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Solenoid or Relay type actuator?
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Solenoids
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| Description |
Solenoids or relays. An article on how to decide whether your application suits solenoids or relays
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| Author |
John Homer
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From coin-drop mechanisms to mains safety trips, the world is full of electrically-operated actuators of one kind or another. John Homer of BLP offers some guidance about selection and design.
There are many engineering solutions for moving a component from A to B, but if the distance involved is short -- between about 1mm and 25mm -- the electromechanical actuator is likely to be among the front-runners for consideration.
There are two basic types: the direct-acting solenoid, in which a moving core or plunger is drawn into a coil when current flows; and the relay-type actuator in which a hinged plate is pulled against the end of the coil. Why would you use one type rather than the other? Often the most suitable choice will be clear after considering just four key factors:
• The force being moved
• The range of movement
• Electrical power available
• Space available
In general terms, moving core-solenoids are capable of providing a greater mechanical force and longer stroke.
Relay-type actuators tend to be used where limited power is available, and they can be faster (depending on other factors). However, there are numerous applications where either type could do the job. In this case I would advise considering the solenoid first. You are much more likely to find a suitable device among standard ranges, or one requiring very little customisation.
Relay-type actuators are far less standardised and custom design tends to be the norm. Solenoid characteristics. Selecting the right moving-core solenoid principally involves matching it to the required force/stroke curve, bearing in mind the available electrical power and space. Most devices are available for either pull or push action, and they range from fist-sized heavy weights capable of lifting kilograms to PCB-mounted miniatures that scarcely cover a 5p coin.
For such apparently simple devices, the characteristics of moving core-solenoids are surprisingly dependent upon their detailed design. For example, varying the end style of the plunger between acutely conical and flat can increase the pull force by as much as five times in some situations.
Duty cycle and hold
A solenoid will always rise in temperature to some extent when operating, depending mainly upon electrical power and duty cycle. It can provide a much greater force for a 10% duty cycle than for continuous operation. If an extended hold is required, a self-latching solenoid may be the answer. These incorporate permanent magnets to provide zero-power hold. They are operated by a short electrical pulse and release by a reverse-polarity pulse.
AC or DC operation
Solenoids are available for either AC or DC operation, the DC type being by far the most common.
AC solenoidstend to be more powerful in the fully-open position than their DC counterparts. They may also be specified where a DC power supply is ruled out on grounds of space or cost. It is important to ensure that an AC solenoid cannot become stuck in the open position since there is a high risk the coil will burn out. And like any other AC device, it may generate some mains hum if not well designed and installed.
Specifying a relay-type actuator
Devices of this type may be used in preference to solenoids where limited electrical power is available. Maximum operating speeds can be higher, and the voltage thresholds can more easily be adjusted by means of the integral return spring. They are not suitable where a relatively high force or long stroke is required. Specification involves much the same kind of considerations as for solenoids. In general, a custom design will be necessary, but it will often be possible to use a standard coil and frame assembly with a customised armature.
Who is responsible for specifying and designing-in these devices -- the mechanical or electrical engineer? Unfortunately, both must be involved, and the supplier's nightmare scenario is when the two engineers do their work in isolation. It is not uncommon for the resultant specification to be impossible to realise in any practical device. They need to talk together from the start. And even better, they should talk at a very early stage to the supplier's technical support engineers, who know most of what there is to know about solenoids and actuators and can save everybody no end of time, trouble and expense.
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