Part 18(d)—Electrical Systems and Control, Part 4
By Ed Butts, PE, CPI
April’s installment of The Water Works introduced the concept of starting an electrical motor with an overview of the
basic terminology and methods for full voltage starting. We will wrap up the topic this month with a discussion on reduced voltage starting methods.
Also please note since this is a continuation of a topic we started with April’s column (and its Figure 1 and Tables 1, 2,
and 3), the figure and table numbers for this column follow in order with Figure 2 and Table 4.
Reduced Voltage Starting
Although a voltage dip often causes various problems, a controlled and momentary reduction in voltage at the motor
terminals can be useful and beneficial to generator or electrical service sizing, but only when an associated reduction in motor torque is acceptable.
Since the available motor torque from a reduced voltage starter generally exceeds the torque required to start and accelerate a centrifugal pump at rest, this is often an acceptable method of motor starting. Reducing the motor starting kVA can also reduce the required size of the genset, reduce the voltage dip, and provide a softer start for both the motor and its load—an important consideration on a deep well vertical turbine (lineshaft) pump.
When sizing a reduced voltage starter, the designer must first determine the acceptable level of motor torque required
during starting, or the pump may accelerate slowly or even fail to reach full speed, ultimately causing motor and/or pump damage. Many types of reduced voltage starters are available for use with motors, but not all will necessarily work with every application.
Autotransformer starters
(Figure 2), also called auto compensators, reduce the voltage to the motor terminals for starting by using a transformer action and are available for both high and low voltage motors.
The autotransformer has a reduced voltage winding tap which is removed from the circuit and connected directly to
the line as the motor approaches its rated speed. Autotransformer starters typically possess three taps available to select from with varying degrees of output torque with each tap: 50%, 65%, or 80% of line voltage are the most common.
Using a transformer function, current drawn from the source will vary as the square of voltage applied at the motor terminals. Thus, when the 80% tap is selected, the line current will be 80% squared (0.802) or 0.64 = 64% of line current that would have been drawn at a full voltage value.
Autotransformers are one of the heaviest and bulkiest reduced starting methods and can be built in either open or closed transition starting.
Resistor starters
(Figure 3) reduce voltage to the motor by inserting resistance into each leg of the circuit and then shorting out the
respective devices when the motor approaches full operating speed.
The resistance type of starter uses two contactors, a timer, and a power resistor as the starting device. Allowable starting time is 5 seconds to prevent overheating of the components. It is the least flexible in application of all starting devices but smooth in acceleration and priced somewhat in the middle of all reduced voltage starters.
The added resistance in a resistor starter acts as an inline voltage-drop device during motor starting as it adds loss to the circuit and imposes an added load on the generator. This method provides smooth acceleration and continuous operation as the starting circuit is removed without the need to momentarily disconnect the motor from the line.
The line current equals the motor current, resulting in a lower torque-to-source kVA ratio for starting than with autotransformers. For example, with 80% of line voltage applied to the motor terminals, the motor current will be 80% of the normal full voltage current and extract the same 80% of current from the line. Contrast this to 64% of line current with an 80% tap autotransformer.
Primary reactor starting
(Figure 4) also uses two contactors, a timer, and a reactor as the starting device. Allowable starting time is 15 seconds. Series reactor starting is commonly used on larger motors as its application flexibility is limited to only the high voltages and currents.
After receiving a start signal, the controller energizes the start contactor to place the reactor in series with the motor and power source. This added series reactive load increases the total circuit impedance and decreases the power factor, thereby reducing the starting current and real power drawn from the power source.
Once connected to the power source through the reactor, the motor starts accelerating the pump under reduced voltage conditions. After an adjustable period of time delay, transition takes place by energizing the main or run contactor. When the main contactor closes, the reactor is bypassed by applying full voltage directly to the motor. The reactor remains connected during transition to maintain a closed transition operation. When the reactor is completely bypassed, the pump accelerates to its normal operating speed.
Once connected to the line through the 50% or 65% reactor taps, the motor respectively draws 50% or 65% of its locked rotor current. As the speed increases, the starting current decreases according to the reactor speed-current curve for the specific tap selected.
When transition occurs, the motor current jumps to the full voltage current at the speed transition took place. From here, the current quickly reduces to its normal running value. When energized through the 50% or 65% reactor taps, the motor respectively produces 25% or 42% of its locked rotor torque to begin accelerating the pump. As the speed increases, the torque dips slightly before it rapidly increases according to the reactor speed-torque curve for the selected tap.
When the transition occurs, the motor torque jumps to the full voltage torque value and accelerates the pump to its full running speed.
Part winding starting
(Figure 5) requires use of a special motor with two parallel stator windings successively connected to the line as the motor speed increases. Full line starting is applied to a part of the motor’s winding. Current and developed torque are reduced to that of a single winding while starting.
Torque characteristics are better if the motor’s stator is designed for part winding starting, but standard dual-voltage motors are often used. Two contactors, each equipped with overload relays and a timer, are the parts of this starter. However, additional cost and possible delay in delivery of the motor itself must be taken into consideration since it may be up to 50% more expensive and take up to a month longer to obtain than a standard squirrel cage motor.
Although this technique can produce a good torque to kVA ratio, it does not allow for a smooth start as the motor will rapidly accelerate from partial to full speed in just a few seconds. This method is not suitable for small high-speed motors. Acceleration time is typically between 5 and 15 seconds depending on the type of motor used and the driven load.
Star or wye-delta starting
(Figure 6) requires the motor have all six or 12 motor leads brought to the connection box. The motor starts as a wye- (star-) connected motor and is then switched to run as a delta-connected motor.
When connected as a wye configuration, voltage is impressed on each individual winding at 58% of the full value of a delta connection. Starting torque and current are 33% of the full voltage value.
