H-Bridge Configuration With a Go-Cart

[fzn10]

I wana use an H-Bridge like the one attached for my Controller for a Go-Cart. Basiclly if want the motor to move forward ill apply a PWM to Q1, and make sure that Q4 is turned on. The thing that im intreseted in is to regenerate power into the battery when the go-cart is just cruising i.e No PWM is applied to Q1. Is regeneration happening automaticlly. Since Q1 is off i have current going through the diode across Q1 and into the battery.

 

[jrz126]

Those must be some pretty hefty mosfets to drive a go-cart. just out of curiosity, what are the specs on those mosfets? I would think some IGBTs would be a better solution.

I dont think it will start to brake automatically. With all of the mosfets turned off, there will be no current through the field to excite the armature. With no current in the field, there wont be any voltage generated at the armature. With ~0 volts on the armature, the diodes on the fets wont turn on.
I could be wrong though.

 

[Hero999]

Regnerative braking has always interested me. Does the motor have to be spinning faster than it would be when it's connected to the battery?

For example, I connect a 12V motor to a 12V battery and it spins at 3000rpm. If I dissconect it and use it as a generator by spinning it at 3000rpm it won't actually generate 12V, it's likely to be 10V. So in order for regenerative braking to work it needs to spin faster than 3000rpm to generate a higher voltage than 12V inorder for the battery to charge properly.

Is this correct? Because if it is, it means that regnerative braking will only work if you're going down hill or you change down a gear.

 

[Nigel Goodwin]

Regnerative braking has always interested me. Does the motor have to be spinning faster than it would be when it's connected to the battery?

For example, I connect a 12V motor to a 12V battery and it spins at 3000rpm. If I dissconect it and use it as a generator by spinning it at 3000rpm it won't actually generate 12V, it's likely to be 10V. So in order for regenerative braking to work it needs to spin faster than 3000rpm to generate a higher voltage than 12V inorder for the battery to charge properly.

The output voltage will be dependent on the load (to some extent), spin the motor at 3000 rpm and you will get MORE than 12V from it with no load. Likewise, feed 12V to the motor with no load and it will spin faster than 3000 rpm.

I suggest you would probably have to do tests to find out how well the idea works in practice?.

 

[Styx]

Whether IGBT's or FET's are best suited is dependant on a bit more thought

FET's have the benefit of their drive-cct not being so demanding and that they switch fast and lower on-state losses. BUT their SOA is actually pretty shite

IGBT's have higher on-state losses, switch slower BUT have a more rectangle SOA (this one fact makes them GREAT!!!)

General rule of thumb is anything below 50V (maybe 100V dependant on current level) then FET's, IGBT's come into their own with higher voltage blocking, saying that look at what yr current is demanding and yr SOA, you may find you may need to goto IGBT's


The first problem I see is the field winding in series with the bridge.
1) a H-bridge IF operating as a voltage-source (from what you are saying yrs is) NEEDS to be a voltage-source, that means big capacitance close to the silicon of the H-bridge.
Inductors == Current-source
Capacitors == Voltage-source

Inductance in the DC-link is VERY BAD!!!! so bad infact...
as it stands as you start to PWM: you turn an diaganal pair of switches on and current builds up in the motor AND the windings. WHAT happens when the PWM turns the switch's off, yes the diodes of the H-bridge will allow the motor free-wheel current to flow BUT what path is there for the winding-inductor..

The path of It will jack the voltage up and break over the silicon, path completely destroying yr bridge within 1-PWM cycle

IF you can make it a parallel excited arrangement then you are onto a winner. Get a DC-link capacitor as well (is needed) and then re-gen can start being a reality.

say you have the DC-link at 12V (well 13V from a lead-acid) and you stop PWM'ing AND the machine's rotor is still turning (say just inertia or because down a hill) what will happen is when the backEMF of the machine (which is proportional to speed) exceeds the DC-link the rectifier-aspect of the H-bridge will kick in and it will start to "pump" the link up.

With a 12V battery there as well that will start to take the charge, thus charging the battery up.
What I would do is put a brake-cct across the DC-link.

The car-battery will only be able to obsorb (via charging) so much re-gen energy, anymore and it will just keep pumping the link up, so put a comparator cct across the link to trip at say.. 15V and reset at.. 14V, this then fires another IGBT/FET that is across the DC-link (with a resistor in series with it) to dump the extra energy

 

[faomari]

Those must be some pretty hefty mosfets to drive a go-cart. just out of curiosity, what are the specs on those mosfets? I would think some IGBTs would be a better solution.

I dont think it will start to brake automatically. With all of the mosfets turned off, there will be no current through the field to excite the armature. With no current in the field, there wont be any voltage generated at the armature. With ~0 volts on the armature, the diodes on the fets wont turn on.
I could be wrong though.

Considering the other posts, defienetly made me think, i never thought about the problem that motor voltage has to be bigger than the battey voltage for it to charge which is quite obivious, mabye use a boost or something like that, but thats just complicating things.