OT. EV project. Choosing an AC Drive.

elevmike

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Detroit, MI
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Ok so some of the PLCs.net vetrens may remember we/I started an electric go-cart project, but for various reasons it died on the vine. However, it's about to be re-born in a bigger and more elaborate fashon... I hope.

The plan is to construct a small two seater sporty looking car on a tubular alum frame powered by a 5hp 230v 3ph ac motor. Suspension, stearing, braking and the other chasis related components will be supplied by my good friend Kevin (who owns the junkyard two blocks down the street). I plan on spending a few (3 or 4)weekends this summer salvaging good looking used components from Horizons & Geos and the like.

To the meat of the issue:

The AC Drive: I've never applied DC to an AC drive, and am not quite sure how to select a drive for this type of application. I've figured that it's going to take 20-12volt batteries = 240vdc.

What's going to happen when I apply 240vdc to the drive?

What shuold be the low voltage cut-off?

How can I apply the regen from coasting & slowing or stopping?

RE-Charging? Should I recharge the batteries with 240v or 12v?

Budget for entire project ~$5K, much of will be eaten by the batteries, body & framework.

Open for discuession... Thanks, Mike.
 
Keep in mind that a 240 VDC bus will only let you get about a 170 VAC RMS equivalent at the motor, assuming zero loss through power conversion. This will have an impact on top speed of the motor.

I don't know that you need to specifically worry about a low voltage cut-off unless the batteries will have an issue with that. All a lower voltage will mean to the drive/motor is that you can't go as fast.

Regen will kind of take care of itself if you use a four quadrant drive. If you run in speed control with proportional gain only the accelerator response will act surprisingly similar to an IC engine. However, your engine braking will be more aggressive as you let off the gas. I have never driven an electric car so I don't know how they act when you back off the pedal. The good thing about a system like this is the design simplicity. Leave the brakes as-is and do all the regen braking with the accelerator.

You may be able to use an off-the-shelf drive from someone like Advanced Motion Control. They, and others, make drives that will accept a wide range DC bus supply and will control an AC motor. Building your own inverter, while entertaining, may be more than you want to take on.

Keith
 
elevmike said:
... powered by a 5hp 230v 3ph ac motor. ...

To the meat of the issue:

The AC Drive: I've never applied DC to an AC drive, and am not quite sure how to select a drive for this type of application. I've figured that it's going to take 20-12volt batteries = 240vdc.

What's going to happen when I apply 240vdc to the drive?

What's going to happen when you apply 240VDC to a 230VAC motor? Absolutely nothing (except heat, I think). The motor shouldn't move. To brake an AC motor, you inject DC. So.... how are you going to generate AC?

And if you recharge 240V of batteries with 12V, they'll always be dead. Unless you have someway to charge one at a time.
 
Ozee

1. He's applying the 240VDC to the input of a variable frequency drive (AC Drive).

2. If charging with 12 VDC he will disconnect the batteries from their series connection and charge them individually. Lowrider (hydraulically actuated car lift dancing and jumping performers) have to disconnect their batteries, which are usually hooked in series to provide 48VDC (4 cells) or 72VDC (6 cells) to the pumps in the cars, in order to individually charge them.
 
Last edited:
bernie_carlton said:
Ozee

2. If charging with 12 VDC he will disconnect the batteries from their series connection and charge them individually. Lowrider (hydraulically actuated car lift dancing and jumping performers) have to disconnect their batteries, which are usually hooked in series to provide 48VDC (4 cells) or 72VDC (6 cells) to the pumps in the cars, in order to individually charge them.

Knowing nothing about battery charging circuits, it was my thought to charge them with 240vdc. However considering Keiths note that 240 will only give me ~170vac on the motor, I'll need more batteries & a higher voltage charging circuit. If it's not advisable to charge the batteries while in the series configuration, then obivously this would require a set of contactors to switch the configuration so each battery would be charged with a 12vdc supply.

Re-gen & braking:

My thought here was to use the re-generated current to charge the batteries for extending the range. A panick stop would apply the mechanical brakes, but normal stopping & slowing would be done with the drive. The throttle pedal would act somewhat like like a hydrostatic drive. Lifting your foot from the trottle control would in effect cause breaking via the drive. In short my challage here is to cause overhauling conditions to provide some re-charging of the batteries. Any ideas on this would be appreciated.
 
