Motors duty cycle

This is a type of duty in which load and speed both vary non-periodically, unlike the periodic duty cycles noted above. The motor now supplies variable load demands at varying speeds and varying overloads, but within the permissible temperature rise limits. It is a duty similar to duty cycle axcept that sometimes the overloads may exceed the full load but are within the thermal withstand limit of the motor (Figure 3.9) ... 

The nearest standard rating to this is 7.5 kW, and a motor of this rating will suit the duty cycle. To ensure that it can also meet the torque requirement of 10.5 kW, it should have a minimum pull-out torque of 10.5/7.5 or 140% with the slip at this point as low as possible so that when operating at 140%... 

This output coixesponds to a continuous duty of drive. It must be suitably corrected for the duty cycle the motor has to perform (see equation (3,11)), i.e. 

Duty types. S, S, and. S5 as discussed in Chapter 3 are normally applicable to crane and hoist motors. For duty types S4 and 5. the duty cycle per unit time is greater than S, . The most important factor is the number of switching operations per hour. A temperature rise in the motor occurs during acceleration, braking and reversing. 

Duty cycles Continuous duty (CMR) (S ) Periodic duties Factor of inertia (FI) Pleating and cooling characteristic curves Drawing the thermal curves Rating of short motors Equivalent output of short time duties Shock loading and use of a flywheel... 

In order to select the most economical drive for an Injection molding machine, the duty cycle must be calculated (3). This calculation must be made for each specific application to determine the applicable motor rating. A typical duty cycle Is shown In Figure 3. The motor rating Is defined In terms of 1) Nameplate horsepower and service factor (SF) and 2) Breakdown torque In terms of full load. One first calculates the RMS HP via Equation (1). ... 

Table 1 summarizes these calculations for the typical duty cycle shown In Figure 3. The required motor horsepower Is given In Equation (2). 

Torque requirements must also be considered. The motor must be capable of carrying the peak horsepower (torque) value from the duty cycle at 90 percent voltage. Motor breakdown torque Is reduced by the square of the voltage. The relationship Is given In Equation (5). 

A tabulation of prices and ratings would lead one to the proper motor selection. The motor should be sized to operate at high efficiency over the anticipated duty cycle. This allows for a minimum power factor correction. In the low duty part of the cycle, a smaller motor Is more efficient. It Is strongly recommended that the power factor correction be used to make the motor operate at Its peak efficiency over the total cycle. Buyers have a tendency to overspecify the required HP which results In less than peak efficiency. 

It should be emphasized that special duty cycles for motors, e.g. arduous starting conditions with high moments of inertia or intermittent operation with short duty cycles, restrict the field of application for e -motors (or generally exclude e -motors). 

Controlled molecular motion was achieved in several synthetic systems,including DNA machines that use a conformational change of the double helix to develop torque, and propeller-like molecules that can be rotated in a specific direction on a molecular axle by a sequence of chemical transformations. The hybridization and dissociation of DNA strands was also used to make DNA motors that extend and contract like pistons in response to DNA "fuel." One such device, unlike many artificial molecular motors, is capable of free ranning rather than requiring intervention at different points in the duty cycle. ... 

Some of the factors that engfneets consider when selecting a motor for an application are (a) motor type, (b) motor speed (rpm), (c) motor performance in terms of torque output, (d) efficiency, (e) duty cycles, (f) cost, life eaqpectancy, (h) noise level, (i) maintenance and service requirements. Most of these are sdfeaqjlanatoiy. We discussed speed and torque in previous chapters next we will discuss motor types and duty cycles. 

A variation on PWM is pulse position modulation (PPM), also known as pulse period modulation or pulse frequency modulation (PFM). In this case, the duty-cycle pulse remains on for a fixed time while the base period is varied. The frequency of the pulses (how close together the pulses are) determines the voltage level. The neuromuscular system is an example of a pulse position modulation system. A muscle is made up of many discrete motor units. A motor unit has an all or nothing response to a nerve impulse in much the same way that a nerve impulse is a nonlinear (thresholded) all-or-nothing event. The level of sustained force output of a motor unit is dictated by the frequency of incidence of the nerve impulses, with the motor units dynamics [mechanical properties—inertial and damping properties (acts as a mechanical filter)] holding the force output smooth between incoming impulses. The motor unit is pulse frequency modulated by the nervous system. 

