The Multilec Power Requirement Calculator helps you size your new generator based on your overall power consumption.
IMPORTANT NOTICE: Our Power Requirement Calculator is a very advanced system, which is used by private individuals and professionals (engineers, electricians, etc). If you need assistance, please do not hesitate to speak to us or request a free quote and we'll assist you with the calculation and provide you with a formal quote.
Before deciding on a generator, it is important to consider ALL the appliances you may want to operate, how many will be used simultaneously and the TOTAL current consumption at any one time. Most appliances have a descriptive plate stating the current, usually in Watts. The power consumption in KW multiplied by a Power Factor of 1.25 totals the kVA needed for your Generator.
For calculations for emergency STANDBY requirements, ONLY consider the least possible appliances that will be used simultaneously. It is however important to note that any automatic start-up items should be isolated from the Generator Power Supply. In most instances your DB board needs to be altered in so far as change-over switch gear is concerned. A STANDBY (Backup) generator delivers power directly to your home's electrical system, backing up your entire home or just the most essential items during power blackouts or load shedding. Determine what needs to run during an outage.
Important - Various electrical applications have very different power requirements, so it is essential to understand each particular need. For example TV sets, cookers, kettles and lamps have resistive loads, this means that their start -p and operating currents are virtually the same - and as stated on the manufactured descriptive plate. Buying the right generator can seem like a formidable task. But in just a few easy steps you can effectively and confidently decide and choose which generator is right for you.
Motor starting is an important consideration when buying a generator. Keep in mind that induction type motors, like those that run sump pumps, refrigerators, compressors, pressure washers typically require 5 to 7 times their listed running watts to start. Please note that power tools using a universal commutator do not require any additional start up power. The Power Requirement Calculator lists tools and appliances you may want to use with your generator. With each tool or appliance listed the approximate running and start-up watts are calculated automatically. All applicable power factors and start-up, as well as three phase allowances are built into the Calculator.
The Power Requirement Sizing Calculator are to help you pre-determine the size generator that would best suit your needs based on what type of business you are running. Please be advised that there are a multitude of factors to take into consideration when generating a generator size as well as the complexity of electrical calculations. The calculator are mere a helpful tool in order to do your sizing and limited budget planning. The assistance of one of our certified electrician is recommended for further detailed design calculations.
Most electric loads in homes don't run on out-of-phase power. Appliances such as stoves, ovens, toasters and electric heaters use in-phase voltages and currents to produce heat. For appliances with motors, such as fridges, washing machines and dryers, the motors are either too small to make it worthwhile for the utility to charge for extra apparent power, or the motors already have compensating circuits built in. As a result, homes are typically only charged for real power in KW, not for KVA power. In most domestic instances the power supply is single-phase.
In industrial applications power supply to induction motors of high power normally is provided in the form of three-phase electrical supply.
Actual Power
Therefore we can refer to kW as actual power; it is the amount of power that is converted into an output.
Apparent Power
On the other hand kVA is a measure of apparent power: it describes the total amount of power being used by a system, for example in a 100% efficient system kW would equal kVA exactly. However in reality electrical systems are not 100% efficient and so not all of the systems apparent power is being used for useful work output. Fundamentally one kVA is equal to 1,000 volt amps. Whereas a volt is for measurement of electrical pressure and amp is a way of electrical current measurement. A term called apparent power (the absolute value of complex power, S) is equal to the product of the volts and amps.
kVA Power Rating
Power ratings are expressed in different forms such as WATTS and KILOWATTS, AMPERES or AMPS, VOLTS, and also in kVA. Outside the diesel generator industry, the term Kilo-volt-amperes (kVA) is not well known. A Kilowatt (kW) is much more common a term and is how electrical items in your home/business are rated.
Apparent power (kVA) x power factor (pf) = actual power (kW)
Fundamentally one kVA is equal to 1,000 volt amps. Whereas a volt is for measurement of electrical pressure an amp is a way of electrical current measurement. A term called apparent power (the absolute value of complex power, S) is equal to the product of the volts and amps.
Normally; Calculation for motor run kVA = 1kW / [motor efficiency 0.95 x running pf 0.8] = 1.316kVA
The unit KVA (kilovolt amperes) is a measure of the power in an electrical circuit. The power depends on the voltage and the current at a given time with the KVA value being the power generated or consumed by the circuit at that moment. For most residential applications, the voltage and current in AC circuits are in phase, and the KVA power is equal to the kilowatts (KW). The KW over time give kilowatt-hours (kWh), which represent the energy used over a given time period.
