Jump to content

Motor Braking Methods


Push_Luck

Recommended Posts

i have read the theory about the motor braking methdos:

it said that got 2 ways to brake the motor, Plugging and Braking with direct current.

 

for plugging: Done by interchanging two stator leads creating opposite direction mmf

 

Braking with direct current: circulating dc current in the stator windings. Stationary field produces braking

 

my question is, do the methods mean that we need to brake the motor "internally" =(inside the motor)???

let take the plugging method, does it mean we need open the casing of motor and interchange the stators to create mmf???How can we change the stator since stator is permanently fixed. Sorry for my stupid question, i have no idea how it happen... i do hope somone can guide me to understand the concept. Thanks in advance for your knowledge and your time to spend on replying my question. thanks.

 

 

Link to comment
Share on other sites

Regarding Plugging:

Are you familiar with how 3 phase motors are reversed? If so, that is "Plugging" This is EXTREMELY hard on the motor! During braking, the stress on the mechanical components is 3 or more times that of normal. Unless your system was specifically over designed to accommodate plug reverse braking, do not attempt it. An analogy would be to have your car driving forward and shifting into reverse. Something is going to give!

 

If you are not familiar with 3 phase motor control, the rotation direction is simply a matter of the phase order of the power lines feeding the motor, i.e. A-B-C. If you use a second contactor that has the wires swapped in sequence A-C-B, then the motor will turn in the opposite direction.

 

DC Injection Braking is also done outside of the motor with a DC power source (rectifier) that detects when the motor is switched off and pumps DC into one set of windings. The downside to that is that the kinetic energy of the rotating mass is converted into heat, mostly in the rotor of the motor, where is has the most difficulty being dissipated. So DCIB is useful only for low duty cycle applications.

 

There is also Dynamic Braking, the process of converting the kinetic energy of the rotating motor into electric power, then applying a load, usually a resistor bank, to that power to dissipate it rapidly. Any 3 phase motor can also be a generator, as long as the windings are "excited" or magnetized from some source. A Dynamic Braking system provides that excitation current and maintains the motor's status as a generator. The faster a motor is rotating, the more power it can generate so the faster is can be stopped, so Dynamic Braking is an ideal method for stopping high inertia loads. But it is a problem of diminishing returns because as the motor slows, the braking power drops and at some point cannot completely stop the motor. So Dynamic Braking systems often use DC Injection Braking at the very end of the cycle to finish the job.

 

VFDs are capable of doing this (with the proper accessories such as braking resistors), but there are also stand-alone dynamic braking systems as well. An off-shoot of Dynamic Braking is called Regenerative Braking, where that energy is actually fed back into the power source so that no resistors are necessary. This has the advantage of having the highest possible duty cycle and actually can save a little bit of energy, but it is by far the most expensive technology available because it entails the equivalent of 2 back-to-back VFDs.

"He's not dead, he's just pinin' for the fjords!"
Link to comment
Share on other sites

Hi Push Luck

one other option regarding regenerative braking, the disapated energy does not necessarily need to be fed back into the supply. If there are adjacent loads that are VSD controlled and driving then the DC busses can be linked and the energy used to drive these loads. In practice you would also have a resistor to disipate any excess energy not required by the other loads.

I agree 100% regarding plugging. Not only hard on the motor but also all associated equipment. And the unfortunate maintenance sparky who will be forever repairing contactors etc.

 

Ken

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
Link to comment
Share on other sites

If you implement plug braking by reverse jogging the motor down to zero speed, then you are certainly applying more stress, however if you use an automatic ververs plug braking system with a reduced voltage starter such as a soft starter, then the stresses should be no higher than during a start.

 

A automated system will use a sensor to detect zero speed and stop the motor from accelerating back up to full speed in reverse.

 

The torque produced by a reverse rotating motor is not too different from a forward rotating motor. The major issues are that the kinetic energy of the load is dissipated in the rotor of the motor during stop, if a manual jogging system is used to plug break, there are reclose effects, contactor opperations etc to consider.

 

DC injected stopping can alos be used effectively, but there are some constraints to consider.

The kinetic energy of the spinning load is dissipated in the rotor.