DEVELOPMENT OF DUTY CYCLE FOR INDUCTION MOTOR CHARACTERISTICS... 

Duty cycle calculations where motor current is derived directly from the motor torque requirements are accurate for separately excited DC motors and synchronous motors, where torque current and armature current are equal. A double drum mine hoist operating at depth will typically have a duty cycle as represented in Figure 1. This duty cycle shows a transition from motoring to regeneration in the full speed zone of the shaft. 

Figure 1. Hoist duty cycle for induction motor with 25% magnetizing current...

For induction motors, the total current can be approximated by the vector sum of the torque producing component and the magnetising or flux producing component of motor current. These two current vectors are in quadrature, and as the hoist torque approaches zero, the motor current approaches magnetising current. Electrical RMS calculations for hoist motors must be based on motor current, and so the calculation based on torque current only is not accurate for induction motors, and under-estimates the induction motor current. For greatest accuracy the duty cycle must be calculated with the correct motor characteristics. 

In the duty cycle shown in Figure 1, for an induction motor with 25% magnetizing current, the RMS error when using only the torque current plot for RMS calculation is 2% below the actual RMS current. 

Figure 4. Hoist duty cycle showing motor current and drive input kVA...

For the duty cycle example of a double drum production hoist as shown in Figure 4, the following are the selected motor parameters ... 

SEPARATE DUTY CYCLE MODEL FOR DRIVE INVERTER AND MOTOR... 

Typical RMS duty cycles for drum hoists assume a continuous process with infinite time constant. Drive equipment time constants are not normally taken into account in the Duty Cycle calculation commonly used in the industry, which almost universally sets the hoist motor RMS and peak power requirements. 

In practical terms, the typical production hoist duty cycle is of the order of 200-300 seconds. With an estimated motor time constant of 20 minutes, a steady state RMS calculation is a good approximation. Drive power equipment consisting of switching devices, heatsink assembly and copper bus have a shorter time constant. Typically a time constant of 5 minutes is used to more accurately approximate the heating in the drive power components and heatsinks. The hoist duty cycle length is the same order of magnitude as the time constant of certain drive power components, and so the steady state model typically used cannot accurately determine the cychcal thermal duty of these components. 

Hoist RMS Power calculations must therefore be different for motor and drive components. Motor current is provided by the drive inverter, which is subject to the same duty cycle as the motor. A model giving the 5 minute RMS as well as the 20 minute RMS duty is required in order to correctly model the heating for both the drive inverter (output) section and the motor. By calculating the 5 minute and 20 minute RMS thermal duty over a period of 30 minutes, a more accurate estimate of the component thermal loading can be studied. 

Figure 7 shows the 5 minute and 20 minute RMS calculations for the same duty cycle. As can be seen in Figure 7, the 5 min RMS has a range which varies significantly from the Constant RMS model. The 20 minute RMS also shows more accurately how the motor heating takes place, and the variations that occur throughout the hoisting cycle. 

The AC drive has a shorter thermal time constant than the induction motor, and a separate RMS duty cycle calculation is required for both the drive and the motor using the applicable thermal time constant. 

Using two motors on the Rock Hoist, which are interchangeable with the motor on the Personnel/Material Hoist, provided a motor rating in excess of optimal duty cycle requirements for the Rock hoist. Calculations have been produced to estimate the effect of using overrated versus optimcd size motors on the rock hoisting system power consiunption. 

Motor-driven timers provide acceptable performance for SIF applications, such as burner purge timing. Most motor-driven timers require a locking device or appropriate modification to eliminate tampering with critical settings. Motor-driven timers are limited in timing resolution and the ability to handle high duty cycles. 

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