brake of a permanent magnet syncronous motor

[mottt]

I´ll use a propeller with permanent magnet syncronous motor.. and i´ll like to brake with short circuit resistence...how could i define the value of the resistence? I think the value must be determined by the inertia of the load and the magnetism of the rotor...but i´ll like to know if it´s posible to have a easy rule to define it or other way for braking?
thanks

 

[sreid]

Three resistors connected in a Wye. Each resistor valve =

R = Peak Phase BEMF at RPM / Maximum Peak Current

For maximum breaking.

Peak phase voltage = Peak Phase-Phase voltage / SQRT 3

 

[mottt]

sorry, but what is BEMF?? and the power? cause as this will have a short time brake.. maybe this resistence taking the peak phase voltage/sqrt 3 / maximum peak current.... is for continuous braking in the theorical way that this values is no decreasing.... sorry for my badly english.

 

[Skogsgurra]

Hello Explosion!

BEMF is Back Electro-Motoric Force. That is the voltage you get from the motor when you turn it.

There is a slight ambiguity in the Peak Voltage and Peak Current terms. It shall not be interpreted as peak of the sinewave, but peak permissible short time current. That is usually (safely) around twice the motor rated current. If you have the motor data available, it can easily be more than the thermal rating of the motor.The limit is when the current gets so high that you risk demagnetizing the rotor. But two times rated thermal current is safe.

You can either connect the resistors between phases (Delta) or in Wye. If you go for Delta, then calculate the resistors according to R = (U/(2xIrated))xsqrt(3). Example: motor rated voltage is 400 V and rated line current is 10 A. Then three resistors with R = (400/(2x10))x1.732 = 35 ohms are needed. The closest standard value is 33 ohms, which you can use without too much concern.

Resistor power will be U^2/R, but this will be at the beginning of the braking so something like a fourth of that value - probably less - can be used if your load does not have a large flywheel attached.

Gunnar Englund

http://www.gke.org

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

 

[sreid]

Depending on the motor (sounds like a small electric trolling motor) you may be able to simply short the three phases together. The only danger is this can result in high currents that could de-mag the magnets. But smaller motors usually have enought winding resistance and there is enough design margin in the magnetic circuit that de-mag won't happen.

 

[LionelHutz]

There is very little information.

I'll agree with Gunner except we have used 3 times rated motor current in the past to brake large synchronous motors. So, try a resistor bank that would produce 2 to 3 times rated motor current when rated motor voltage is applied. Get resistors with adjustable taps if you want to be safe.

Shorting could work but remember that the motor becomes a generator when braking this way. Supplying a resistor that will dump power (and therefore use the kinetic energy from the motor) will likely brake quicker than shorting. Also, with a short, almost all of the rotational energy in the motor has to be dissipated as heat in the motor windings.

 

[Skogsgurra]

A bank of incandescent lamps is very useful for this kind of breaking. Lamps have a rather low cold resistance compared to full voltage resistance. So braking current is kept quasi-constant as the motor voltage decreases during braking. Lamps also emit heat efficiently.

Gunnar Englund

http://www.gke.org

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

 

[mottt]

the motor is a 100 kw syncronous permanent magnet... and the voltage will be 400 volts... 24 poles... I still wondering how much power i´ll need in the resistance.. other idea is to take 2 phases rectify it and put in the other phase and in other of the phases i rectify... what do you think?... maybe this is more dificult and the blake is no so fast....

if i use inductive resistors, maybe i could get a higher brake in the rotor? how to calculate?

 

[Skogsgurra]

Fine! More input. 100 kW definitely needs big resistors.

The RL idea is not bad. Keep the impedance fairly constant by selecting wL so that you get, say three times rated load at rated frequency. Then add R to reduce current to two times rated load. At these rather high levels, I would not risk demagnetization by going above double rated current. The motor manufacturer should have some idea where the limit is.

As motor brakes, voltage and frequency goes down, wL follows frequency down, so current would stay constant if no R in circuit. With R in circuit, the current drops, of course.



Gunnar Englund

http://www.gke.org

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

 

[sreid]

Infra red (IR quartz) heating elements make good high power resistors for this type of application.

 

[mottt]

sreid, have you some link to this IR quartz heating elements??
thanks all of you

 

[sreid]

Motorxplosion,

Not really. Search the internet. They are what you will find in a Toaster Oven that sits on you counter top at home. You just need bigger, industrial heating versions.

 

[LionelHutz]

Do you not have some type of electronic drive operating this motor to accelerate it to speed? Why not use it to decelerate the motor?

Feeding DC into the motor will not brake it. You would essentially be slipping poles which would cause large current and torque transients that could be damaging.

Maybe you really want to know how much energy needs to be dissipated in the resistors? Calculate the rotational energy in the system. Reduce it a certain percentage to account for the losses that brake the system. If it's a flywheel type load then there won't a lot of losses. You could calculate that around 10% would be lost to the system. So, the remaining 90% of the rotational energy is the energy that must be dumped into the resistors.

 

[sreid]

I assumed (probablly wrongly) that the requirement was for a "Dead Man" circuit. If power is lost to the motor, a relay drops out causing the resistors to be connected to the motor to despin the motor fast for safety reasons or to avoid some kind of distruction to the machinery connected to the motor.

 

[Skogsgurra]

This type is used to dry coatings and base paper in fast paper machines. You can easily get several megawatts - if you need it.

http://www.infrared-dryers.com/series-f.html

Gunnar Englund

http://www.gke.org

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

 

[mottt]

yes, the normal stop will be with the driver... but this is for braking for safety reasons.... and this is a propeller that moves a little ship.

thanks a lot.