Power Supply

The available power supply and its characteristics must be understood by the motor salesman in order to properly apply an AC induction motor and controls. This section discusses the following power supply factors:

• Voltages
  • Power Distribution Systems
• Frequency
• 50 Hz Operation of 60 Hz Motors Power
• Regulations and Rates
• Power Supply Variations
  • Voltage variation from nominal values
  • Frequency variation from rated values
  • Unbalanced voltage between phases
  • Combined variation of voltage and frequency

Voltage

The supply voltage must be known in order to select the proper motors and controls for an application. The supply voltage should normally exceed the nameplate voltage by a slight amount, however, some voltage variance is allowable (see "Power Supply Variation" below). Generally, generator and transformer secondary windings are rated the same value as the nominal system voltage. Motors, motor controls and other power utilization equipment are listed lower than nominal system voltages to compensate for system voltage drops.

NEMA ⁽¹⁾ and the Edison Electric Institute have recommended voltage standards for power generation and distribution equipment, and for motor nameplate values which are as follows:

Three Phase
Nominal System Voltage			Suitable + or - 10% Motor Nameplate Voltage
50 Hz	60Hz		60 Hz	50 Hz
	120*		115*	220
	208		200	380
	240		230
	480		460
	600		575
	2400		2300
Single Phase
	120		115	110
	240		230	220

(1) MG1-10-30, "Voltages"
*Applies to motors rated 15 hp and smaller.

The common 3 phase supply voltage for a 4-wire network is 208 nominal volts. Many induction motors of 220 volt designation are unusable on 208 volt network, however, the operating characteristics will be modified (see "Power Supply Variation"). Motors rated 230 volts will not operate satisfactorily on 208 volt networks.

See "Power Supply Variation" for effect of voltage variations for rated value.

Many motors are offered with connections for two different voltages; for example 230/460. See footnotes in price book for dual voltage identification. Motors specified as dual voltage will operate successfully at either voltage if connected properly.

Multi-speed motors are almost always single voltage rated.

Some Reliance motors will have a tag attached which will remind the motor user of the suitable operating voltages of the motor on a 60 Hz power supply. Some of these stickers are shown below and can be obtained from Stationery Stores in Cleveland.

Power Distribution Systems

The electrical power distribution systems in most common usage today are Radial and Network Systems.

Radial Systems distribute electrical power in a form similar to the spokes of a wheel. On this system distribution to each spoke is independent of distribution to other spokes, thus voltage deviations of ±10% can occur.

Network Systems are stiffer than Radial Systems since they are constructed similar to a grid with two or more busses supplying the network. Individual segments of the network thus support distribution to other areas.

120/208 Volt Systems can be either Radial or Network Systems and are usually found when there is a high lighting load with respect to power load.

Figure 1 120/208 Volt - 4 Wire System

The line to neutral voltage on this "Y" system is 120V, i.e.: (208) single phase and is used for lighting loads. The line-to-line voltage (208V) is used for 3 phase power loads.

Since each 1% increase in voltage over 120V decreases light bulb life 10% (5% increase reduces life 50%), these systems are arranged to prevent over voltage on the 120V system. Hence the 3 phase output power voltage is set for 208V maximum. Voltage deviations of 208V - 10% occur on 120/208 volt systems. To meet these conditions NEMA has adopted a 200V standard to replace the 208V motor of the past.

The Electric Motor Industry's change from 220/440 volts does not result from a recent significant change in power distribution voltages. Distribution voltages of 240/480 have been standard in most areas for over 20 years. The previous motor voltage standards of 220/440 were correct for these nominal power system voltages of 240/480, since voltage regulation practice and location of transformers and switchgear with respect to load, resulted in greater line voltage drops than exist today.

Today's higher voltage at the load results from improved regulation and closer proximity of transformers to load which decrease line voltage drop.

Table 1 shows the voltage ranges at the load found in industry, taken from the 1964 NEMA survey and conclusively demonstrates the logic of the new motor standard of 230/460 for 240/480 nominal power systems.

Table 1

Today's stiffer distribution systems are illustrated by Table 2 which shows that median voltages at the transformer are only a few percent higher than voltages at the load.

Nominal Power System Voltage Median Voltage At Transformer At Load 240 233 228 480 470 462

Table 2

Phases

Three-phase power is the most commonly used throughout the world since three times the power transmitted by single phase can be obtained by using three wire, three phase system rather than a two wire, single phase system. The addition of the third wire and use of 3 phases makes economic sense. Single phase, if necessary, can be obtained from a three phase supply simply by tapping into any two lines of the three wire system.

Two phase power supply is found in only a few scattered areas around the U.S. and the world. Two phase motors are not generally available in stock; Check AC Products in Cleveland for availability. Two winding multi-speed motors are difficult to wind for two phases, therefore, transformation to allow three phase operation is recommended.

