Ac motor brake

[hesham]

Hi all
I have just finished rerofitting a milling machine .The client ask for an electrical break by DC injection to the spindle motor when stopped.So i want to calculate the crruent consumed by the motor while breakong(our while applying Dc to the rotor) in order to select the diodes suitable for this.The spindle motor is 7.5 KW.
Any one can help.
Thanks

 

[randylud]

Motor full load

Hesham,
The 7.5 KW motor will have a full load current rating on the motor name plate. If it is dual voltage, then pick the current for the wiring of the windings that you are using. Do not exceed this value for your injected DC. Doing this on one of the three windings will provide a good brake for the application you state. You should obviously size your diodes for a value greater than that you expect to need full time. Good luck!

 

[hesham]

re

randylud
Thanks for help,
Is the injected DC will be applied to only one winding of the three or to two windings (u-v,u-w,v-w).
What i am afraid from is that the current consumed from the DC bridge will be more higher then the motor rated current and i expect so.
Another point how can i make sure that the current applied will not exceed a value and could cause a damage to the motor winding.
BR

 

[Steve Bailey]

Are you using a variable frequency drive for the spindle? Many, if not most, include DC injection braking in their llist of features. They will have done all the necessary engineering to properly size the components and included the protective circuitry to prevent damage from excessive current. If you weren't planning to use a VF drive for the spindle, the customer requirement of DC injection braking may be a good reason to reconsider that decision.

 

[randylud]

Is the injected DC will be applied to only one winding of the three or to two windings (u-v,u-w,v-w).

When I said one winding, I meant your choice of any two of the three wires coming from the motor. In your statement, it will not matter which two you pick, so use what is most convenient to you.

What i am afraid from is that the current consumed from the DC bridge will be more higher then the motor rated current and i expect so.

Another point how can i make sure that the current applied will not exceed a value and could cause a damage to the motor winding.

You will have to proved for a limiting resistance between the bridge rectifier and your motor winding. The winding its self will have a certain value of resistance so if you use a rheostat or potentiometer of sufficient wattage, you can adjust the voltage from the bridge by using the resistance of the pot as a voltage divider. Adjust the voltage from the bridge while monitoring the current to the winding and then once it is set, you can remove the ammeter.
p.s. I posted after Steve did so you should heed his suggestion. The VFD option is certainly simpler and probably less expensive than a built up circuit.

 

[hesham]

No VFD

Steve
We are not using VFD to this motor ,the motor is conected DOL.And the client don't want to invest money in this Item he says i don't need to change this motor speed .

randy
Limiting resistor is agood solution i think ,but there will be a heat dispated from it beside the panel space requerments.
I will try to find a variable resistor with high wattage for this purpose.and i will try to find a formula to calculate exactly the current ,

 

[Steve Bailey]

Hesham,

Unless the customer explicitly refuses to accept the machine with a VF drive, you should compute the cost of the starter plus your engineering time to design the braking system plus the cost of the braking components. Compare that with the cost of the drive. Then tell the customner that the box that looks like a drive is, in reality, a brake. You could also tell the customer that he can achieve longer tool life if he takes the time to learn how to apply the adjustable speed feature of the "brake".

 

[JesperMP]

How much is the rated current (In) on that spindle motor ?
Small VFD's are much cheaper than they used to be.

 

[DickDV]

Hesham, you do not need a VFD to take advantage of electronic braking. Simply contact your local source for electronic soft-starters and ask for a stand-alone DC injection brake. They are designed to operate in conjunction with a magnetic starter.

You do not want to design one yourself. To do the job properly, you will need adjustable braking current up to 150% of motor nameplate (not 100%), plus zero speed detection, plus a timer for stopping the DC after an adjustable amount of time.

I have seen stand-alone DC brakes from Benshaw, ABB, Square D, Motortronics, and Allen-Bradley. Surely there are others. They are all rather inexpensive and do a good job of decelerating a spinning load. They are not suitable as a holding brake. That's the job of a mechanical brake.

 

[hesham]

I didn't find

Dick
Yes i tried to find a stand alone DC injection breaking but i didn't find it Stocked at any of our local suppliers ,this why i want to design on with my self.
So according to your post i should design my rectifier with current rating 150% of the rated current .
BR

 

[elevmike]

Hesham,

I've never tried this device on an an ac motor, but IPC Automation makes a DC motor field control. You can adjust/control the output current, and voltage via your controller. http://www.thomasregister.com/olc/32177651/hoi_mtr.htm

 

[juad]

dc injection break

hi all
dear hesham
the attached file here is an application of dc ingection break from NRM company to fabric cutter line that mesure lenth but in fact it did
not work well in ouer factory for excact breaking, it is work to
slow down the speed ,when we used it we face the problem that the motor either over heated or bad break( according to the value of resistance). what I try to say that you must take in mind the actual inerita to get good break.but i failed to calculate it well. i hope to here your voice if you finished your project .
thanks

 

[juad]

sorry
here the attached file again