So make it easy... read the previous section on the Primary-Resistor Starter and every time you come across the term "Resistor", replace it with "Reactor".

Auto-Transformer Starters

When a three-phase motor is started at reduced voltage by the line-resistance or the line-reactance method, the line and motor currents are, of course, the same. The starting current will be less than that existing with full-voltage starting only to the extent that line resistors or line reactors incur voltage drops. The autotransformer method of starting, on the other hand, supplies the motor with reduced voltage by transformer action, and this implies that the line-side, or primary current, will be reduced during the start sequence just as the secondary voltage is reduced.

Two or more contactors may be used to provide reduced voltage starting of a motor. By using an autotransformer, a lower voltage is present at the motor terminals, reducing starting torque and inrush current. Once the motor has come up to some fraction of its full-load speed, the starter switches, and applies the full voltage to the motor terminals. Since the autotransformer only carries the heavy motor starting current for a few seconds, the devices can be much smaller compared to continuously rated equipment. The transition between reduced and full voltage may be based on elapsed time or triggered when a current sensor shows the motor current has begun to reduce. Autotransformer starters were patented as early as 1908.

Autotransformer starting uses tapped autotransformers in an open delta connection to reduce the motor voltage. Standard voltage taps are at points that yield motor voltages of 50, 65, and 85 percent of full rated value.

The wiring diagram above shows the internal wiring of an autotransformer starting circuit. Note that the START(S1) - STOP(S0) station and its relays and contactor coils are operated from one of the line voltage phases, and a neutral (or second phase of the line voltage). In most cases, this scheme would be replaced by adding a "control transformer", whose primary is wired to two of the phase lines and the secondary (low voltage) feeds the control power to the pushbuttons and relay coils.

Because of the "three contactors" requirement of this method of starting, two of the contactors, START(K2) and RUN(K3), are "mechanically interlocked" in addition to being "electrically interlocked". This adds a layer of phase protection to keep short circuits from occurring should the electrical interlocks fail.

With the pressing of the START(S1) button, the K5(Contactor Relay) and K4(Timing Relay) are energized and sealed-in by a contact on the K5 relay. An instantaneous contact on the K4 timer closes, and energizes contactor K1, the transformer "star" shorting contactor. An auxiliary contact on K1 closes, energizing the K2 (Transformer) contactor, which energizes the transformer and applies starting power to the motor.

During this set of contact closures, the timing relay K4 is running and time is declining. When the timing cycle completes, the "timed contact" closes, energizing the MAIN RUN contactor, K3. Contactor K3 energizes and its auxiliary contact opens the circuit to the START contactor, K2. Because K2 and K3 are "mechanically" interlocked, when K3 operates, K2 de-energizes and opens the starting power to the transformer. Contactor K3 is now energized and supplying FULL LINE VOLTAGE to the motor.

When the STOP(S0) button is pressed, it opens the coil circuit to relays K4 and K5, which in turn opens the coil circuits to all of the contactors, and the motor stops.

Wye-Delta (Star-Delta) Starters

 

First, let's explain the Wye-Delta reality that there are two different starting "methods" within this one type of reduced-voltage starting type.

The first and probably the most common method is the "Open Transition" method of Wye-Delta starting. This requires three (3) contactors to complete the process. When switching from the Wye (starting) to the Delta (running) sequence, power will be completely removed from the leads of the motor and there will be an instance of "coasting" during this no-power period. Without getting too technical, the rotor and the stator of the motor will each produce a magnetic field. These fields will NOT necessarily be in sync. If this is the case when the Delta (run) contactor is closed, these competing magnetic fields will cause havoc with the power line and the windings of the motor. And while this sounds terrible, this is still the most common of the Wye-Delta starters.

The second method of starting is called "Closed Transition". In this design, a fourth (4) contactor is added to the power circuit along with some power resistors. Then, when the switchover occurs, between the Start and Run phases of the process, the fourth contactor is energized and the resistors absorb the damaging current caused by the out of sync magnetic fields. The motor is NEVER without power being applied to its windings, so the effects of the damaging power line disturbances are minimized.

Wye-Delta Control and Power Diagram

 

 

Operation & Workings of an Automatic Wye-Delta Starter

In the diagram above, we see that the control power is connected to the line power from phase B and C. In most cases, companies will insert a control power transformer in this configuration, to reduce the control power from line voltage to 120VAC or 24VAC as a safety measure.

The operation of the starter is described as follows:

The operator presses the START button. Power will then flow through the closed contact of the STOP button, through the (now) closed contact of the START button, through the coil of the MAIN contactor, and the closed contact of the overload relay. This energizes the MAIN contactor and it closes it's power contacts to three of the leads of the motor. When the MAIN contactor closes, it's auxiliary contact (M-1) closes in parallel with the contact of the START button which "seals in" the circuit of the main contactor, and the operator may now remove his/her finger from the momentary contact START button.