If there is any AC component in the DC, there can be problems with the motor synching on to the AC component and not stopping. i.e. this is a coommon problem with DC braking and soft starters where two SCRs in two phases are used to half wave rectify the applied AC. If this raw DC is applied to the motor, there will be a braking torque from the DC component and a rotating torque field from the AC component. Appply too much DC too soon and the motor will continue to rotate at line speed. A shorting contactor across one winding is a means of reducing the AC component.

 

Best regards,

Link to comment
Share on other sites

Regarding Plugging:

Are you familiar with how 3 phase motors are reversed? If so, that is "Plugging" This is EXTREMELY hard on the motor! During braking, the stress on the mechanical components is 3 or more times that of normal. Unless your system was specifically over designed to accommodate plug reverse braking, do not attempt it. An analogy would be to have your car driving forward and shifting into reverse. Something is going to give!

 

If you are not familiar with 3 phase motor control, the rotation direction is simply a matter of the phase order of the power lines feeding the motor, i.e. A-B-C. If you use a second contactor that has the wires swapped in sequence A-C-B, then the motor will turn in the opposite direction.

 

DC Injection Braking is also done outside of the motor with a DC power source (rectifier) that detects when the motor is switched off and pumps DC into one set of windings. The downside to that is that the kinetic energy of the rotating mass is converted into heat, mostly in the rotor of the motor, where is has the most difficulty being dissipated. So DCIB is useful only for low duty cycle applications.

 

There is also Dynamic Braking, the process of converting the kinetic energy of the rotating motor into electric power, then applying a load, usually a resistor bank, to that power to dissipate it rapidly. Any 3 phase motor can also be a generator, as long as the windings are "excited" or magnetized from some source. A Dynamic Braking system provides that excitation current and maintains the motor's status as a generator. The faster a motor is rotating, the more power it can generate so the faster is can be stopped, so Dynamic Braking is an ideal method for stopping high inertia loads. But it is a problem of diminishing returns because as the motor slows, the braking power drops and at some point cannot completely stop the motor. So Dynamic Braking systems often use DC Injection Braking at the very end of the cycle to finish the job.

 

VFDs are capable of doing this (with the proper accessories such as braking resistors), but there are also stand-alone dynamic braking systems as well. An off-shoot of Dynamic Braking is called Regenerative Braking, where that energy is actually fed back into the power source so that no resistors are necessary. This has the advantage of having the highest possible duty cycle and actually can save a little bit of energy, but it is by far the most expensive technology available because it entails the equivalent of 2 back-to-back VFDs.

regerading plugging, let take an example on single phase induction motor; the motor got 4 terminal ( let say A,B, C, D) to make it can rotate in forward and reverse direction. So does plugging mean then when the motor is rotating in forward rotation (i connect the terminal A and B) and suddenly i change the terminals to C and D (it will going to rotate in reverse direction) ?

thanks for ur time.

Link to comment
Share on other sites

thanks kens and marke as well.

 

 

How if i have a magnetic brake that installed in the end of motor's shaft; the moment i give currents to the magnetic brake then motor will stop. does it a type of braking methods? so what's the name(such as plugging, regenerative braking....) that given to this type of braking methods?

thanks.

 

Link to comment
Share on other sites

Hi push luck, the magnetic brake on the end of your motor is most likely a brake for holding the load, not decellerating it. If you imagine a hoist motor it will have some form of braking which it uses to control the load as it lowers it but it will also have some form of mechanical brake that is used to hold the load and also as a safety device in case of power failure. Usually these types of brake requires power to it to release.

Ken

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
Link to comment
Share on other sites

Hi push luck, the magnetic brake on the end of your motor is most likely a brake for holding the load, not decellerating it. If you imagine a hoist motor it will have some form of braking which it uses to control the load as it lowers it but it will also have some form of mechanical brake that is used to hold the load and also as a safety device in case of power failure. Usually these types of brake requires power to it to release.

Ken

so could it count as a type of braking method?

or braking methdos only consist of Plugging, Dynamic Braking, DC injection, and Regenerative braking. so totally only 4 types?? In addition to that, are these four types apply to both AC motor and DC motor? :rolleyes:

thanks.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...