Require improved accuracy for your bulk materials weighing application?

Have you been told by a sales guy recently “we can provide you with a unbalanced/rotary vibrator motor feeder that will do the job, we will just fit an inverter”?

Yes this solution maybe a lower cost solution and yes it will get a sale for the sales guy, but it may leave you with many years of headaches trying to get the correct accuracy.

“So let’s get back to basics”

The use of a electromotive drive system for driving a vibrating feeder is one of the original solutions to provide driven forces to a vibrating feeder. Over the years many types of electromotive drive systems have been utilised, from mechanical rod linkages (thrust cranks) to the use of Unbalanced/Rotary motors. The use of unbalanced vibrator motors is now the most common solution when specifying a vibrating feeder for conveying bulk materials.

“How do they work”

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As a forced drive system, unbalanced vibrator motors are commonly used in pairs. Each motor is orientated so it lies in the same plane as the other motor. Often they are located at opposite sides of a feeder or beside each other, fixed to a motor beam either below or above the feeder trough. For vibrating feeders, motors are generally installed at between 25° and 30° relative to the base of the feeder trough. Both motors rotate in opposite directions to each other so that the motors produce a force along the action line upwards towards the trough. This linear periodic force causes the trough to oscillate. If the maximum trough vertical acceleration is larger than the acceleration of gravity (g), the bulk solid will detach from the trough and start to fly. The bulk solid is forced to perform ballistic flight in the conveying direction when the trough is moving backwards. Gravity then causes the bulk solid to fall down towards the trough base. The contact phase will last till the next bulk solid throw.

“How do the motors produce the force”

Each motor has an eccentric mass fixed to either end of the motor shaft, as both motors contra rotate the resultant forces produce an oscillation amplitude along the force action line. Unbalanced motors can be supplied in different operating speeds, the design of the vibrating feeder has to be specially chosen so to ensure the feeder is strong enough and to therefore prevent premature failure of the feeder trough structure.

“What if I want to adjust the material flow rate”

With unbalanced motors it is possible to adjust the amount of centrifugal force generated by each motor by stopping and isolating the machine, removing the end covers and then resetting the eccentric weights. This is more of a commissioning task and is not recommended for general everyday adjustments.

“What about in-service adjustments to material flow rate”

The use of an frequency controller (inverter) can provide a suitable solution for reducing the flow rate during operation. Flow rate is reduced by reducing the machines frequency and therefore reducing the rotational speed of the motor. must be noted that it is only possible to reduce the electrical frequency from 50Hz to 20Hz. Therefore as an example, a 4 pole motor that rotates at 1500 RPM can only be reduce to 600 RPM. It is worth noting that the lowered frequency can lead to an unreliable flow rates depending on material properties. If the machine frequency is tuned to close to the natural frequency of the springs, the resonance vibration leads to instable flow rate.

“Why do these machines jump around when starting and stopping”

Due to the fact unbalanced motors create vibration by rotating this means that their operating frequency is only achieved once the motors are running at full speed. This therefore means that the as the motor accelerates up to top speed the frequency of the motors will be equal to the supporting elements natural frequency at some point during start up or stopping. When this occurs the phenonium known as resonance occurs.

“So what is resonance”

Resonance and in the case mechanical resonance occurs when the forced frequency that is produced by the two unbalanced motors becomes equal to the natural frequency of the supporting element. When this occurs the vibrating feeder will seem to become unstable and jump around some people say that the units has reached its “hypercritical operation”. If the forced frequency were to remain equal to the systems natural frequency this effect is often called “disastrous resonance” as if this were to occur within a building structure severe damage could occur. Luckily this does not occur with unbalanced motors, as resonance is only experienced for a very short time, normally less than a second.

“What about when the machine stops”

When an unbalanced motor feeder is stopped the complete feeder has to run down through all the frequencies until the motors have stopped. This therefore means the feeder will run through resonance as it did at start up. During this time the material being conveyed will still fall from the end of the trough as there is no way of stopping the movement until the motors have come to a complete rest.

“How can this be improved”

It is possible to utilise a braking unit to reduce the effect of resonance and run down time for the vibrating feeder. The braking unit applies a DC voltage to the motor windings and in effect works like an mechanical friction brake stopping and then holding the eccentric weights. If using an inverter this function will be part of the inverter parameters and function.

“Problem solved then”

Well the use of a braking unit will improve the stopping time and effect of resonance but it will not prevent all the material from falling from the end of the trough or what is called “in flight” material.

This is where the use of another type of vibrator maybe be more suitable.

“So what can we use instead”

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When improved accuracy is required for handling of bulk materials the use of an electro magnetic vibrator drive unit should be utilised. These drive units are a spring mass system, the units are operated by making use of the resonance of the oscillation system. In the same way as the unbalanced motors a single electro magnetic drive unit is installed in the same action line on a vibrating feeder. The drive unit is connected to a controller unit that must be selected to match the drive unit. The drive unit has two physical sides, one called the working side that is fastened to the vibrating tray and the other called the free side. The working side is firmly connected to the trough and contains an electrical coil. The free side contains vibrator weight and auxiliary weights. Between these two sides is a leaf spring pack, which as a whole creates the spring mass system.

 “But how does it work?”

Without going into the mathematical formula surrounding the operation of the drive unit, basically the drive units are tuned to suit the weight of the vibrating feeder, this can either be completed in the factory or on site by adding or subtracting the auxiliary weights on the free side of the drive unit. Once tuned the system will be operating to the correct natural frequency as specified for the specific drive unit. The drive unit operates by the controller providing a voltage to the magnetic coil. When operating the coil energises and therefore brings the coil and armature together. This is possible due the movement provided by the leaf spring pack. When the controller de-energises the coil, the free mass is released. This allow the free mass side to be propelled away in one direction and the working side (including the vibrating trough) to be propelled in the other direction. This operation occurs 50 times a second for a 50Hz drive unit for example.

“What about adjusting material flow rate”

As a electro magnetic drive unit operates at a fixed frequency, adjustment can be made by reducing the voltage transmitted by the drive unit controller. This reduction in voltage therefore reduces the distance the free mass moves and therefore reduces the amplitude of the vibrating feeder trough. This method of adjustment allows for accurate adjustment of flow rate, which is especially important for many process applications including weighing and dosing.

“What about starting and stopping”

Unlike the unbalanced motor feeders, electro magnetic feeders do not suffer from the effects of resonance. This is due to the fact that the drive is a fixed frequency system which, once started will operate instantly and in the same way when stopped, the vibration will stop instantly. This therefore reduced in flight material falling from the end of the vibrating trough considerably. With the correct material bed depth being selected the amount of in flight material for a weighing application and be minimised significantly and if a customer requires even greater accuracy then it is possible to install pneumatic gates to adjust the material bed depth to a minimum towards the end of the weighing process. So therefore improving accuracy even further.

If you want to know more about what different types of vibrating feeder AViTEQ can provide please visits:- www.aviteq.com/en/products/conveying-technology/

Email:- [email protected] or Call:- +44 (0) 7419 765 133