When voltage and current are in phase, the kilovolt amperes (KVA) equal the kilowatts (KW), or the power used in the electrical circuit. When voltage and current are out of phase, the KVA are higher than the KW and give the apparent power that has to be multiplied by the power factor (pf) to get the KW.
As long as the voltage and current rise and fall together, they are in phase and produce real power. In such a case, the KVA power, obtained by multiplying the voltage and current together and dividing by 1,000, is equal to the KW power. This is the case for home appliances for which power consumption is normally given in KW.
For some electric loads, such as large industrial motors, the voltage and current are not in phase. Instead, the voltage in a typical AC circuit rises, but the current is held back by the magnetic field of the motor. When the voltage and current are out of phase, they produce less real power although the electrical circuit still contains the same voltage and current values. As a result, the KVA power, or the apparent power based on the voltage and current, is higher than the real power. To compensate, the KVA power is multiplied by the power factor (pf), expressed as a decimal between zero and one. Typical power factors for large industrial loads are 0.8; meaning that the KVA power times 0.8 gives the real power in KW. Thus’ kVA=1.25 x kW
Induction motors of high power require a suitable device for starting because they take an excessive amount of starting current. There are various starting methods used for starting induction motors because Induction Motors draw very high starting current (5-7 times more) compared to the full load current of the motor when it’s started. In calculation of the power supply needed in industrial three-phase supply it is important to consider the motor starting methods applicable in the equipment utilized.
The high starting current of induction motors results in a high starting torque. Unnecessary high starting torque, even when not required by the load, increases mechanical stress on systems such as the rotor shaft, bearings, gearbox, coupling, chain drive, connected equipment and others, leading to premature failure and plant downtime.
The first kind of device used for starting the induction motor is called the Direct On Line Starter (DOL). Direct On Line starter is the simplest kind of starter that connects the motor directly to the power supply through a three-phase contactor. The method is characterized by less investment, simple equipment and small quantity. Although the starting time is short, the torque is smaller at starting and the current is large, which is suitable for starting small capacity motors. The Direct On Line Starter usually consists of a Contactor, Circuit Breaker and an Overload relay for protection against any damage.
Between the mains supply terminals and motor terminals the DOL starter main terminals are connected and with the two terminals of the three-phase power supply the control circuit is energized.
Current will flow through one phase to the control circuit and the contactor coil to the other phase when the start button is pressed. Contactor coil gets energized due to this current that makes to close the contacts of the contactor and in the result of this three-phase supply is connected to the motor. The start button is sometimes called as push button because when this button is released the control circuit of DOL starter still provide the power supply via Hold-On-Contact. The current path via contactor coil will break when the stop button is pressed and due to this the contactor contacts drop out, thus breaking the power supply to the motor. We can also say that the overload relay coil operates.
Depending on the heating effect of the load current thermal overload protection relay operates because the heating effect is directly proportional to load current. More the load current more is the heating effect. Spring loaded contact in the control circuit trips out only when bimetallic strip used inside it expands due to heat. This all happens when the amount of load current is too much that it has the ability to heat the thermal coil. Relay operating speed is defined by current adjustment. Normally it must be 3-5 times the rated current of the motor.
Remotely operated DOL starters are also used for control the switching of the motor from any desired place. But there is a condition for remote control based switching that one should know, that all the remote “OFF” push buttons always be connected in series with “OFF” pushbuttons of the starter and vice versa (all the remote “ON” push buttons always be connected in parallel to “ON” pushbutton of the starter).
DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts where high inrush currents do not cause any harm. If the high inrush current of the motor does not cause an excessive voltage drop in the supply circuit a direct online starter can be used.
The reduced-voltage starting method can be introduced into medium and large size induction motors to restraint the starting current. When the motor finishes the starting, it will resume to full pressure working. However, the result of reduced-voltage starting will lower down the starting torque. Therefore, the reduced-voltage starting is only suitable for starting the motor under no-load or lightly loaded condition. Some common reduced-voltage starting methods are as listed below.
3.2.1 Stator circuit series resistance starting
A three-phase electric reactor is inserted into the circuit of motor stator windings. The electric reactor can be simply considered as coil, which can produce induced electromotive force to reduce the direct input power frequency voltage.
3.2.2 Star-Delta Starter
The star delta starting is a very common type of starter and extensively used, compared to the other types of the starters. This method used reduced supply voltage in starting. In the normal operation, 3 phase induction motor whose stator winding is stipulated to link in delta connection can be started in star while starting, to reduce the voltage of each phase of motor and then reduce the starting current. After finishing the starting, then it is connected in delta.
