Industrial customers install capacitors to cancel the inductive component of motor loads to improve their power factor. The measure of real power (kW) divided by total power (kVA) is defined as the "power factor." The highest power factor achievable is 1 or "unity" power factor and is often expressed as a percentage with 1 equal to 100%. If you are billed for kVA, then you are paying for the reactive power component and you are not getting any useful work. A facility with a very low uncorrected power factor is indicative of a significantly high number of under loaded motors. Therefore lightly loaded motors are said to have a lower power factor than a fully loaded motor. The reactive component stays essentially constant whether a motor is lightly or heavily loaded. The vector sum of real and reactive power is called the apparent power and is expressed in kVA. The supply distribution system provides both real and reactive power to operate the motor. Useful mechanical work is developed from real power supplied by the supply system and is measured in kilowatts (kW). Measured in kVARs, reactive power does not provide any mechanical work. Most AC motors require reactive power from the supply system to develop magnetic fields. Motor speed varies directly with the frequency of the power supply. However, this should never be a problem if the system is supplied from a utility.
Voltage unbalance is calculated as follows:įigure 7-1: 3-phase Squirrel Cage Induction Motors Derating Factor Due to Unbalanced VoltageĪ motor should not be operated if the phase unbalance is greater than 5%.įrequency variation of up to 5% is permitted for normal motor operation. These effects occur due to negative sequence currents flowing in the motor. A phase unbalance of 3.5% results in a temperature rise of 25% and a current increase of 6-10 times the voltage unbalance. Phase voltage unbalance must be less than 1% for proper motor operation. A motor of the proper voltage rating should be used, or a transformer should be installed to supply the correct voltage. While the maximum allowable voltage for the motor is 484 V (110% x 440) there is no allowance for an upward supply voltage variation (for example, the utility can supply 500 volts and be within accepted tolerances) as the motor is already operating at its upward supply voltage limit. For example, a motor with a nameplate voltage of 440 V is sometimes connected to a 480 V system. The use of a motor with a non-standard or incorrect utilization voltage from the supply system should be avoided. The following table (Table 7-1) shows the relation between motor nameplate voltage and the correct supply voltage for that motor. The nominal supply voltage of the power system and the utilization or nameplate voltage on the motor often differ. An example of this is low voltage 400 Hz motors that are used in the aircraft industry, as well as some mine tool applications. Motors can be specified to operate on voltages and frequencies other than standard. For 125 HP and up, they are available for 460, 600, 2400 or 4160 volts at 60 Hz. 3-Phase:ģ-phase motors up to 100 HP are available for 200, 240/460, 460 or 600 volts at 60 Hz. Single phase motors are rated for 120/240 volts at 60 Hz. Therefore, it would not be economically practical to provide motors beyond a certain HP rating for a given voltage when the conductor size becomes too large both in the supply to and within the motor. For example, a 50 HP motor will require 150 Amps to operate at 208/120 volts, but requires only 50 Amps at 600/347 volts. The limit to the supply voltage is dependent on the current required to operate the motor. Table 7 1 provides a cross comparison of nominal system voltage to show what one might find on a typical motor nameplate. The electrical supply distribution system must supply the correct voltage and have sufficient capacity to start and operate the motor load.