During this sequence, a control circuit is also completed to the coil of TIMER (TD1). When the coil energizes it begins it's timing cycle. At the same instance, a circuit is completed through a closed timed contact (TD1-2) of the timer, and a normally closed auxiliary contact (R-1) of the RUN (R) contactor, to the coil of the START (S) contactor. Through that circuit, the START contactor is energized, it's power poles close, and the windings of the motor are connected in a Wye configuration to the power lines. The motor STARTS!

Having started the timing cycle, when the START button was pressed, it has been running during the acceleration of the motor starting process. After it completes the cycle, usually between 5 to 10 seconds, (depending on load characteristics), the timing contacts TD1-1 and TD1-2 change state. TD1-2 opens and removes the circuit to the coil of the START (S) contactor which causes it to open the power circuit to the connected leads of the motor (the Wye connection). The motor is now without power and will "coast". At the same time, however, timer contact TD1-1 has closed and completed a circuit to the coil of the RUN (R) contactor. This energizes the RUN contactor causing it's power poles to close and connect the motor to the power line in a Delta configuration. The motor start sequence is now complete and the motor is running at full power.

Please note in the schematic that the circuits to the coils of the RUN and START contactors are through "normally closed" auxiliary contacts on the opposing contactor. This is a safety to keep them from being energized simultaneously. In some manufacturer's designs, these two contactors are also "mechanically" interlocked as an extra precaution.

Advantages...

Based on the number and types of components required to assemble this starter, it is somewhat less expensive than some other types, like the auto-transformer, or electronic solid-state starter as examples.

A Wye-Delta starter will reduce the inrush (starting) current required to one third (1/3 - 33%) of the "locked rotor current" (LRA). Additionally, the voltage is reduced (based on the difference between a wye and delta connection in the motor) to approximately 58% of the voltage applied, and finally, the torque output of the motor will also be reduced to 33% of full load torque. While these are considered "advantages" of this type of reduced-voltage starting, they can also be a detriment, as well.

Disadvantages...

As stated above, while we consider the lower inrush current (33%), the lower voltage (58%) and lower starting torque (33%) advantages, we must also keep in mind that some loads simply cannot be started with these lower values. For instance, if a specific load requires at least 50% of the motor's full-load torque, we simply won't be able to start it with a Wye-Delta starter.

One must also realize that with this type of starter the MOTOR must have the winding ends brought out to the terminal box so the proper start and run connections can be made "externally". In many industrial plants, the maintenance personnel prefers their motors to be 3-lead motors to minimize connection errors by technicians.

We mentioned above that most "switch-over" timing relays are in the 5 to 10-second range. This is usually sufficient time to get the load rolling and up to "near operating" speed. However, if the time is too short and the motor does NOT reach at least 90% of rated speed at the time of "switch-over", the current spike may be as high or higher than FULL LOCKED ROTOR AMPS, thus causing issues with burned power contacts in the contactors and basically no advantage to the power company's demand criteria.

Part Winding Reduced Voltage Starters

The circuit for a Part Winding Starter is rather simple since there are only 2 contactors involved.

In the diagram above the sequence of operation is as follows:

When the START button is pressed, the START (S) contactor is energized, and power is fed to the winding of the motor connected to terminals T1, T2, and T3. Because the coil of the timing relay TR is connected across two of the phases, this timer coil energizes. By energizing this relay, the "timed contact", TR, in the control scheme OPENS thus preventing the RUN contactor from operating. Operation of the TR timing relay and the START (S) contactor also closes auxiliary contacts across the START button, thus "sealing" it in and allowing the operator to remove their finger from the start button. This completes the "STARTING" cycle and the motor energizes on "part" of its windings.

After the timing cycle completes, the TR (timed) contact closes and the RUN contactor energizes. When it energizes, it closes the power lines to the motor winding connected to terminals T7, T8, and T9. This action places the two windings of the motor in PARALLEL and the motor is now running with the full line voltage applied to the complete stator winding.

Note that there are two sets of overload relays in the system, each protecting a different set of phase windings within the motor.

Electronic Reduced Voltage (Soft Start) Controllers

     

Electronic Reduced Voltage Starters (Soft Starts) are becoming more popular in today's industry. One of the benefits, of course, is that there are no moving parts. No mechanical items to "wear out". And they are pretty much all in one small package.

The only external items that are needed are the pilot devices like the Start and Stop buttons. The overload relay is part of the electronic circuitry, as well as the timing relays, and power devices. The device is one compact unit, ready to "Soft Start" your electric motor with reduced voltage.

And so many features are "programmable" too. Like acceleration and deceleration time. In some of our automatic electro-mechanical reduced voltage starters, we used a timing relay to control the time between transitions. In this electronic design, these timers are built right into the control board. They are adjustable through the programming language and a small HMI (Human Machine Interface) device that accompanies each starter.