If a change of the direction of rotation of a driven machine would be disastrous, the power system should be checked to determine the likelihood of phase reversal. If necessary, phase-failure relays (nonreverse couplings) may be installed.

Frequency

The standard frequency in the United States is 60 cycles (Hz and cps). In foreign countries, 50 cycle systems have been used extensively in traction and steel mill applications, but the modern trend is tow 60 cycle in these applications. The use of 40 and cycle systems are isolated and demand is relatively small, so stock motors are not available.

Some 60 Hz motors having a 1.15 service factor may successfully be operated at 50 Hz at a reduced voltage and horsepower rating (see "50 Hz motor applications" for details).

To order 60 Hz rated and nameplated motors, refer to price adder in price book.

Higher than 60 Hz frequencies are obtainable for special high-speed motor application by using induction frequency converters or alternators.

For information on the effect of frequency variation from rated, see the "Power Supply Variation" section below.

50 Hertz Operation Of 60 Hertz Motors

Basically the 60 hertz motor will operate at 50 hertz on selected voltages at 85% or 80% of the 60 hertz rated horsepower. Because of the increasing number of requests for operation of 60 hertz motors on 50 hertz power supplies, we are making available a self adhesive label, RE 491A3, a sample of which is reproduced here.

The label may be used on all non-modified motors, cast iron or rolled steel, 48T thru 440T frames subject to the following conditions.

1. The label will not be applied by our plants or stock centers.
2. Use for customer incidental requirements - the best motor for 50 hertz operation is still one designed specifically for that service.
3. Standard NEMA Design B - 2, 4, 6 and 8 pole, 3 phase, Design C, Design D, multi speed motors.
4. Motors may have less than NEMA torques. Care must be exercised in using for hard-to-start and hard-to- accelerate loads.
5. Not to be used on motors with inherent thermal protection.
6. Not to be used on explosion-proof motors.
7. Temperature limits for "de-rate factor" loads will apply (900 C rise for Class B insulation).

The label is available now without charge through Stationery Stores.

Power Regulations And Rates

Power companies sometimes indirectly regulate the amount of motor starting power that a customer can use. This is due to the power companies capacity to serve a given installation. The power capacity of a plant is limited by the size and number of incoming power lines, transformers, and other distribution equipment. If the power system is not "stiff enough" to handle the normal load and the in-rush load due to motor starting which will be 4-1/2 to 7 times full load current depending on motor design, reduced voltage starters might be needed to keep the high motor starting current from exceeding the systems capacity. See "Reduced Voltage Starting" in the Starting Methods section, page 0-1 for more information on starters.

Note that most motors today, however, are started across the line, especially those under 100 hp.

** All Reliance motors are electrically and mechanically capable of across-the-line starting.

To protect their systems, electric utilities sometime establish limitations on starting of large motors. These limitations⁽²⁾ generally take one of the following three forms:

1. A maximum allowable current or power consumption per motor horsepower during any portion of the starting period. This protects the power company in a general situation. Such limitations are defined in terms of amperes per horsepower and kilovolt-amperes (kva) per horsepower.
   
   
2. A maximum permissible motor horsepower, or a maximum allowable current or power consumption during any portion of the starting period. Generally, this recognizes the specific size of motor to be used, application of the motor, capacity of the power distribution system serving the motor, and location of the motor in the power distribution system. Such limitations are defined in terms of amperes and kva.
   
   
3. A maximum allowable increase in current or power consumption per unit of time during the starting period. This type of limitation generally occurs where a power company's automatic voltage regulator is capable of maintaining relatively constant voltage at the distribution point, providing a large change in load is not suddenly applied. Such limitations are defined in terms of amperes, or kva, per unit of time, in which case the unit of time is given in terms of seconds or fractions of a second. Generally, there is no definite limit to the ultimate value of the inrush current.

The cost of energy to the customer may effect his choice of motors and controls. Motor efficiencies can vary widely, particularly when speed adjustment is involved and the customer may purchase the motor with the highest operating efficiency simply due to the savings in power expense over the life of the motor. This factor in motor selection becomes more important as the size of the motor and power utilized increases.

Power rate structure varies with each power company and each power company has a large number of rate schedules which are applied to different customers depending on what load they use. Large power users sometimes negotiate a power rate with the power company on an individual basis if other established rates do not apply to them. Power rates are usually based on four major functions which are total power consumption, largest demand requirement, the power factor, and the cost of coal.

The total power consumed may be measured in either kWh or kva and in some cases the rate schedule may state that a set amount of power per month is without charge.

