The formulas and calculations which appear below should be used for estimating purposes only. It is the responsibility of the customer to specify the required motor Hp, Torque,
and accelerating time for his application. The salesman may wish to check the customers specified values with the formulas in this section, however, if there is serious doubt
concerning the customers application or if the customer requires guaranteed motor/application performance, the Product Department Customer Service group should be contacted.
Rules Of Thumb (Approximation)
At 1800 rpm, a motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3-phase motor draws 1 amp per hp
At 460 volts, a 3-phase motor draws 1.25 amp per hp
At 230 volts a 3-phase motor draws 2.5 amp per hp
At 230 volts, a single-phase motor draws 5 amp per hp
At 115 volts, a single-phase motor draws 10 amp per hp
Symbols
I = current in amperes
E = voltage in volts
KW = power in kilowatts
KVA = apparent power in kilo-volt-amperes
HP = output power in horsepower
n = motor speed in revolutions per minute (RPM)
ns = synchronous speed in revolutions per minute (RPM)
P = number of poles
f = frequency in cycles per second (CPS)
T = torque in pound-feet
EFF = efficiency as a decimal
PF = power factor as a decimal
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:
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
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:
a. Inertia reflected to the motor shaft.
b. Torque load on the motor during all portions of the duty cycle including starts, running time, stops or reversals.
c. Accurate timing of each portion of the cycle.
d. 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.
e. 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.
f. 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, keyseating,
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
the sales engineer to refer to the Product Department for motor sizing with a duty cycle application.
