Edit: Help understanding motor braking

[Ajvr]

Hi all,

Id like to use some automotive alternators to make a load dyno for testing for small engines up to around 15hp.

Reading all the great info on here of alternators converted to motors it seems running them at approx 50V & 80-100A seems achievable.

Would this mean that I could expect each alternator to be able to provide approx 4kW of resistance with the voltage regulators removed & manual rotor voltage control?

Is the maximum voltage limited by the windings in the stator and their ability to dissipate the heat?

My plan would be to hook up the output to some form of heating elements to aid dissipating the heat & providing load.

I would then look to adjust the rotor voltage up to 12V depending on load required & then use a load cell type system to measure the torque.

Thanks for the help!

 

[stepus]

Hi, yes alternator can be used as a dyno, but its braking power depends on RPMs and current at rotor coil. I thing having a watercooled alternator, and short its phases togeter will work good, and then control braking force only by small current to rotor.
Adding some resistive load on output - like you mentioned to have a voltage at output 50V will need to increase current to rotor, and it adds heat to rotor. Or use a gearing to increase RPMs at alternator - but i dont know for how much RPMs are alternators designed

 

[Ajvr]

Hi all,

So as is easy to do I got a bit carried away and purchased a Honda IMA motor in the hope of being able to use a single motor to provide enough resistance,

This is a BLDC with permanent magnets with approx 20kW drive capability,

With the alternator it was fairly easy to get to grips with adjusting the rotors voltage to change the field strength and adjust the braking capability but im struggling to get my head around how I could potentially use this motor for braking.

Im hoping someone may be able to help clarify my understanding & some questions on the different methods.



Braking method:

DC Injection

How it works: Creates a magnetic field in the stator that the rotor will try to align itself with.

Control: Send DC to the stator & vary the voltage to control the braking strength.

Problem: Energy is converted to voltage/current in the coil and can quickly overlaod the motor so is not useful for long load tests.

Question:

Would it be possible to use this method and create some form circuit to dump the excess energy above that supplied by the DC supply into "braking resistors" allowing this method to be used for longer periods of providing a braking load?



Dynamic braking

How it works: A voltage is generated as a magnetic field passes through the coil. Another field is generated in the coil which opposes that of the movement and results in the torque required to move the assembly.(turn the generator)

This voltage is from EMF. So is related to the change of flux(field strength) - the rate of change (faster is stronger) & the number of turns in the coil.

Generally it appears that the target rotational speed is set just below the actual speed to create the braking force. If the difference in speed is too great too much current will be dissipated damaging the motor/control electronics.

Problem: This energy needs to be dissipated back into either a power supply (Regen) OR braking resistor. Amperage must be monitored again to protect the motor and control electronics

Questions:

Can this provide as much braking torque as DC injection braking?

With this method how do we vary the braking force - is it by adjusting the difference between the target rpm & actual?

 

[mxlemming]

Few options for a bldc as a brake. Two easy ones here:

1) get a controller and use it for Regen which you can pump back into your battery.
2) rectify the output with a 3 phase bridge rectifier and then sink the power from that bridge into a variable resistor or a fixed resistor with a low side pwm and recirculating diode/synchronous MOSFET.