Arduino Forum

Using Arduino => Motors, Mechanics, and Power => Topic started by: jacko91 on Oct 15, 2018, 07:54 pm

Title: Winches and selflocking
Post by: jacko91 on Oct 15, 2018, 07:54 pm
Good day,

I am trying to figure out how 12/24 DC winches work. Looking at cheap 230V winches: They have usually a friction brake which gets loose as soon as the current magnetizes the rotor. Say if a winche is mounted to the ceiling and some weight is being lowered. Due to the AC driven magnetic field the weight doesnt pull down the motor as the rotor is following the magnetic field.

Now how exactly would this be done with DC supply, so that when the brake is loosen that the weight is not  automatically unwinding?

Thanks
Title: Re: Winches and selflocking
Post by: hammy on Oct 15, 2018, 11:09 pm
The type of motor has no effect , the motor is not stalled to hold the weight .
A lot of winches use a worm gear arrangement , with this you cannot drive the cable drum from the cable end.
Title: Re: Winches and selflocking
Post by: terryking228 on Oct 16, 2018, 12:27 am
Hi,

A DC motor can be slowed a lot (When it is being driven by external force as a generator) by short-circuiting it's terminals.  Or it can has resistances connected for a desired slowing. This is called "dynamic braking".. 

But it can't STOP the motor completely..
Title: Re: Winches and selflocking
Post by: WattsThat on Oct 16, 2018, 05:25 am
Common hoists* have brakes that clamp the shaft when unpowered. AC or DC, it doesn't matter, they are functionally the same. The friction brake unlocks with a solenoid coil that has the same rating as the hoist input voltage.

* A hoist is bidirectional, typically used for raising and lowering a load. A winch is usually a drum upon which a cable or chain is wound and is unidirectional with its loadability.
Title: Re: Winches and selflocking
Post by: jacko91 on Oct 16, 2018, 08:51 am
Thanks for all the input.
So I opened up a 230 Winch and it has a spur gears. After removing the brake, it is quite easy to move the shaft from the drumside. So I am wondering how is it, that when lowering some weight, the weight doesnt effect the speed of the unwinding.
Title: Re: Winches and selflocking
Post by: outsider on Oct 16, 2018, 10:32 am
An AC induction motor resists overspeeding by regenerating, pushing the excess energy back up the mains.
Title: Re: Winches and selflocking
Post by: jacko91 on Oct 16, 2018, 10:57 am
Alright, makes sense.
But this is not the case with a DC motor, right. So the only solution would be self locking gears, right?=
Title: Re: Winches and selflocking
Post by: Paul_KD7HB on Oct 16, 2018, 07:43 pm
Alright, makes sense.
But this is not the case with a DC motor, right. So the only solution would be self locking gears, right?=
No! The answer is to power the motor in reverse and control the speed.

Paul
Title: Re: Winches and selflocking
Post by: jacko91 on Oct 16, 2018, 08:26 pm
But for that approach you would need to know the exact weight of the load before releasing the brake.
Title: Re: Winches and selflocking
Post by: Paul_KD7HB on Oct 16, 2018, 09:08 pm
But for that approach you would need to know the exact weight of the load before releasing the brake.
Why? If the motor is powerful enough pick up the load it is powerful enough to lower the load. Are you adding to the weight? If so, the answer is the same.

Paul
Title: Re: Winches and selflocking
Post by: jacko91 on Oct 16, 2018, 09:13 pm
Hey,

But lets say I want to control speed of the motor while lowering. Before I open the brake I would need to apply just the right amount of voltage so that after releasing the brake, the weight will not move and then I could start lowering. Or what am I missing?
Title: Re: Winches and selflocking
Post by: MarkT on Oct 17, 2018, 02:04 pm
You ideally want a servo controller to drive the motor, with position or speed feedback.  Such controllers are 4-quadrant
and thus happy to actively brake and hold position.

For a simple winch there is only full speed for the motor, which means constant speed in
either direction with minimum sensitivity to load torque.  Overload such a motor and it will lose
control.  Rather than active braking and position holding the brake mechanism is used.

A failsafe mechanical brake is often a mandatory requirement anyway.
Title: Re: Winches and selflocking
Post by: jacko91 on Oct 18, 2018, 09:08 am
Looking at simple winches: constant speed in either direction with minimum sensitivity. I understand how this is reached using AC Motors. But how is it done with DC Motor. The Load will definitly influence the speed quite a lot.

Thanks
Title: Re: Winches and selflocking
Post by: jackrae on Oct 19, 2018, 09:31 am
Speed of a shunt wound DC motor is proportional to voltage.  The friction of a high reduction gear train tends to act as a brake (try turning the shaft of such a system).  If the motor overspeeds the voltage created by the rotor is in excess of the battery voltage and hence feeds current back into the battery such that regenerative braking controls the motor speeds.


However, most cheap hoists are simple ON-OFF type devices and down-speed will generally be somewhat higher than up-speed.