Abstract:
A solenoid includes a magnetic frame, a bobbin having a length, a hold coil, a pick up coil having a length, a fixed pole, a movable armature having a length, and a return spring biasing the armature away from the pole. The solenoid includes a pick up state when the armature and the pole are separated by a magnetic gap, and a holding state when the armature and the pole are proximate each other. The pick up coil is wound around the bobbin for a portion of the length of the bobbin and the hold coil is wound around the bobbin for a remaining portion of the length of the bobbin. The length of the pick up coil is about the same as the length of the armature and is less than the length of the bobbin.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The disclosed concept pertains generally to electromagnetic actuators and, more particularly, to solenoids. 
         [0003]    2. Background Information 
         [0004]    Electromagnetic actuators, such as solenoids, are used for many different applications. A solenoid provides an electromagnetic force in response to electrical power applied to its terminals, Solenoids can include an air core or an iron core. In iron core solenoids, a magnetic frame cooperates with magnetic flux produced by a coil in order to provide a closed, low reluctance magnetic path for the magnetic flux. The coil is wound on a bobbin and mounted inside the magnetic frame. Solenoids also include a moving core or armature and a fixed core or pole. The magnetic flux completes a path from the pole through a magnetic gap to the armature to the magnetic frame and back to the pole. In this complete travel of the magnetic flux, there is some amount of magnetic flux (i.e., a leakage flux) which does not reach the armature. This leakage flux is wasted and cannot contribute toward producing a magnetic force. Therefore, for effective and efficient use of solenoids, the amount of leakage flux should be minimized, in order that the magnetic force can be maximized. 
         [0005]    Referring to  FIG. 1 , a solenoid  2  includes a magnetic frame  4 , a hold coil  6 , a pick up coil  8 , a bobbin  10 , a fixed core (pole)  12 , a moving core (armature)  14 , a return spring  16  and a plunger  18 . Solenoids, such as the solenoid  2 , have two extreme positions including a first position (or pick up state) when the armature  14  and the pole  12  are separated by a maximum possible gap (or magnetic gap  20  of  FIGS. 1 and 2 ), and a second position (or holding state) when the armature  14  and the pole  12  are proximate (e.g., almost touching) each other (as shown in phantom line drawing in  FIG. 1 ). The solenoid pick up state occurs when an electrical power supply (not shown) is not provided to the coil terminals (not shown) for the hold coil  6  and the pick up coil  8 . After the electrical power supply is provided to the coil terminals in the pick up state, the coils  6 , 8  carry some amount of current depending upon the solenoid state, the coil impedance and the number of coil winding turns. The number of turns (N) and the current (I) carried by the coils  6 , 8  determine the total NI across the coil terminals. The amount of NI across the coils  6 , 8  and the magnetic gap  20  determine the value of the magnetic flux in the solenoid  2 . 
         [0006]    The pick up coil  8  and the hold coil  6  can be wound either in series or in parallel. Normally, there is no electrical connection between the coils  6 , 8  in the solenoid  2 , and they are electrically connected in series or in parallel through an “economizer” circuit (not shown). A suitable “economizer” or “cut-throat” circuit (not shown) can be employed to de-energize the pick up coil  8  in order to conserve power and minimize heating in the solenoid  2  in the holding state. The economizer circuit can be implemented by a timing circuit (not shown) which pulses the pick up coil  8  only for a predetermined period of time, proportional to the nominal armature operating duration. This is achieved by using a dual coil arrangement in which there is a suitable relatively low resistance circuit or coil and a suitable relatively high resistance circuit or coil in series with the former coil. Initially, the economizer circuit allows current to flow through the low resistance circuit, but after a suitable time period, the economizer circuit turns off the low resistance path. This approach reduces the amount of power consumed during static states (e.g., relatively long periods of being energized). 
         [0007]    The example winding approach employed in  FIG. 1  is such that the pick up coil  8  is wound first across about the entire height (with respect to  FIG. 1 ) of the bobbin  10  and then the hold coil  6  is wound over about the entire height (with respect to  FIG. 1 ) of the pick up coil  8 . 
         [0008]    There is room for improvement in solenoids. 
       SUMMARY 
       [0009]    According to one aspect, a solenoid includes a magnetic frame, a bobbin having a length, a hold coil, a pick up coil having a length, a fixed pole, a movable armature having a length, and a return spring biasing the armature away from the pole. The solenoid includes a pick up state when the armature and the pole are separated by a magnetic gap, and a holding state when the armature and the pole are proximate each other. The pick up coil is wound around the bobbin for a portion of the length of the bobbin and the hold coil is wound around the bobbin for a remaining portion of the length of the bobbin. The length of the pick up coil is about the same as the length of the armature and is less than the length of the bobbin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0011]      FIG. 1  is a vertical cross-sectional view of a solenoid in which the height of the pick up coil is about the same as the height of the bobbin. 