Star-delta starting is widely used because of its advantages including simple starting equipment, low cost, more reliable operation and easy maintenance. The method achieved low starting current by first connecting the stator winding in star configuration, and then after the motor reaches a certain speed, throw switch changes the winding arrangements from star to delta configuration. By connecting the stator windings, first in star and then in delta, the line current drawn by the motor at starting is reduced to one-third as compared to starting current with the windings connected in delta. At the time of starting when the stator windings are start connected, each stator phase gets voltage VL/√3, where VL is the line voltage. Since the torque developed by an induction motor is proportional to the square of the applied voltage, star-delta starting reduced the starting torque to one–third that obtainable by direct delta starting.
3.2.3 Soft Start Starter
Soft starter is a new type control device whose main advantages include soft starting, light load and energy saving, and quickness. One of the most important features is that the electronic circuit is conducted in the silicon controlled rectifier of motor under the tandem connection of power supply. Using the soft starter to connect the power supply with the motor and different methods to control the conduction angle in silicon controlled rectifier can make the input voltage of motor increase gradually from zero and transfer all the voltage to motor from the beginning to the end, which is called soft starting. When starting in this way, the torque of motor will gradually increase with enhancive speed. In fact, the soft starter is a voltage regulator that only changes the voltage without altering the frequency at starting. In technical terms, a soft starter is any device which reduces the torque applied to the electric motor. It generally consists of solid state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.
3.2.4 Auto Transformer Starter
Autotransformer reduced-voltage starting refers that the reduced voltage of grid power is attached to the motor stator windings until the speed approaches to a steady value and then the motor is connected to the power grid.
At starting, the switch is pulled to the “start” position, and the autotransformer is linked to the grid followed by connection to the stator windings of motor to achieve reduced-voltage starting. When the rotation speed approaches to the rated value, the switch will be pulled to “running” position, and the motor directly access to the grid under full pressure operation through cutting off autotransformer.
Autotransformer reduced-voltage starting is introduced into the star connection for the large capacity motor or normal operation with certain load starting. According to the load, transformer tapping is chosen according to receive required starting voltage and starting torque. At this moment, the starting torque is still weakened, but not reduced by one-third (compared with the star-triangle reduced-voltage starting). However, the autotransformer is large-sized and light weight, the auto transformer starter is more expensive, more complicated in operation and bulkier in construction when compared with the star–delta starter method.
The operation principle of auto transformer method is similar to the star delta starter method. The starting current is limited by (using a three phase auto transformer) reduce the initial stator applied voltage. But an auto transformer starter is suitable for both star and delta connected motors, and the starting current and torque can be adjusted to a desired value by taking the correct tapping from the auto transformer. When the star delta method is considered, voltage can be adjusted only by factor of 1/√3.
3.2.5 Rotor Impedance Starter
This method allows external resistance to be connected to the rotor through slip rings and brushes. Initially, the rotor resistance is set to maximum and is then gradually decreased as the motor speed increases, until it becomes zero. The rotor impedance starting mechanism is usually very bulky and expensive when compared with other methods. It also has very high maintenance costs. Also, a considerable amount of heat is generated through the resistors when current runs through them. The starting frequency is also limited in this method. However, the rotor impedance method allows the motor to be started while on load.
3.2.6 Variable Speed Drive
A variable-frequency drive (VFD) or adjustable-frequency drive (AFD), variable-voltage/variable frequency (VVVF) drive, variable speed drive (VSD), AC drive, micro drive or inverter drive is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage.
The variable frequency power supply uses solid state components to produce a pulse-width modulated current that varies the power and frequency supplied to the motor. This enables accurate control of the motor speed over a broad range. VSDs are used in connection with pump and fan applications to vary the pump or fan speed according to demand, often with large savings in energy use. Using a VSD to reduce the speed of an AC motor by 20% reduces the energy consumption by around 50%. Using a VSD to reduce the pressure in a pump by 20% reduces the energy consumption by around 28%. However, in VSD motors the starting method is between 2.5 to 5 times more expensive than low inrush current motors
Both the Frequency Start and the Excitation Start methods of motor starting enable a Generating Set to start a larger induction motor than would normally be possible with the available Generating Set, because of the typically high levels of inrush (Locked Rotor) current associated with a normal starting procedure for this size of motor. However, both are dependent on the motor not having to accelerate to speed a coupled load that has a high torque requirement. Both systems have their own subtle merits, making it difficult to offer exact guidance regarding which would be best for certain individual applications.
In calculating the required power from a Generator needed for application, we must understand and therefore the calculations must allow for the various starting methods utilized in industrial supply. There is also a distinct difference in starting a motor under load or not loaded at all during start-up power demand. To this extend the Power Calculator have built-in factors to allow for excessive amount of starting current required, compared to the full load current of the motor when it’s started.
