MOTOR FORMULAS AND CALCULATIONS

For a detailed explanation of each formula, Click on the links below to go right to it.

Rules Of Thumb (Approximation)

Torque in lb.ft. =	HP x 5250 rpm	-----	HP =	Torque x rpm	-----	rpm =	120 x Frequency No. of Poles
				5250

Temperature Conversion

Deg C = (Deg F - 32) x 5/9
Deg F = (Deg C x 9/5) + 32

Temperature Conversion Formula ºR = 1.8 K + 0.6º .K = 5/9 (ºR-0.6º) ºF = 1.8ºC + 32º   ºC = 5/9 (ºF-32º) ºR = ºF + 460º  .K = ºC + 273º     ºC = Celsius, degrees ºF = Fahrenheit, degrees  .K = Kelvin ºR = Rankine, degrees           to ºC Temp. to ºF -17.8 10.8 37.8 65.6 93.3 0 50 100 150 200 32.0 122.0 212.0 302.0 392.0 121.0 148.9 176.7 204.4 232.2 250 300 350 400 450 482.0 572.0 662.0 752.0 842.0 260.0 287.7 315.6 343.3 500 550 600 650 932.0 1022.0 1112.0 1202.0 to ºC Temp. to ºF 371.1 398.9 426.7 454.4 482.2 700 750 800 850 900 1292.0 1382.0 1472.0 1562.0 1652.0 510.0 537.8 565.6 593.3 621.1 950 1000 1050 1100 1150 1742.0 1832.0 1922.0 2012.0 2102.0 648.9 676.7 704.4 732.2 1200 1250 1300 1350 2192.0 2282.0 2372.0 2462.0 to ºC Temp. to ºF 760.0 787.8 815.6 843.3 872.1 1400 1450 1500 1550 1600 2552.0 2642.0 2732.0 2822.0 2912.0 899.9 927.7 955.4 983.2 1011.0 1650 1700 1750 1800 1850 3002.0 3092.0 3182.0 3272.0 3362.0 1038.8 1066.6 1094.3 1121.1 1900 1950 2000 2050 3452.0 3542.0 3632.0 3722.0

High Inertia Loads

t =	WK2 x rpm 308 x T av.	-----	WK2 = inertia in lb.ft.2 t = accelerating time in sec. T = Av. accelerating torque lb.ft.
T =	WK2 x rpm 308 x t

inertia reflected to motor = Load Inertia

Load rpm Motor rpm

2

Synchronous Speed, Frequency And Number Of Poles Of AC Motors

ns =

120 x f P

-----

f =

P x ns 120

-----

P =

120 x f ns

Relation Between Horsepower, Torque, And Speed

HP =

T x n 5250

-----

T =

5250 HP n

-----

n =

5250 HP T

Motor Slip

% Slip =

ns - n ns

x 100

Symbols

Equivalent Inertia

In mechanical systems, all rotating parts do not usually operate at the same speed. Thus, we need to determine the "equivalent inertia" of each moving part at a particular speed of the prime mover.

The total equivalent WK² for a system is the sum of the WK² of each part, referenced to prime mover speed.

The equation says:

WK2EQ = WK2part Npart Nprime mover 2

This equation becomes a common denominator on which other calculations can be based. For variable-speed devices, inertia should be calculated first at low speed.

Let's look at a simple system which has a prime mover (PM), a reducer and a load.

The formula states that the system WK² equivalent is equal to the sum of WK²_parts at the prime mover's RPM, or in this case:

WK2EQ = WK2pm + WK2Red. Red. RPM PM RPM 2 + WK2Load Load RPM PM RPM 2

Note: reducer RPM = Load RPM

WK2EQ = WK2pm + WK2Red. 1 3 2 + WK2Load 1 3 2

The WK² equivalent is equal to the WK² of the prime mover, plus the WK² of the load. This is equal to the WK² of the prime mover, plus the WK² of the reducer times (1/3)², plus the WK² of the load times (1/3)².

This relationship of the reducer to the driven load is expressed by the formula given earlier:

WK2EQ = WK2part Npart Nprime mover 2

In other words, when a part is rotating at a speed (N) different from the prime mover, the WK²_EQ is equal to the WK² of the part's speed ratio squared.