           [0012]      FIG. 2  is a plot showing leakage flux for the solenoid of  FIG. 1 . 
           [0013]      FIG. 3  is a vertical cross-sectional view of a solenoid in accordance with embodiments of the disclosed concept in which the pick up coil is wound near to the armature and the height of the pick up coil is about the same as the height of the armature. 
           [0014]      FIG. 4  is a plot showing leakage flux for the solenoid of  FIG. 3 . 
           [0015]      FIG. 5  is a simplified cross-sectional view of the bobbin, pick up coil and hold coil of  FIG. 3 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
         [0017]    As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. Further, as employed herein, the statement that two or more parts are “attached” shall mean that the parts are joined together directly. 
         [0018]    The disclosed concept is described in association with an example solenoid, although the disclosed concept is applicable to a wide range of different solenoids. 
         [0019]    The disclosed concept employs a dual coil arrangement in a solenoid for effective and efficient reduction of the amount of leakage flux. 
         [0020]      FIG. 2  shows the corresponding flux distribution in the solenoid  2  of  FIG. 1 . There is a relatively high amount of leakage flux  22  from the pole  12  to the magnetic frame  4 . Because of this relatively high leakage flux  22 , the useful flux reaching the armature  14  is not sufficient to move the armature towards the pole  12  (since it does not produce sufficient force) which results in a greater NI requirement. The increased requirement of NI for a given number of turns of the coil can be achieved by providing more current through the coil (and a higher pick up voltage). This relatively higher leakage flux  22  reduces the overall efficiency and effectiveness of the solenoid  2 . 
         [0021]    At the start of the travel of the armature  14  in the pick up state, the magnetic gap  20  is maximum which, in turn, results in a maximum reluctance of the corresponding magnetic circuit. The solenoid  2  of  FIG. 1  produces the minimum magnetic flux for a given NI in the pick up state which, in turn, results in the minimum magnetic force. In order to produce sufficient NI in the pick up state, the pick up coil  8  has to carry a relatively higher amount of current (resulting in a relatively higher pick up voltage), The magnetic flux completes its path from the pole  12  through the magnetic gap  20  to the armature  14  to the magnetic frame  4  and back to the pole  12 . In this complete travel of the magnetic flux, there is some amount of the magnetic flux (i.e., the leakage flux  22  of  FIG. 2 ) which does not reach the armature  14 . In the pick up state, the magnetic flux produced by the pick up coil  8  is minimum for a given NI, such that it becomes very important to minimize the amount of flux leakage. 
         [0022]    As the armature  14  starts travelling toward the pole  12 , the magnetic gap  20  starts to reduce, which results in less magnetic reluctance and more magnetic flux. This phenomenon is valid until the holding state and it gradually reduces the NI needed to hold the armature  14  in the holding state. The amount of flux leakage from the pole  12  to the magnetic frame  4  is more in the pick up state than the holding state since the magnetic gap  20  is reduced in the holding state. As a result, it becomes very challenging to control the leakage flux  22  ( FIG. 2 ) in the pick up state in order to get the desired useful magnetic flux (passing through the armature  14 ) and the resulting magnetic force. Otherwise, the solenoid  2  will need more Ni across the pick up coil  8  to drive the armature  14  if the leakage flux  22  is greater. 
         [0023]    There are multiple ways of winding coils around a bobbin. Depending upon the winding approach, the magnetic reluctance for the magnetic flux is changed which, in turn, changes the amount of the leakage flux from the pole to the magnetic frame. 
         [0024]    Referring to  FIG. 3 , in accordance with the disclosed concept, a dual coil arrangement of two direct current (DC) coils  32 , 36  is employed by a solenoid  30 . A first or pick up coil  32  has a relatively low resistance and employs relatively lower AWG coil windings. A second or hold coil  36  has a relatively higher resistance and employs relatively higher AWG coil windings. Initially, in the pick up state, only the pick up coil  32  carries the current, while in the holding state, the electrical power supply (not shown) is switched to the hold coil  36  through a suitable circuit (e.g., without limitation, an economizer electronic circuit, which functions like an RC timer) (not shown). In the pick up state, only the pick up coil  32  carries current; and, in the holding state, either the hold coil or both coils (depending upon the electrical connection in the economizer electronic circuit) carry the current. The solenoid  30  is in a non-energized position (ready for pick up) with a return spring  42  forcing an armature  40  upward (with respect to  FIG. 3 ) to a stop  48  in order to provide the maximum possible gap (Magnetic gap  50  between the armature  40  and pole  38  of  FIGS. 3 and 4 ), There is also a plunger  52  connected to the armature  40  and protruding through an opening  54  in magnetic frame  34 . 
