BLDC motor as generator to power eddy current brakes

[digglesby]

Hi everyone - here's a concept that I'd like a few opinions on:

As part of a project, I'm looking at using a brushless DC motor mounted on a high-speed vehicle as a generator. Once the vehicle hits its top speed, the batteries should disconnect from the motor and the vehicle's own velocity will cause the motor to generate an AC current (through wheel rotation). This current would then be rectified and used to power EM eddy current brakes that slow the vehicle down far enough that disc brakes can be used. The end goal here is to calculate a braking force curve as a function of motor rpm.

A couple questions:
1. I have no idea currently on how to calculate how much power a given motor would generate at a given velocity v. What's the best way to tackle this? Calculate P = w * torque with a torque curve? What's missing is some sort of manufacturer's data.
2. Some sort of special controller would be needed - I'm a bit of an ignoramus here.

Thanks for reading and for your time!

Jon

 

[berkeman]

the vehicle's own velocity will cause the motor to generate an AC current

Welcome to the PF.

How does a DC motor generate an AC current? And how are you planning on converting the DC output into the AC needed to generate eddy currents?

 

[digglesby]

Welcome to the PF.

How does a DC motor generate an AC current? And how are you planning on converting the DC output into the AC needed to generate eddy currents?

Thanks for the reply! I'm completely mistaken about the DC motor generating an AC current - it would be DC output. The eddy current electromagnets... my gut tells me that using them like permanent magnets in a linear eddy current brake would work, much as is seen in some trains. Thanks for the questions - should I make an edit in the original post to reflect this?

 

[berkeman]

I think the original post is fine, since you are clarifying it. Can you post more about the linear eddy current braking system? I guess if you put a DC magnetic field across only a part of the rotating disk, that will create the changing spatial magnetic field zone that does give eddy currents. How would you optimize such a geometry to give the best braking force? How are you planning on cooling the discs? From your New Member Introduction post, it sounds like you will be slowing down a train that is moving pretty fast...

 

[JRMichler]

There's two kinds of DC motors - brush and brushless (BLDC). BLDC motors use internal encoders to send the rotor position back to the drive. The drive uses the rotor position to send the appropriate phase currents to each stator phase. Most BLDC motors have two cables, one each for position and power. Some newer motors combine all wires into one cable.

The industrial servo drives that I am familiar with are all four quadrant drives. The four quadrants are driving forward, driving reverse, driven forward (braking in the forward direction), and driven reverse (braking in the reverse direction). These drives have everything that a vehicle needs - drive forward, drive reverse, motor braking forward, and motor braking reverse. The driving torque curve is the same as the braking torque curve.

You need the manufacturer's data sheet for your combination of motor and drive. The drive is the electronic box that runs the motor. The motor torque curve has several limits - peak torque, RMS torque, base speed, peak power, RMS power. Each combination of motor and drive is affected differently by those limits.

Make sure that the drive can handle the kinetic energy from braking. Braking pumps energy back into the system, and not all drives will put that energy back into the supply lines.

The hobbyist electric vehicle builders usually use brush type DC motors. Those operate completely differently.

 

[Tom.G]

Simplification:
The motor(s) obviously have enough power to accelerate the vehicle, therefore they have enough power to decelerate it (which is what you are proposing with the eddy current brake.) Just put a resistive load on the de-energized motor and use that as a brake. It's called dynamic braking. Do a Google search using that term. You may even get away with just short-circuiting the motor windings thru a relay.

 

[digglesby]

I think the original post is fine, since you are clarifying it. Can you post more about the linear eddy current braking system? I guess if you put a DC magnetic field across only a part of the rotating disk, that will create the changing spatial magnetic field zone that does give eddy currents. How would you optimize such a geometry to give the best braking force? How are you planning on cooling the discs? From your New Member Introduction post, it sounds like you will be slowing down a train that is moving pretty fast...

The eddy current brakes will put a DC magnetic field across a rail moving at high velocity (so no discs) - the same rail which our craft is riding on/attached to.
So more like this geometry:

or

Cooling then isn't as big of a concern right now because the rail is rather massive and long.
I am worried about outrunning the eddy currents in the rail and thus losing braking force

The industrial servo drives that I am familiar with are all four quadrant drives. The four quadrants are driving forward, driving reverse, driven forward (braking in the forward direction), and driven reverse (braking in the reverse direction). These drives have everything that a vehicle needs - drive forward, drive reverse, motor braking forward, and motor braking reverse. The driving torque curve is the same as the braking torque curve.

Make sure that the drive can handle the kinetic energy from braking. Braking pumps energy back into the system, and not all drives will put that energy back into the supply lines.

A question: I want to clarify in my head the output voltage of a motor being driven as a generator. Say it's rated from 320-600V DC input - are there any conditions (besides resistive load) about what voltage it outputs when the motor's being run as a generator? I.E., if I crank the shaft on a motor, exactly which output voltage range can I expect?

 

[jim hardy]

if I crank the shaft on a motor, exactly which output voltage range can I expect?

You can expect voltage about equal to what it takes to drive the motor when unloaded to the same speed as your cranking.

Brushless ? As in multiphase computer controlled permanent magnet type ? You'll have to tell the computer to reverse the torque. The motor will follow its own speed-torque curve back down to zero speed, just torque reversed. The energy can be returned to your battery.