So how do they work...? LIKE MAGIC!!! Well, not really but pretty close. The 3-phase line voltage is fed to the incoming line terminals of the starter. That needs to come from a circuit protective device like a circuit breaker or switch and fuses, to protect the line. But once power is applied to the line side of the starter, it is fed to a set of solid-state diodes that converts the AC power to DC power. The DC power is then manipulated by way of the control board to the output SCR's (Silicon Controlled Rectifiers) or Power Transistors. These output devices are triggered by the control board in such a manner as to "re-create" a sinusoidal waveform that replicates an AC Sine Wave.

When the control board triggers these power devices to turn on or off, it has allowed the voltage to increase to a specific amount at each infinite step in the sine wave. This is how the voltage is increased or decreased in the starter. So the increase in voltage is totally "step-less" and very smooth. Since we can control the acceleration time, motor loads can be controlled very evenly. And the same is true when the motor is asked to stop... we can adjust the deceleration time to whatever time frame suits the application.

The overload system is handled by electronic overload sensors that monitor the current going to the motor, just like the mechanical overload. Except, the electronic device is more accurate and reacts quicker to overload than the heater of the mechanical overload. And sensing of a "single-phase" condition is quicker and more accurate, too.

Another feature of this starter is the "voltage boost" or starting voltage at the low end. If we have a load that is particularly hard to get rolling, it's possible to add some additional voltage at the starting point to get things moving. Normally we would see the voltage start at ZERO (0) volts and accelerate to the full voltage over the desired time frame. But instead of starting our motor at ZERO volts, we can adjust the starting voltage to some other value, like 150VAC (or any other voltage) on a 480VAC system. This will help get that "hard to start load", rolling.

A final note on the Solid State Reduced Voltage Starter is something that has become more commonplace in recent years. During the acceleration of the load, the voltage is built from ZERO to full voltage (say 480VAC) over time. Once it reaches full voltage, we no longer need the solid-state power devices in the circuit. And while they are extremely efficient, there is some loss within the package. So more recent designs have those power devices being removed from the power circuit and the motor is connected directly across the full voltage line of the power system. Sometimes this is done with a contactor and sometimes electronically. Either way, the power devices are removed from the circuit and additional electrical efficiency is realized with no heat-generating loss.

Electronic Variable Speed Drive (Inherent Feature)

  

The "AC Variable Speed Drive" is NOT a "true" reduced voltage starter. But how it operates meets the criteria of a reduced voltage starter and the outcome is the same. So we've added it to this topic as the final type of reduced voltage starter.

An AC Variable Speed Drive (AKA: VFD or Inverter) is used for controlling the speed of an AC Induction motor throughout it's running cycle, not just during the starting process. VFD stands for "Variable Frequency Drive" because that's what the electronics are doing, varying the frequency.

In a VFD the AC incoming power is first sent to a full-wave rectifier bridge that converts the power to DC. The DC is then manipulated by the control board electronic circuits to recreate an AC Sine Wave. Does this sound familiar? It should because that's exactly what the Electronic Solid State Reduced Voltage Starter we talked about earlier does! But in this case, the electronics circuit creates a sine wave with varying "frequency", not just a varying voltage. We begin with an incoming AC Power Line that is "probably" 60 Hz (maybe 50 Hz outside the USA), and after converting to DC it is reformed into a "manufactured" frequency from ZERO to 60 Hz, or higher, that through the control board is "totally adjustable".

Because there is a correlation between frequency and voltage, the newly created sine wave has adjustable frequency AND voltage. Due to the internal resistance of the motor windings, and other considerations, it is necessary for the voltage to be varied as the frequency changes. A motor designed to operate at 60 Hz and 480VAC, will overheat if the power being fed to the motor is 480VAC at 15 Hz. So the control board has a "defined" ratio between the frequency and voltage, referred to as "volts/hz" (Volts per Hertz). Using the 480VAC and 60 Hz as our example, we end up with a V/Hz ratio of 460/60, or 8 Volts per Hertz. Theoretically then, as the drive gets energized and begins its acceleration, the frequency starts at ZERO and increases over time to the full frequency of 60 Hz. Using our 8 V/Hz constant, when the drive is supplying 30 Hz to the motor, the voltage is 8 V/Hz x 30 Hz or 240 VAC. This allows the motor to increase in speed with a frequency that matches the voltage being applied.

An Adjustable Frequency Drive has the same, and more, features as the reduced voltage starter. Such as acceleration and deceleration ramps, voltage "boost" during starting, electronic overload protection, connections to pilot devices to start and stop the motor, and other appropriate and useful additions. So the VFD, while not considered a true "Reduced Voltage Starter", does, in fact, do all the same things, and adds some additional useful features.

So talk to the folks at A.R.&E. when you need to apply a "Reduced Voltage Starter" or if you have a question about their application. We'd be happy to discuss what's best for your application and help you put the package together.