         [0025]    As a non-limiting example, the relatively low resistance pick up coil  32  has a resistance of about 4.5 Ω at 25° C. and NI of 2000 AT (ampere-turns), and the relatively high resistance hold coil  36  has a resistance of about 40 Ωat 25° C. and NI of 4100 AT. 
         [0026]    For efficient operation of a solenoid, such as the solenoid  30  of  FIG. 3 , a maximum flux should pass through its armature  40  in order that the magnetic force on such armature  40  can be maximized with a given NI. Since there is relatively more leakage flux  46  ( FIG. 4 ) in the pick up state than the holding state because of the greater magnetic gap  50 , the position of the pick up coil  32  with respect to the armature  40  is very important. Hence, the pick up coil  32  is preferably wound as close as possible to the armature  40  in order to minimize the leakage flux. 
         [0027]    The solenoid  30  of  FIG. 3  employs a dual coil arrangement in order to improve efficiency. The pick up coil  32  is first placed around the bobbin  44  for a portion of its height (with respect to  FIG. 3 ) but not across the complete height (with respect to  FIG. 3 ) of the bobbin  44 . Then, the hold coil  36  is placed below the bottom end  56  (with respect to  FIG. 3 ) of the pick up coil  32  in the remaining space across the bobbin height (with respect to  FIG. 3 ). Finally, the remaining turns of the hold coil  36  are wound across the complete height (with respect to  FIG. 3 ) of the bobbin  44  after the hold coil  36  and the pick up coil  32  come to the same radial level. 
         [0028]    This can be understood from  FIG. 5  and from the following non-limiting example. If the available width (W) in the bobbin  44  for the coil windings is 1.2 in. and the available height (H) is 1.3 in., then the pick up coil  32  is wound across a height (H 1 ) of 0.5 in. and a width (W 1 ) of 0.7 in. (e.g., without limitation, depending on the number of turns, the coil current, the coil resistance and the winding AWG). Then, the hold coil  36  is wound for the remaining height (H 2 =H−H 1 ) of 0.8 in. (i.e., 1.3 in. −0.5 in. in this example) and a width (W 1 ) (i.e., 0.7 in. in this example) equal to the width (W 1 ) of the pick up coil  32 , After this, the remaining turns of the hold coil  36  are wound across the complete height (H) of 1.3 in. and the remaining width (W 2 =W−W 1 ) of 0.5 in. (i.e., 1.2 in.−0.7 in. in this example). 
         [0029]    The flux plot for the solenoid  30  of  FIG. 3  is shown in  FIG. 4 . Here, the leakage flux  46  is significantly improved with respect to the leakage flux  22  of  FIG. 2 . Reduction in the leakage flux  46  results in relatively more magnetic flux passing through the armature  40  which, in turn, provides relatively more magnetic force on the armature  40 . As a result, the solenoid  30  needs relatively less NI in order to operate which results in a relatively lower pick up voltage. 
         [0030]    The height (with respect to  FIG. 3 ) of pick up coil  32  around the bobbin  44  may vary depending upon the desired force on the armature  40  and other factors, such as for example and without limitation, bobbin envelope size, AWG of the coil winding conductors, coil resistance, allowable current through the coils  32 , 36 , number of winding turns, current carried through the coils  32 , 36 , and pick up voltage. Although the height (with respect to  FIG. 3 ) of the pick up coil  32  can vary, it is preferred to wind this coil  32  having a height (with respect to  FIG. 3 ) as close as possible to the height (with respect to  FIG. 3 ) of the armature  40 . 
         [0031]    The disclosed winding method of the pick up coil  32  and the hold coil  36  around the bobbin  44  reduces the ampere-turns (NI) of each of the coils  32 , 36  and reduces the pick up voltage of the pick up coil  32 . As a result, the solenoid  30  needs less NI to operate, which results in a lower heat loss in the solenoid  30 , and reduces the weight and the overall size of the solenoid  30 . 
         [0032]    The reduction in the leakage flux  46  results in relatively more magnetic flux passing through the armature  40  which, in turn, provides relatively more magnetic force on the armature  40 . As a result, the solenoid  30  needs relatively less NI and a relatively lower pick up voltage in order to operate. 
         [0033]    While specific embodiments of the disclosed concept have been described in detail, it will be appreciated b r those skilled in the an that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.