Most rate schedules have a demand charge. This is a rate based on maximum demand in a given period of time, as well as energy consumed. For example, a customer is charged a power rate which is determined by his peak load during a given time for all the power he consumed even though the peak load may have existed for only a short period of time.

Rate schedules also incorporate a power factor clause, which adjusts the rate proportionally to the power factor and/or specifies a penalty or bonus charge to the customer if the power factor is below or above a stated lagging value. For example, if the P.F. is below 80% a penalty is charged or if the P.F. is above 90% a bonus is awarded. For more information on the definition and correction of a power factor, see the "Power Factor" section.

A coal clause is usually written into a rate schedule which allows the power company to increase the users rate as the price of coal increases. This clause is meant to protect the power company from the inflation effects on coal which is the main energy source for the power companies. For example, the cost of coal per ton was about $5.50 in 1965 and has risen to over $24.00 per ton today. This is one of the major reasons for the increasing cost of electric power in recent years.

Power Supply Variations

An ideal power supply would have constant voltage, frequency and phasing, but in reality they fluctuate around the nominal values specified for the particular system. Voltages will usually range from 10% above to 10% below the nominal values and sometimes more in an industrial environment. Frequency is usually closely controlled whether power is purchased or generated in a private plant, but it also can vary. Phasing, which is balanced when the voltage in each phase of a polyphase system is equal, can be unbalanced by 1, 2, or more percentage points.

The effect of this power supply variation on electric motor performance is different for each type of motor (an AC induction motor is different than a synchronous is different than a DC). This section will discuss only the effect on AC induction motors.

Each type of power variation (voltage, frequency or phasing) has a different effect on the operation of an AC induction motor. For ease of understanding, each power variation and its effect will be discussed individually below. In an actual industrial environment, however, it is common to simultaneously experience a combination of the three different power supply variations. When this occurs the effect on the motor is the result of the combined effects superimposed on one another.

Voltage Variation from Nominal Values

Applicable for 180-449 frames.

NEMA has established a standard for AC induction motors which concerns variation from rated voltage. The standard is MG1-12.43 (Jan. 1993) Which states that motors shall operate "successfully" under running conditions at rated load with a variation in the voltage up to ±10% with rated frequency. "Successfully" does not mean that the motor will operate at rated performance. For example, the motor may not be able to start and accelerate the driven load under a voltage variation condition since the speed-torque curve will change.

The major effects on motor operation due to a voltage variation are:

1. increased temperature rises
2. reduction in starting torque
3. reduction in running torque

1. increased starting and running torques
2. higher starting current
3. decreased power factor

See Table A at the end of this section for quick reference to the effect of voltage variation on motor operation.

Voltage variations are caused by changing amounts of current flowing through a distribution system which contains impedance. The change in current results in a change in IR drops in the system, and therefore, a variation in the voltage levels throughout the system and at the utilization equipment terminals. Voltage variations are described as voltage spread" regulation, or flicker.

Frequency Variation from Rated Value

NEMA standard MG1-12.44 states, "alternating current motors shall operate successfully under running conditions at rated load and at rated voltage with a variation in the frequency up to 5% above or below the rated." "Shall operate successfully," does not mean the motor will run at rated performance; motor current, torque, efficiency, power factor and full-load speed will be affected by a deviation of frequency from nameplate values. The approximate changes in motor performance due to frequency variations are shown in Table A at the end of this section.

Unbalanced Voltage between Phases

A polyphase induction motor is designed to operate on a power supply where the voltage is equal in each phase of the supply. When the voltage is unequal, a small rotating magnetic field is created which moves in the opposite direction as the main field. This bucking magnetic field produces induced voltages and thus high currents. A large current unbalance can result from a slight voltage unbalance, and for a given % unbalance, the current will range from a large value at no load to a lesser value at locked rotor condition.

Motor temperature is also significantly affected by a slight unbalance. The percentage increase in temperature rise will be approximately two times the square of the percentage voltage unbalance or:

As an example, 3.5% voltage unbalance will cause an approximate 25% motor temperature increase.

Other effects of a voltage unbalance are that a marked reduction in motor efficiency can be expected, locked-rotor torque and breakdown torque will decrease slightly, full-load speed will decrease slightly, and full-load power factor will decrease.

See Table A below for the approximate changes in motor performance due to voltage unbalance.

Combined Variation of Voltage and Frequency

Applicable for 180-449 frames.

NEMA standard MG-1-12.45 (Jan. 1993) states that an "AC motor shall operate successfully under running conditions at rated load with a combined variation in the voltage and frequency up to 10percent above or below the rated voltage and the rated frequency, provided that the frequency variation does not exceed 5 percent."

***Reliance motors meet or exceed the operating performance specified in the NEMA standards concerning power supply variations.***

Table A

Figure 2