Dynamic braking of DC motor 1

[Masbibe]

Hello all,

In attach you can find standard (recomended) circuit diagram of DC drive with dynamic braking.

Drive is ABB DCS800, 4Q, 3300A, 690VAC
Motor is 700 VDC, 2300A, 225 rpm, separate excitation.

Dynamic breaking will be used in case of drive or network failure so question is:
I DONT plan to put breaker Q2 (in DC circuit) and I dont see any problems but maybe somebody will tell me that I am wrong.

I know what are advantages using DC breaker but in my case it will be to expensive so I am only interesting will there be problems during dynamic braking because in ABB recomandations breaker first disconnect drive from motor armature before close contactor K1. My plan that contactor is switching by NC contact of main AC breaker (Q1).

Thanks in advance,

Milovan Milosevic

 

[Masbibe]

One additional information: There is no possibility that load speed up the drive during dynamic braking and cause some overvoltage on drive terminals.

Milovan Milosevic

 

[eeprom]

MilovanSRB,
Dynamic brakes do require some form of protection, but not necessarily a breaker. ABB drive manuals (AC) provide typical circuit diagrams for the use of dynamic braking. These diagrams include provisions for protecting against fire in the case of brake resistor overloads. I don't know that DC drives would be identical, but I would assume that they would be similar.

Most dynamic brake resistors (such as PowerOhm) will have a thermostat on them. These are usually wires in series with a contactor on the supply side of the drive. So if the brake resistor is overheating, then the drive is completely shut down.

EE

 

[Masbibe]

eeprom,

Thanks for reply.
I think in this case it is not neccessary to implement OL protection for resistor.
In our case we have 4 motors like this one. This is cold rolling mill application. 2 motors are for stand and another 2 are for coiler and uncoiler.
It is important that in case of network or drive failure motors stops as soon as possible and maximum energy that resistor can recieve is kinetical energy of coils (or stand). So if resistors are dimensioned well there should be no problem.

Milovan Milosevic

 

[iop95]

ABB recomand Q2 braket to protect their converter. Q2 may be eliminate and use K1 to conect brake. A failure in converter in short-circuit mode will transform converter in brake resistor and will brake motor at hifh current. If there are often start-brake operations, at such high armature currents, any braker will have rapid contacts used.
Regarding Q1, same problems regarding contacts life. I think best operating mode is by electonic command in ABB converter to disable output voltage and after this close K1; reverse at start-up, open K1 and enable converter output.

 

[Masbibe]

iop95,

Thanks for response. Dynamic breaking will be used very seldom. I am planing to control Q1 from drive so this mean that drive is switching-off Q1 after he disable both bridges (current fall to zero).

Milovan Milosevic

 

[enggvijay]

generated energy dissipated as heat inside of the rotor, which increases the likelihood of damage and eventual failure. Therefore, some systems transfer this energy to an outside bank of resistors,contactor k1 connect resistance to motor for enery dissipation,but if u r using high enertia rotor,than may be we can not control enery dissipation in resistor, and may b also damage the convertor, so it must necessary to use braker Q2....
'i thinkkkkkk''

 

[Drivesrock]

Hi all.

I understood that a high-speed DC breaker (at Q2) is used to protect the thyristors in the event of a mains failure when regenerating. The thyristors will not be able to turn off and the motor will be regenerating back to the supply transformer as a short-circuit until line side contactors open, if there is one, or Q1 opens on instantaneous. The dc breaker opens sooner. Cheaper DC semiconductor fuses could be used but you'll have to replace them everytime they blow.

I was wondering though, about the motor field. When the mains fails then we'll lose the field supply just when we want to brake. Is this acceptable? Or does anyone recommend a UPS for the field supply?

Thanks in advance.

 

[Masbibe]

Drivesrock,

Thanks for response.

Partly you are right. High speed DC breaker is used during regeneration in case of network failure. But breaker or contactor on AC side will not help in this case because short circuit is formed in drive between plus and minus. This phenomana is called comutation failure (2 thyristor on same lag are conducting).

In our case field is not suplied from UPS (but it is possible). It is suplied from different network.



Milovan Milosevic

 

[Skogsgurra]

I think that Drivesrock points to a very important aspect of dynamic braking. You cannot brake a motor dynamically if there is no magnetic field!

If braking takes several seconds, most of the motor field will be gone in less than a second and there will be no braking torque left in the motor. So, if you REALLY need to do dynamic braking, you really NEED to keep excitation up for the whole braking period.

A UPS or, perhaps, a huge capacitor is what you need. Or a battery fed excitation winding.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Masbibe]

Skogsgurra,

Thanks for response.
Of course if you dont have field there is no braking.
You can make some connection to archieve self excitation (connecting armature over some voltage divider to field) but efect of braking will not be so good because voltage on armature will drop with speed.
In our case we supply field from different network. Using UPS in our case will be too expansive and also we didnt predict this during sales phase of this project.
I was only wondering is there possibilitie to damage somehow thiristors or something else in drive if armature is connected during braking and drive is OFF like it will be in my case.



Milovan Milosevic