In the example, the result can be obtained as follows:

The WK² equivalent is equal to:

WK2EQ = 100 lb.ft.2 + 900 lb.ft.2 1 3 2 + 27,000 lb.ft.2 1 3 2

Finally:

The total WK² equivalent is that WK² seen by the prime mover at its speed.

Electrical Formulas     (For more formulas see "Formulas")

To Find	Alternating Current
	Single-Phase	Three-Phase
Amperes when horsepower is known	HP x 746 E x Eff x pf	HP x 746 1.73 x E x Eff x pf
Amperes when kilowatts are known	kW x 1000 E x pf	kW x 1000 1.73 x E x pf
Amperes when kVA are known	kVA x 1000 E	kVA x 1000 1.73 x E
Kilowatts	I x E x pf 1000	1.73 x I x E x pf 1000
kVA	I x E 1000	1.73 x I x E 1000
Horsepower = (Output)	I x E x Eff x pf 746	1.73 x I x E x Eff x pff   746

I = Amperes; E = Volts; Eff = Efficiency; pf = Power Factor; kVA = Kilovolt-amperes; kW = Kilowatts

Locked Rotor Current (IL) From Nameplate Data

Three Phase: IL =	577 x HP x kVA/HP E		See: kVA/HP Chart
Single Phase: IL =	1000 x HP x kVA/HP E

EXAMPLE:

Motor nameplate indicates 10 HP, 3 Phase, 460 Volts, Code F. IL = 577 x 10 x (5.6 or 6.29) 460 IL = 70.25 or 78.9 Amperes (possible range)

Effect Of Line Voltage On Locked Rotor Current (IL) (Approx.)

IL @ ELINE = IL @ EN/P x	ELINE EN/P

Basic Horsepower Calculations

Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute. When work is done by a source of torque (T) to produce (M) rotations about an axis, the work done is:

radius x 2 x rpm x lb. or 2 TM

When rotation is at the rate N rpm, the HP delivered is:

HP = radius x 2 x rpm x lb. 33,000 = TN 5,250

For vertical or hoisting motion:

HP =

W x S 33,000 x E

Where:

For fans and blowers:

HP = Volume (cfm) x Head (inches of water) 6356 x Mechanical Efficiency of Fan

Or

HP = Volume (cfm) x Pressure (lb. Per sq. ft.) 3300 x Mechanical Efficiency of Fan

Or

HP = Volume (cfm) x Pressure (lb. Per sq. in.) 229 x Mechanical Efficiency of Fan

For purpose of estimating, the eff. of a fan or blower may be assumed to be 0.65.

For pumps:

HP = GPM x Pressure in lb. Per sq. in. x Specific Gravity 1713 x Mechanical Efficiency of Pump

Or

HP = GPM x Total Dynamic Head in Feet x Specific Gravity 3960 x Mechanical Efficiency of Pump

For estimating, pump efficiency may be assumed at 0.70.

Accelerating Torque

The equivalent inertia of an adjustable speed drive indicates the energy required to keep the system running. However, starting or accelerating the system requires extra energy.

The torque required to accelerate a body is equal to the WK² of the body, times the change in RPM, divided by 308 times the interval (in seconds) in which this acceleration takes place:

ACCELERATING TORQUE = WK2N (in lb.ft.) 308t

Where:

Or

TAcc = WK2N 308t

The constant (308) is derived by transferring linear motion to angular motion, and considering acceleration due to gravity. If, for example, we have simply a prime mover and a load with no speed adjustment:

Example 1

The WK²_EQ is determined as before:

If we want to accelerate this load to 1800 RPM in 1 minute, enough information is available to find the amount of torque necessary to accelerate the load.

The formula states:

TAcc = WK2EQN 308t or 1000 x 1800 308 x 60 or 1800000 18480

In other words, 97.4 lb.ft. of torque must be applied to get this load turning at 1800 RPM, in 60 seconds.

Note that T_Acc is an average value of accelerating torque during the speed change under consideration. If a more accurate calculation is desired, the following example may be helpful.