This type of starter has the longest allowable acceleration time of 45-60 seconds. When wye-delta starting is used with a limited capacity generator, the additional loss in the motor’s starting torque due to a significant transient voltage dip
often results in the motor failing to accelerate to near rated speed prior to making the transition to running mode. Thus, the power source is loaded as if it was directly on full voltage power, which can lead to motor stall or failure.
Solid-state starters
For the purposes of this discussion include variable frequency drives (Figure 7a) and electronic soft starters (Figure 7b). Solid-state starters can adjust output torque, acceleration ramp time, and current limit to create a smooth and continuous motor start, resulting in a controlled acceleration.
Solid-state starters offer smooth and step-less motor starting by varying the conduction angle of the silicon-controlled
rectifiers (SCRs). This angle varies from 20% to 100%, which in turn controls the motor voltage from 40% to 100%.
They have the advantage of operating without mechanical parts and large electrical switching contacts. This provides a
smooth application of power.
For the purposes of sizing a genset, the current limit adjustment reduces the inrush current and may also be used to
reduce the starting kW and kVA requirement on the generator. The range of available current limit settings is typically from 150% to 600% of full-load current.
A 600% current limit setting on a solid-state starter results in a genset sizing that is basically the same as full voltage starting. However, a 300% current limit setting reduces starting kVA by 50%. Use of the current limit setting also reduces the torque available to the motor.
From a genset sizing perspective, an extended acceleration ramp time and low current limit setting (if appropriate for the motor and the driven load) would result in the least impact on voltage and frequency dips.
One downside to using solid-state motor starters is their integral SCRs will likely cause voltage distortion. To compensate for this, the generator should be oversized. The basic recommendation is to apply two times the running kW load except when an automatic bypass is used.
If the solid-state starter has an automatic bypass, the SCRs are only present in the circuit during starting. Therefore once the motor is running, the bypass contactor closes and shunts the SCRs. In this case, the voltage distortion during starting can be ignored, and added generator capacity is usually not needed.
The operating sequence of a solid-state soft starter is fairly simple. Upon receipt of a start signal, a soft starter motor controller begins ramping up the voltage, typically beginning at 40% and steadily rising up to 100%. After approximately 10 seconds, the bypass contactor closes to provide full voltage to the motor, which shunts the SCRs.
When a stop signal is received, the bypass contactor will open and the soft starter will begin ramping down until the baseline low voltage is reached, when the motor will then shut down. If a start demand is received during a ramp down, the soft starter will immediately ramp the motor back up to full speed.
This type of operating sequence is often referred to as a “water hammer package” and is used to control the transition of many deep well and booster pumps. Soft starters are typically available with adjustable ramping up (and down, if so equipped) time and rate as well as starting voltage levels.
All versions of variable frequency drives (VFDs) are current limiting and reduce starting kW and kVA. The current drawn by these drives is nonlinear (having harmonics), which causes a distorted voltage drop across the reactance of the generator.
Since VFDs are nonlinear, you must include an additional generator capacity sizing factor to keep voltage distortion to a reasonable level of approximately 15% total harmonic distortion or less. The larger the generator, the greater the reduction in impedance of the power source (generator), which in turn reduces the effects caused by harmonic current distortion.
For six-pulse VFDs, a typical generator sizing factor would be twice the running kW of the drive. This offsets any possible reduction in starting kW and kVA. If it is the pulse width modulated (PWM) type (or includes an input filter to limit current distortion to less than 10%), then you can reduce the sizing factor down to 1.4 times the running kW of the drive.
VFDs start at zero frequency and ramp up to a set point. Variable voltage drives start at zero voltage and ramp up to a selected point. Both are under a current or torque limit to avoid large inrush current.
Generally, when these drives represent more than 25% of the total load on the generator set, it may require a larger generator. VFDs require large generators. They are current limiting and reduce kW and kVA.
Non-linear current is drawn which has harmonics. This causes a voltage drop across the reactance of the genset. When
total harmonic distortion exceeds 15%, additional generator capacity may be needed.
Larger generators have greater reduction in impedance of the generator; this reduces the effects of the harmonic current distortion. For six-pulse VFDs, twice the running kW of the drive is a typical sizing factor used to offset any reduction in starting kW or kVA. If an input filter is used to limit current to less than 10%, the sizing factor can be reduced down to 1.4 times the running kW of the drive.
Passive filters may be used to reduce the impact of harmonic distortion. However, the possible effects of leading
power factor with tuned inductor/capacitor filters at start-up or with light loads may affect voltage regulation of self-excited generators.
Single-phase motor starters
(Figure 8) simply connected to a single-phase supply will not rotate as the windings do not produce a rotating magnetic field. For one half cycle of the AC waveform, torque will be produced in one direction and then in the opposite direction for the next half cycle, thereby cancelling out all rotor torque.
Thus, to initiate motor starting, a rotating magnetic field must be established. Single-phase motors are typically regarded as full-voltage start motors, using pressure switches or manual motor starters for fractional and less than 2 HP motors and magnetic starters for 3 HP and larger.
The internal wiring connections and winding methods control the method of starting. There are a few different ways to affect a single-phase motor connection that will result in a rotating magnetic field as follows (the schematic connections for each motor type are shown in Figure 8):
- Split phase or resistance start (≤1½ HP)
- Capacitor start-induction run (⅓-2 HP)
- Permanent split capacitor (≤ 1 HP)
- Capacitor start-capacitor run (≥ 3 HP)
- Electronic starter for single-phase motor (all sizes)
Table 4 provides basic information on the various motor starting methods.
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This concludes this edition of The Water Works. We will continue our discussion on electrical systems and controls in October by beginning an overview on control systems.
Until then, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.