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Electric Motors/Motor current release device


Hi, I'm looking for the correct name for a device that would be used to only release the power/current from a motor (in this case, to raise a bed) when the ON button is activated, kind of a power block I guess, and then cut off the power again when you release the button. I hope I explained that well enough. What's the correct name for that, so I can find one. Thank you in advance.


Almost but not quite enough information.  First I thought momentary switch, were pushing start activates the contacts, starts the motor, but does not "seal" or mechanically latch,   so that the single button when depressed starts the motor, and then on release opens the contacts [the speed of the release varies with the type/design of switch]

Then you threw me, with "power block" and that got my attention telling me you are needing a softer start, or NON ACROSS The line start, so it is a smoother operation, and maybe it doesn't over shoot the desired position.

Now when you say bed, not a bed like a mill bed but like an adjustable bed or hospital type bed?

So I am going to go on the assumption that this is a bed as for sleeping, but you want to adjust the back or all of it

"when the ON button is activated, kind of a power block I guess, and then cut off the power again when you release the button."

your statement above other than power block is a momentary type switch. With an adjustable bed, most are operated through a gear reducer of some sort, for reasons of torque, and for reasons of a controlled fixed speed that is fairly slow    

ie  1800 RPM motor is going to wind up very fast at full voltage, where a gearbox say 100 to 1  18 RPM.

Then I thought is, he may be talking limit switches but the power block sorta got me off that thought.

I would guess we are talking single phase as far as the motor?

Single phase motors are very hard to speed control, other than a PSC type motor   permanent split capacitor where the cap is in the circuit the entire time,  can be slowed a little, with some sort of voltage drop, rheostat or even a fixed transformer or resistors,

Yet the problems that come with reduced voltage even with PSC is the loss of torque is on a real sharp curve compared to the applied voltage, so it might slow it down some, but the struggle to produce torque can wear that motor out in no time  not a great option,   

The 2nd BEST Option for speed control is a DC or servo motor, and some beds do use permanent magnet DC motors, which hold torque while the voltage is being adjusted or ramped up to whatever acceleration levels you need,   matched with a gear reducer  there is no centrifugal over run to deal with,  and you can pretty much tweak things right in for an application such as a bed,  

Another more crude method is to use a selector switch with different values of dropping resistors at each position say a drum switch. Where a 90 volt Dc motor might have three speeds,  dropping 10 volts on one, 20, then 30 or wiring so they add in series or parallel,  but often the current is fairly high and resistors are expensive by amperage,  and home or hospital beds are low voltage high current  [relative]

The absolute best control is three phase and often not that much fuss to change out,   If the motor is a c-face to a gearbox there is likely a three phase motor with the same flange,  Im talking single to three phase not dc to three phase,   

If there is a way to install a three phase motor, then the use of a VFD   which controls frequency, will give you on a very uncomplicated VFD drive, user settings that control speed, infinite, control torque to a larger range, dynamic braking, acceleration , overload protection, reversing to lower the bed, fixed time run, fixed time run with periods of different speeds to start slow accel, then decel and even brake  Plus the drive converts the wall single phase into real three phase,  all in one controller,     cost American for say a half horsepower is somewhere in the less than $100 for the motor and now days some of the lesser lines of vfd drives around that HP rating can be had for less than $100 also,   

Can you tell me more about the specifics, I think I know where you need to go, and why  I think you need slower acel and controlled stop,  where even a momentary switch might not react quickly enough,    it all depends on the power train, are we using gears, pulleys what?

If you can tell me the type of motor   geared or not, what the actual problem is you are trying to deal with  in either a repair or a design,  I can most likely point you to the simple fix    I am really thinking you have all you need just a way to soft start it,  but I really need to know if this is a fix or design issue,   and then we can simplify things   but from deluxe control to maybe something as simple as a momentary switch  might be all that is needed and when I know the exact issue  lots of choices,   .

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Three phase/ AC DC single phase motors, controls, any problems or failures, motor installation, performance issues, connections. All other electric motors/gearboxes/apparatus. Specialty repair concerns, obsolete motors and solutions. Other mechanical or specialty equipment. See my profile under Home/electrical at this site


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