Synchronous motor used to support lowering of vertical gate. 2

[RickyEE]

I have a vertical gate that is raised by a synchronous motor via a gear box. Once the vertical gate is fully open a brake is engaged to hold the gate in the open position. To lower the gate the brake is released and a resistor load bank is used to absorb the reverse emf that is generated when the gate is being lowered.

Will this still work if normal power is lost to the synchronous motor? For instance, If normal power is lost and the vertical gate needs to be lowered, we can manually release the brake and connect the resistor load bank to the motor, Will the vertical gate fall on its own or will the torque from the synchronous motor help coast the gate down gracefully?

Any thoughts would be appreciated.

Thanks,

 

[ScottyUK]

If the motor has no excitation on the field then it can not behave as a generator and it will deliver no power into the resistor bank, and therefore will develop no braking torque (or any other kind of torque).



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If we learn from our mistakes I'm getting a great education!

 

[waross]

Well, windage may provide a little braking, as in "Too little, Too late."
You need the field energized as well as the resistor bank connected before the motor will provide regenerative braking.
You maybe able to cobble something together with batteries on an old, slip ring motor, but with a new motor with a brushless exciter, you will have to energize the motor normally before the field will be energized.

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

If the motor has enough residual magnetism on the rotor, it likely possible to self-excite a slip ring motor. In the most basic form, you rectify the 3-phase output back into the rotor. The motor will produce a little voltage when it begins to spin - use that voltage to build the field, which will then build more output voltage. Once the output voltage rises enough, apply the load resistors.

 

[ScottyUK]

Lionel,

Are you gonna volunteer to stand under the gate while it is braked by residual magnetism?

Slightly more seriously, if there is any likelihood of a safety hazard arising because this gate may come down under a power fail condition then you need to seriously think about at a proper magnetic hold-off brake or equivalent.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Scotty, when I read the origional post again the bigger question becomes why are resistors used when lowering normally? I'm still not clear why the motor would be line connected and regen back into the power system. The only reason I could think of was the resistors loaded the motor stator enough that it ran slower than synchronous speed.

So, the next question is - exactly what type of synchronous motor is this?

 

[ScottyUK]

Possibly running with the field energised during lowering and dumping the braking power into a resistor bank? Dunno!


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If we learn from our mistakes I'm getting a great education!

 

[RickyEE]

The load bank absorbs the energy when then gate is being lowered during normal operation.

Thanks,

 

[starkopete]

Hi,
If I understand this right, when lowering, the gate acts as the prime mover driving the motor as a generator. The resistors are used to load the "generator" and thereby control the lowering of the gate. If this is excited by an external source through brushes you will need to have a battery back-up or stand-by generator (as waross suggested) so that you can excite the fields prior to manually releasing the brake.
Thanks

 

[LionelHutz]

So, you won't tell us what type of synchronous motor it is?

 

[ScottyUK]

lionel,

Any synchronous machine with a wound rotor (i.e. not a permanent magnet machine) will behave as I described: if you excite the field it will generate when it rotates. If you connect the terminals to a resistor bank it will act to brake the motor.


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If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Yes Scotty, they would.

A brush motor would be more likely since it will generate at low speed. A brushless could work too though once it picks up some speed.

But the use of resistors seems odd when the motor could just be line connected so it lowers at the same speed as when the gate is raised. So, knowing what type of motor and how the rest of the controls are working might give some clue as to why resistors are being used.

I was first thinking maybe the motor will overspeed and can't be synchronized when lowering but the motor should just run to sync speed plus the slip and then synchronize.

 

[Skogsgurra]

Raising and lowering a gate usually does not need high-power motors. Even a very heavy thing can usually be moved with less than 10 kW because of low opening and lowering speed.

If that is so, a permanent magnet rotor is the most likely motor type (we do our guessing while Ricky keeps the secret from us).

If it is a PM motor, the resistors will work regardless of grid or not. The only thing that has to be considered is that the resistors are connected to the motor via NC contacts so that there will always be a path when power is off.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

we do our guessing while Ricky keeps the secret from us

.

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If we learn from our mistakes I'm getting a great education!

 

[RickyEE]

Ok, I am back online. The existing synchronous motor does not have a permanent magnet rotor. We will consider replacing this motor with a permanent magnet rotor so we can use regenerative braking when lowering the gate during times when normal power is out.

Again, thanks for all your comments.