Electric Motors and Power Systems

AC POWER SYSTEMS

To understand how voltage standards for motors are set it is important to know the basics of the power systems they operate on. In general, utilities that supply power in the USA, and most other 60 cycle countries, are required to provide power to the incoming point of a facility in multiples of 120 volts. Thus incoming equipment, such as circuit breaker panels, are rated in multiples of 120 volts. The common voltages are 120, 240, 480, and 600.

In addition, utilities are obligated by the regional governing authorities, (usually called Public Utility Commissions) to regulate the voltage within a fairly narrow range such as plus or minus 5%.

For example, in most single phase residential systems the voltage is 120/240. It is brought to the building with 3 wires, one being a neutral and the other two having voltages 120 volts different from the neutral wire. The voltage difference between the two “hot” wires is 240 volts.

In 3 phase systems the situation is a bit different. There are 3 phase, 3 wire, ungrounded systems where the voltage between the three wires is 240 volts. The big brother of that system is the ungrounded 3 phase, 3 wire 480 volt system. Ungrounded systems are usually found in older facilities.

In newer installations, the two most popular systems are called 4 wire grounded wye systems. The low voltage version is represented by a 120/208 volt system. The higher voltage version it is a 277/480 volt system. On both of these “grounded wye” systems, the low voltage portion (120 or 277 volts) is only available as single phase. The high voltage (208 or 480 volts) is available as either single phase or 3 phase. It should be noted that in the 4 wire grounded wye systems the high voltage is 1.73 times (the square root of 3) higher than the low voltage. These grounded wye systems are generally felt to be safer and more flexible than the older ungrounded systems. The flexibility comes from the ability to handle single phase lighting circuits, that operate at 120 volts or 277 volts, from the same system that feeds the 3 phase circuits for motors, equipment for heating, air conditioning, elevators, and industrial machinery.

MOTORS

Now to discuss motors that operate on these 60 cycle power systems. In the case of “utilization equipment”, such as motors, the voltage standards have been selected in multiples of 115 volts. For example, 115, 230, 460 and 575 volts. The standards for the “utilization equipment” have been deliberately picked to be slightly less than the utility delivery voltages because in an industrial plant or large commercial building there may be several hundred feet between the incoming service point and the equipment. The distances involved will always lead to some voltage loss (or drop) through the wiring. On short runs this might be very small, even less than a volt, but on long heavily loaded runs it might be as much as 3 or 4% of the operating voltage. So choosing the utilization voltage to be different — and less than — the utility service voltage makes good sense.

There is also another factor that should be mentioned. The design standards for utilization equipment are set so the equipment is able to handle a voltage variation of plus or minus 10% of the nameplate rating. Thus a motor nameplated at 460 volts should be able to be operated successfully up to 460 plus 10% (506 volts) and down to 460 minus 10% (414 volts). If everything is right with the voltage of the system being in multiples of 120 plus or minus 5% and the equipment voltage being multiples of 115, plus or minus 10% then everything fits together like a neat jigsaw puzzle.

There is one oddity in the mix. That is 3 phase motors for the 120/208 volt power systems. For example, if the power system were to be 208 volts minus 5% (approximately 198 volts) and you were using a 230 volt motor, then the 230 volt motor could only go down to 207 volts (-10%) without being in trouble. There would be a discrepancy between the 198 volt low range of the system voltage, and the 207 lowest operating voltage of a 230 volt motor, this could spell trouble. So how can this be addressed?

There are two ways that motor manufacturers have faced up to the problem. The first is to provide motors rated for 200 volts that can operate successfully down to 180 volts, or up to 220 volts. This is an adequate margin to cover the normal range of voltages that could be expected on a 120/208 volt system. But using this approach exclusively would mean that the complete inventory of motors in all sizes, enclosures, mechanical configurations, etc. would have to be duplicated to handle the motor requirements for the 120/208 volt power systems. This would be very expensive and cumbersome, especially with the wide variety of small motors (under 10 HP) that exist.

So most motor manufacturers have taken a different approach to handling these smaller motors. This approach is that by using a somewhat more conservative design on the 230 volt motors it is possible to create a 3 phase, tri-voltage motor with voltage ratings of 208-230/460. With this approach the 230 volt winding ( and connection diagram) is used on the 208 volt power system. When this approach is taken the motor manufacturer is essentially saying that this motor can be successfully operated on voltages as low as 208 minus 10% or 187 volts. This approach usually works very well since 208 volt power systems are normally used in small buildings with relatively short distances between the incoming power service and the utilization equipment. These short runs tend to make 208 volt power systems quite stable so that the limit of the motor’s low voltage capability is seldom tested.

On motors larger than 10 HP the 200 volt motor is generally the best choice, but in many situations 230 volt motors are frequently and successfully applied on the 208 volt systems. In some cases a derate table is provided for the “low voltage” situation. In other cases the motor service factor may be reduced from 1.15 down to 1.0 when it is applied to a 208 volt power system.

Table 1 summarizes this information to show the power system voltage and description along with the motor voltage rating for single and 3-phase 60 Hertz motors.

50 HERTZ POWER SYSTEMS

There seems to be an endless array of possible combinations, but most of them do make sense. In 50 hertz areas virtually all power systems are of the 4 wire, grounded wye type. A typical arrangement would be a 220/380 volt power system. In this case, as in the case of a 120/208 volt 60 hertz system, the (low voltage) 220 volt power is only available as single phase and the 380 volt power is available as either single or three phase.

As a result of the voltage being described as 220/380 we frequently see specifications indicating that 3-phase motors be wound for 220/380. Although feasible to do this, it is unnecessary because the 3-phase motors will only be operated on 380 volt 3-phase power. Some of the most popular voltages are 220/380 and 240/415. Recently European countries have recognized the problem of trying to provide equipment for these two different voltage standards and have come up with a standard that splits the difference. The new standard is 230/400. What this means is that if the motor has an adequate amount of tolerance it can run on either a 380 volt system or a 415 volt system without being damaged. Also in most 50 Hertz systems, unlike the domestic systems, the equipment voltage rating tends to be the same as the supply voltage. In other words, 380 volt motors are used on 380 volt systems as opposed to situation in this country where the equipment utilization voltage is deliberately set lower than the supply voltage.

Table 2 shows some typical supply voltages and the appropriate equipment standards for 50 cycle power systems.

When dealing with foreign voltage requirements it is always desirable to check the specified voltage against the listing of available voltages indicated in a U. S. Department of commerce booklet, Electric Current Abroad*. If the specified voltage and frequency does not match the voltages shown in the booklet for the country and city involved it should be a “Red Flag” that would suggest that the customer be contacted and the voltage confirmed for accuracy. Mistakes can be very costly!!

*Copies of Electric Current Abroad are available from your local Baldor District Office.

SUMMARY

Matching motors to the power system voltages can be fairly simple if the basis of the systems is understood.