Braking resistor question

[Syp]

Hi!

I have a question on when to use braking resistors with a brushless motor (and amplifier), or even with a brush motor. The braking resistor is there to absorb energy that would otherwise be absorbed in the motor coil, wiring, pcb traces, and power mosfets? Doesn't the power supply capacitance absorb most of this extra energy? How do you calculate what size resistor is needed, if any?

Say I have a 1kW brushless servo motor, and I use 0.5 ohm resistors in series with each coil. Further suppose the motor is loaded at 10 amps at 100V. This means the resistor is dissipating 50 watts and has a 5 volt drop across it. Let us say the power mosfets have a RDSon of 0.010 ohms. So during a high energy braking condition, the current in the motor coils will go much higher right? Like 50 amps or more for a few hundred ms? At 50 amps, the resistor would have 25 volts across it and instantanious power of 1250 watts, the mosfet would have 0.5 volts across drain/source and have an instantanious power of 25 watts.

I guess it is the impedance of the motor coil that is the issue here? The motor coil is something like R + Z, where Z=2*pi*frequency and R is the DC resistance of the coil, which is probably less than 1 ohm? So in a braking condition, the frequency will go to zero, meaning Z goes to zero, and the motor acts like a low ohm resistor, so we suddenly have a condition where 95 volts is across the motor coil which has stopped, generating a large current and the braking resistor is there to limit this current somewhat?

Have I got any of this correct?

And if these braking resistors are needed, why are they not incorporated into the sense resistor...oh, I guess because the sense resistor needs to be small so that it does not heat up significatly (which would dramatically change it's resistance).

Confused!

Syp

[NC Cams]

I believe that power dissipation in a resistor is Power = I*I*R where I is the current flowing and R is resistance in ohms.

Thus if current flow = 15 amps and R= 0.5 then P =112.5 watts. That is a bunch of heat to plan to dissipate if my understanding is correct.

The larger the resistor wattage, the better it will be at absorbing heat without drastically going up in temp. Needless to say, you're still going to want to heat sink it.

Keep in mind that you can parallel resistors to gain power absorbtion capacity. IE 4 @t 2 ohms in parallel = 1 @ 0.5 ohms.

[Syp]

Hi!

Mmm, not to flame, but I know Power = I^2 x R...that was not really what I was asking (I show I know that when I said at 10 amps a .5 ohm resistor has 50 watts to dissipate).

More what I was trying to figure out was a) when is a braking resistor neccessary, and b) how does one calculate the value and power rating of said braking resistor?

Thanks!

Syp

[NC Cams]

No offense take.

My experiences with "braking resistors" was derived via a long and expensive experimentation with electric R/C cars - as they follow the same physical laws, I thought the info would apply.

We used to use servo driven rheostats to control speed. They had two resistors - power and braking. The wiper would swing in the appropriate direction under power or braking command. These were ultimately replace by mosfets that were shunted across the brushes and PWM'd to control "braking resistance" as they were switched on and off.

Testing showed that current flow was solely a function of voltage generated by the motor under braking. The higher the voltage, the greater the current flow. The lower the resistance across the motor, the harder the brakes were applied.

Using E=I*R, we could figure out what we needed in the way of braking resistor for a particular application.

We then started testing motors with inertia dynos. We had to find a way to slow down a 40,000 rpm motor turning a 3" flywheel (ugly when one of those gets loose). We started applying braking resistor technology that we learned from the speed controls on the cars.

Again, it was purely a function of voltage (which we measured) which begat current (which we also monitored) that developed the power whigh we applied across the resistor (which we played with in size and value) to determine how fast we wanted to slow the motor down.

Lower motor speeds (same windings) generated more voltage/current and vice versa. Low resistance braked faster but were a bit harder on the motors (again, highly stressed and abused R/C motors) due to insane current densities.

From my experience, I'd respond to your questions as follows:

a. A lower resistance should slow things down faster than a higher one. (greate ON duty cycle of PWM'd fets= low resistance)

b. If you know the power do dissipate, you have the voltage and current. Now that you have the current and you choose the resistor, plug I and R into the power equation for a resistor wattage rating (the method I used for sizing my dyno braking resistors).

If you undersize the resistor a bit, it will only get hotter quicker and vice versa. If you oversize it, it just won't get as warm as quick.

Yes we were dealing with P/M DC brush motors and yours is a brushless. The way I see it is this: once the current comes out of the motor, the braking resistor doesn't know how it got there. It should treat the current the current purely as a function of Ohm's law and the power equation.

I hope this helps. Feel free to disagree or to take a different approach.... More importantly if I am wrong in my thinking, I"d like to be corrected....

[kaye7877]

I think the answer really depends on how the drive is constructed. I've used servos in machinery that could remove energy from the system(machine components) at 100%+ of full rated torque and I've used servos that could only remove about 30% before needing to add a power resistor to dissipate more energy.

When you are removing energy from the system, the bus voltage in the drive is driven higher. If you try to remove more energy from the system than the drive can handle, it will fault on overvoltage.

If you do not have this information in a manual or can't get your hands on one, leave room for the resistor in your design, but don't include it to save a little $. Then if you are faulting on overvoltage during decel, put a resistor on it. You will need to know how to enable the use of an external resistor in the drive.

Of course drives vary so much that i may be way off. Give me more specifics and I may be able to help more.