How to select  Servo Motor Size    

     

     In this article, we'll examine the core process involved servomotor sizing.(How to select  Servo Motor Size) Sizing is that the process of choosing the simplest motor for a given application. There are many factors that are wont to determine the simplest motor. a number of the factors that determine the best motor are listed here ,

         The motion profile along side this technique inertia determine the speeds, accelerations... ...and torques that the motor must be ready to produce. Regeneration capacity must even be considered. These factors must be balanced against others like cost, encoder resolution... ...environmental ratings, power requirements or limitations in available space.

        While all of those factors are important... ...the core process in servomotor sizing involves four key factors. as long as the inertia ratio and motor speed are appropriate for the appliance ... ...both the max torque and therefore the RMS torque must lie within the motors capability.

       

What is The primary key sizing factor?

     The primary key sizing factor is that the "Moment of Inertia" ratio. the whole moment of inertia during a servo system are often divided into two parts... ...

        1) The motor inertia 

        2) The load inertia. 

         The instant of inertia ratio is that the Load's moment of inertia divided by the Motor's moment of inertia. Any rotating object features a moment of inertia. the instant of inertia may be a measurement of how difficult it's to vary the rotating velocity of that object. 

       When sizing... ...the moment of inertia is usually written with the letter 'J' and is usually mentioned simply as inertia. The motor inertia 'Jm' is a component of the planning of the servomotor... ...and is listed within the catalog. However the load inertia 'JL'... ...often consists of the many components Each component that's moved by the motor contributes to the entire load inertia. the entire load inertia is found by using the right equations for every component. 

       

 How to calulate inertia ratio?

      Inertia ratios around 5:1 are typical for several applications. Performance tends to travel up because the inertia ratio is lowered. Often right down to 2:1, 1:1 or lower. But when high performance isn't as critical... ...ratios of 10:1, 100:1 or maybe higher aren't uncommon. generally , simple control loop tuning and machine performance go up because the inertia ratio goes down. So if all other factors are equal a lower inertia ratio is best .

        However, an excessively low inertia ratio also can indicate an excessively large... ...and therefore expensive and hulking motor with little performance increase. If inertia ratio is that the limiting sizing factor... ...it pays to completely understand the applications performance requirements... ...and choose the inertia ratio accordingly. Several motors that provide an appropriate inertia ratio could also be available. therefore the task remains to seek out the littlest motor... ...that still has the power to supply the speed and torque required for the appliance .

       A motors speed and torque capability... ...is described within the catalog using a private speed torque curve for every motor. The speed torque curve shows the motor speed on the vertical axis... ...and the motor torque on the horizontal axis. First notice that the speed torque curve has two regions... ...continuous and intermittent. If the mixture of torque and speed required by the motor is found within the continuous region... ...then the motor can produce that torque and speed forever with none chance of overheating the motor. If the mixture of torque and speed produced falls within the intermittent region... ...then the motor can only produce that speed and torque for a limited amount of your time . 

     If that point is exceeded the motor will begin to overheat. to stop damage thanks to overheating the amplifier automatically disables the motor... ...and enters an alarm state if the deadline is exceeded. But when short bursts of high torque are required... ...such as during acceleration and deceleration. The motor can run within the intermittent region safely... ...producing the maximum amount as 3 times the rated torque for 3 seconds or longer. The applications RMS torque however, must lie within the continual region. If any combination of speed and torque required lies outside both the continual and intermittent region... ...then the motor isn't capable of manufacturing that combination of speed and torque.

      When selecting a motor it's imperative to make sure that the speed torque curve is employed effectively. The speed torque curve displays several points of interest. "Rated Torque" is that the maximum torque the motor can produce continuously at rated speed and lower... ...and is restricted by motor heating. This rated torque is given the worth of 100% torque. Likewise "Rated Speed" is that the highest to hurry at which rated torque is out there . The motor can continuously run faster than rated speed... ...but the torque available drops significantly the faster the motor runs. The motors maximum attainable speed is listed at the highest of the speed torque curve... ...and the motors maximum torque is at the far right. the utmost torque available is usually 3 times the continual torque... ...and can be applied for 3 seconds or longer. 

     While the motors capability is described by the speed torque curve... ...the application requirements are best illustrated using the speed profile and a torque profile. 

       The speed profile may be a graphical representation of the motor speed versus time... ...and the torque profile... ...is a graphical representation of the motor torque required... ...for the machine to follow the speed profile during that very same time. This graphic illustrates a typical speed and torque profile required to perform a positioning move... ...for a horizontal actuator with no external forces. The speed increases at the start of the move accelerating to a traverse speed... ...and then decreases decelerating to a stop. thanks to its shape, this is often mentioned as a trapezoidal speed profile... ...or a trapezoidal move. The torque at the start of the trapezoidal move is highest... ...because mechanical friction must be overcome... ...and because the load must be accelerated from rest.  now of highest torque is known as the "Max Torque". Once the traverse speed is reached a nominal level of torque must be applied to beat friction... ...and maintain speed. To decelerate the load... ...a reverse torque is required. The reverse torque during deceleration isn't  high because the forward torque during acceleration... ...since friction also helps decelerate the load... ...and when friction torque is high a forward torque could even be required during deceleration... ...so that the motor doesn't hamper too quickly.

       it is vital to form sure that the motor can produce the required max torque at the appliance speed. The max torque of application speed... ...ideally falls within the intermittent region of the motors speed torque curve. It can also fall within the continual region... ...but this could be a symbol that the motor is over-sized. 

     Another torque calculation is critical for sizing the "RMS Torque". RMS torque could also be a time weighted average of the torque during a whole machine cycle... ...and represents an equivalent steady-state torque level.

   as an example 

       a servomotor with 1.2 Nm RMS torque... ...will experience the same heat rise if it produces 1.2 newton meters of constant torque. So it is also important to form sure that the RMS torque at application speed... ...falls within the continual region of the speed torque curve. as long because the inertia ratio speed max torque and RMS torque all fit the motor as described. Then the motor is correctly sized. However a multitude of other factors must even be considered... ...depending on the precise requirements of the appliance ... ...and may require a rather larger or smaller motor to be used.  now of highest torque is known as the "Max Torque". Once the traverse speed is reached a nominal level of torque must be applied to beat friction... ...and maintain speed. To decelerate the load... ...a reverse torque is required. The reverse torque during deceleration isn't  high because the forward torque during acceleration... ...since friction also helps decelerate the load... ...and when friction torque is high a forward torque could even be required during deceleration... ...so that the motor doesn't hamper too quickly.

       it is vital to form sure that the motor can produce the required max torque at the appliance speed. The max torque of application speed... ...ideally falls within the intermittent region of the motors speed torque curve. It can also fall within the continual region... ...but this could be a symbol that the motor is over-sized. 

     Another torque calculation is critical for sizing the "RMS Torque". RMS torque could also be a time weighted average of the torque during a whole machine cycle... ...and represents an equivalent steady-state torque level.

   as an example 

       a servomotor with 1.2 Nm RMS torque... ...will experience the same heat rise if it produces 1.2 newton meters of constant torque. So it is also important to form sure that the RMS torque at application speed... ...falls within the continual region of the speed torque curve. as long because the inertia ratio speed max torque and RMS torque all fit the motor as described. Then the motor is correctly sized. However a multitude of other factors must even be considered... ...depending on the precise requirements of the appliance ... ...and may require a rather larger or smaller motor to be used.