Standard Motor Catalog

Section TR Technical Reference Guide

VOLTAGE UNBALANCE Unbalanced line voltages applied to a polyphase motor result in unbalanced currents in the stator windings. Even a small percentage of current unbalance, thus increasing temperature rise and possibly resulting in nuisance tripping. Voltages should be as evenly balanced as can be read on a voltmeter. If voltages are unbalanced, the rated horsepower of the motor should be derated, based upon the percent unbalance, as shown in the following graph


Unbalanced voltage results in reduced locked-rotor and breakdown torques for the application. Unbalanced voltage results in a slight reduction of full-load speed.

Full-Load Speed:

Current: Locked-rotor current will be unbalanced to the same degree that voltages are unbalanced but locked rotor KVA will increase only slightly. Full-load current at unbalanced voltage will be unbalanced in the order of six to ten times the voltage unbalance. Temperature Rise: A 3.5% voltage unbalance will cause an approximate 25% increase in temperature rise. LOW STARTING VOLTAGE Large motors may experience a considerable voltage drop at the motor terminals when started due to large inrush current values. Motors can be designed to compensate for the drop in voltage. For example, motors in frames 143-5013 can be supplied for operation under low starting voltage conditions down to 65% of nameplate rated voltage. The inertia referred to the motor shaft, the type of load (constant or variable torque), and the expected voltage drop must be provided to allow evaluation of the application for each rating involved. In any case, motors designed for low starting voltage may have higher than standard inrush current when started on full rated voltage. FREQUENCY DEFINITION Frequency can be defined as the number of complete alternations-per-second of an alternating current

Figure 3: Voltage Unbalance is calculated as follows:

Maximum Voltage Deviation

Percent Unbalance = 100 x

From Average Voltage Average Voltage

For instance, a 100 horsepower, three-phase motor operating with voltages of 598, 575 and 552 applied at the motor terminals, Average Voltage = (598+575+552)/3 = 575V Voltage deviation from average Voltage = Absolute (575-598) = 23V; Absolute (575-552) = 23V; Absolute (575-575) = 0V Maximum deviation from average voltage = 23V %Unbalance = 100*23/575 = 4% From Figure 3, the rated output of 100 horsepower should be derated to approximately 82 Hp to reduce the possibility of damage to the motor. Motor operation above 5% voltage unbalance is not recommended. Unbalance voltages will produce the following effects on performance characteristics:


Figure 4

As shown in Figure 4 above, current is said to have been through one complete cycle when it has gone from zero to maximum, to minimum, and back to zero again. Frequency is the number of these complete cycles over the passage of time and is usually expressed as HZ: one HZ equals one cycle per second (cps). Predominant power system

frequency in North America is 60 HZ. FREQUENCY STANDARDS

The predominant power system frequency in the United States is 60 HZ. However, 50 HZ systems are common in other countries. Other systems such as 40 HZ and 25 HZ are isolated and relatively few in number.


Data subject to change without notice. 03/24 •

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