Example 2

The time that it takes to accelerate an induction motor from one speed to another may be found from the following equation:

t = WR2 x change in rpm 308 x T

Where:

The Application of the above formula will now be considered by means of an example. Figure A shows the speed-torque curves of a squirrel-cage induction motor and a blower which it drives. At any speed of the blower, the difference between the torque which the motor can deliver at its shaft and the torque required by the blower is the torque available for acceleration. Reference to Figure A shows that the accelerating torque may vary greatly with speed. When the speed-torque curves for the motor and blower intersect there is no torque available for acceleration. The motor then drives the blower at constant speed and just delivers the torque required by the load.

In order to find the total time required to accelerate the motor and blower, the area between the motor speed-torque curve and the blower speed-torque curve is divided into strips, the ends of which approximate straight lines. Each strip corresponds to a speed increment which takes place within a definite time interval. The solid horizontal lines in Figure A represent the boundaries of strips; the lengths of the broken lines the average accelerating torques for the selected speed intervals. In order to calculate the total acceleration time for the motor and the direct-coupled blower it is necessary to find the time required to accelerate the motor from the beginning of one speed interval to the beginning of the next interval and add up the incremental times for all intervals to arrive at the total acceleration time. If the WR² of the motor whose speed-torque curve is given in Figure A is 3.26 ft.lb.² and the WR² of the blower referred to the motor shaft is 15 ft.lb.², the total WR² is:

And the total time of acceleration is:

WR2 308 rpm1 T1 + rpm2 T2 + rpm3 T3 + - - - - - - - - - + rpm9 T9

Or

t = 18.26 308 150 46 + 150 48 + 300 47 + 300 43.8 + 200 39.8 + 200 36.4 + 300 32.8 + 100 29.6 + 40 11

Figure A
Curves used to determine time required to accelerate induction motor and blower

Duty Cycles

Sales Orders are often entered with a note under special features such as:

-----"Suitable for 10 starts per hour"
Or
----"Suitable for 3 reverses per minute"
Or
-----"Motor to be capable of accelerating 350 lb.ft.²"
Or
-----"Suitable for 5 starts and stops per hour"

Orders with notes such as these can not be processed for two reasons.

1. The appropriate product group must first be consulted to see if a design is available that will perform the required duty cycle and, if not, to determine if the type of design required falls within our present product line.
2. None of the above notes contain enough information to make the necessary duty cycle calculation. In order for a duty cycle to be checked out, the duty cycle information must include the following:
   1. Inertia reflected to the motor shaft.
   2. Torque load on the motor during all portions of the duty cycle including starts, running time, stops or reversals.
   3. Accurate timing of each portion of the cycle.
   4. Information on how each step of the cycle is accomplished. For example, a stop can be by coasting, mechanical braking, DC dynamic braking or plugging. A reversal can be accomplished by plugging, or the motor may be stopped by some means then re-started in the opposite direction.
   5. When the motor is multi-speed, the cycle for each speed must be completely defined, including the method of changing from one speed to another.
   6. Any special mechanical problems, features or limitations.

Obtaining this information and checking with the product group before the order is entered can save much time, expense and correspondence.

Duty cycle refers to the detailed description of a work cycle that repeats in a specific time period. This cycle may include frequent starts, plugging stops, reversals or stalls. These characteristics are usually involved in batch-type processes and may include tumbling barrels, certain cranes, shovels and draglines, dampers, gate- or plow-positioning drives, drawbridges, freight and personnel elevators, press-type extractors, some feeders, presses of certain types, hoists, indexers, boring machines, cinder block machines, key-seating, kneading, car-pulling, shakers (foundry or car), swaging and washing machines, and certain freight and passenger vehicles. The list is not all-inclusive. The drives for these loads must be capable of absorbing the heat generated during the duty cycles. Adequate thermal capacity would be required in slip couplings, clutches or motors to accelerate or plug-stop these drives or to withstand stalls. It is the product of the slip speed and the torque absorbed by the load per unit of time which generates heat in these drive components. All the events which occur during the duty cycle generate heat which the drive components must dissipate.

Because of the complexity of the Duty Cycle Calculations and the extensive engineering data per specific motor design and rating required for the calculations, it is necessary for customer to refer to an electrical engineer for motor sizing with a duty cycle application.