AC Motor Braking 2

[weso]

I need to select a braking unit for my 415V 37KW 3phase induction motor. I have been told that a general rule of thumb for calculating the maximum braking current is 4*KW. This gives me ~150A. So i should select the next size unit. Is this correct, and does anybody have a more technical solution rather than a 'rule of thumb'

 

[VicP]

Brakes should be sized so the maximum braking current does not exceed 400% of motor FLA. However, this does not mean that every brake application should be set to use 400% braking current. The number of starts and stops per hour, the load inertia, etc. also have to be considered. Why not give the application data to the Brake Manufacturer and let him size the Brake?

 

[jbartos]

Suggestion: Visit

http://www.rdmengineering.co.uk/drivloc.htm

http://www.deltaww.com/products/motordrives/vfdb.htm

(for specification, resistors, diagram, etc.)

http://www.northstar-mn.com/ambitech1.html

etc. for more info

 

[Marke]

If you apply DC to the windings of an induction motor, you establish a stationary flux field. If the rotor is spinning, the rotor winding will be cutting through the flux lines generating a rotor current and therefore a rotor field also. Beacuse there are two fields, there will be a torque force between them. The rotor of the induction motor will try to spin at the same speed as the stator field, which with DC applied is stationary. Therefore the stator field will act as a brake trying to slow the motor to reset. The magnitude of the torque, is a function of the motor characteristics, primarily the rotor characteristics, the DC current flow, and the magnitude of the current. If the current is not smooth, as is usually the case with the DC injection function of soft starters, there can be, in addition to the zero speed torque, a full speed torque coming from the line frequency component of the DC.
In the case of pulsed DC, the maximum brakeing torque is quite restricted. The line frequency component can keep the rotor spinning at full speed with no resultant braking occuring.

DC injection results in a high current flow in both the stator and the rotor. This translates into heat and care must be taken not to exceed the motor ratings. The kintetic energy in the driven load is dissipated in the rotor during stop.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[electricpete]

Mark - I never quite understood dc braking before but that makes sense when you explain it. I vote you a star. I'd vote you more if I could.

It provides an interesting parallel to the discussion of rotor heating during an unloaded (inertia-only) start, which was also equal to the [final] kinetic energy of the motor.

In the case of dynamic braking it seems somewhat easier to understand the result from physical considerations. The stator provides a dc stationary field, and the rotor acts like a generator feeding the rotor resistance (load). Neglecting other motor losses (stator I^2*R), it is easy to see there is no work done by the stator current or stator field.

Stator Field Power = Torque x Speed = Torque x 0 = 0.

If there is no work done by the stator field then conservation of energy tells us the rotor kinetic energy has to go somewhere and the only place it can go is to the "generator load" (rotor resistance heat dissipation).

As I said the intuition is a little tougher for the unloaded start. (Using similar assumptions of no stator losses or other losses). In that case there is energy input by the stator field and the amount of that energy by conservation of energy must be TWICE the final kinetic energy of the rotor. (1 times kinetic energy goes to kinetic energy of rotor, and 1 times kinetic energy goes to rotor heating). It doesn't follow intuitively, but maybe it follows somewhat intuitively (?) from analogy with the braking case.

 

[johnnyw]

Has anyone ever considered using reactive current to brake a three-phase AC synchronous machine (i.e., permanent magnet) by injecting d-axis current via the motor drive controls during the forced deceleration? My thought is that you could absorb some of the load inertia as heat into both the stator windings and drive power electronic devices to help slow the rotor. I imagine the controls would be tricky, but if you could regulate line current at the maximum amplitude with all d-axis contribution (no q-axis current to produce torque) and let the speed could decay naturally (based on the losses), then the motor stator and drive shouldn't overheat. Is this ever done using the field supply of a wound rotor synchronous machine? Anyone have any thoughts or experience with this method of braking?