Originally posted by elevmike:

In short my challage here is to cause overhauling conditions to provide some re-charging of the batteries. Any ideas on this would be appreciated.

This will just be the nature of the beast. If you use a four-quadrant AC drive the motor will act as a generator as you try to decelerate. This will cause the drive bus voltage to increase to the point that it is above the battery voltage. At that point current will flow back into the batteries. Assuming the drive is current limited in the forward direction to the point that it won't hurt the batteries the same should be true in the regenerating direction.

I like the idea on the accelerator action. It may feel a bit odd compared to how an IC engine driven car would act. But it is the simplest solution. There are probably more elegent solutions based on accelerator and brake pedal positions and variable torque limits but what you describe is simple to implement.

Keith
 
To be honest, I don't know if you can get an AC drive any other way these days. Four quadrant just means you can produce motoring and braking torque in both motor shaft directions. I have never run into an AC drive that was not four quadrant but that doesn't mean they don't exist.

Keith
 
Ummmmm, hold it a minute here. A four-quadrant drive is a rare thing and involves a bi-directional front end. This provides so that excess DC voltage and energy on the DC bus can be inverted back to AC and injected backward into the power supply for reuse elsewhere. This is not what you want for a DC input configuration.

A standard AC drive is, from the power supply's point of view, only two quadrant meaning that it provides motor forward and backward but no braking capacity. That's what you want for a DC input.

With a battery set connected to the DC bus, you get four-quadrant (motoring and braking in both directions) from an ordinary two-quadrant drive. A 230VAC output drive requires about 330 volts DC on the DC bus. That would also be the battery set voltage. Under motoring conditions, energy would flow from the batteries to the inverter section and then on to the motor. Under braking conditions, the motor will generate current backward to the inverter section and then thru the inverter to attempt to charge up the DC bus. This will cause the batteries to absorb the braking energy and recharge---exactly what you were hoping for.

Be sure to choose a drive which does not have input phase loss faulting or it can be turned off. Also, choose a worldwide drive design which will have a much wider DC bus voltage tolerance. This will allow deeper discharge and heavier recharge.

As to recharging batteries in a series string, if you have one weak cell, it may limit recharging current and hog too much of the voltage. But, due to the difficulties in charging each battery separately, series string recharging is generally done. That's why they always recommend replacing all the batteries at the same time (as in golf carts).

This sounds like a fun project. Wish I had time to fiddle with things like that again. Boo Hoo! Slobber! Grimace!

Anyway, good luck. Let us know how this goes.
 
Ummmmm, hold it a minute here. A four-quadrant drive is a rare thing and involves a bi-directional front end. This provides so that excess DC voltage and energy on the DC bus can be inverted back to AC and injected backward into the power supply for reuse elsewhere. This is not what you want for a DC input configuration.

A standard AC drive is, from the power supply's point of view, only two quadrant meaning that it provides motor forward and backward but no braking capacity. That's what you want for a DC input.

With a battery set connected to the DC bus, you get four-quadrant (motoring and braking in both directions) from an ordinary two-quadrant drive. A 230VAC output drive requires about 330 volts DC on the DC bus. That would also be the battery set voltage. Under motoring conditions, energy would flow from the batteries to the inverter section and then on to the motor. Under braking conditions, the motor will generate current backward to the inverter section and then thru the inverter to attempt to charge up the DC bus. This will cause the batteries to absorb the braking energy and recharge---exactly what you were hoping for.

Be sure to choose a drive which does not have input phase loss faulting or it can be turned off. Also, choose a worldwide drive design which will have a much wider DC bus voltage tolerance. This will allow deeper discharge and heavier recharge.

As to recharging batteries in a series string, if you have one weak cell, it may limit recharging current and hog too much of the voltage. But, due to the difficulties in charging each battery separately, series string recharging is generally done. That's why they always recommend replacing all the batteries at the same time (as in golf carts).

This sounds like a fun project. Wish I had time to fiddle with things like that again. Boo Hoo! Slobber! Grimace!

Anyway, good luck. Let us know how this goes.
 
Regen will increase DC bus voltage which would in turn flow back into the batteries but at what rate? Can that be limited by the drive or will you need another controller for that?

You may want to look into a DC drive such as what is used in electric tow trucks/forklifts. About ten years ago, I helped build an electric cart that performed great using a 36 volt forklift motor with a controller designed for it.

The regeneration during decel made the battery life outstanding compared to other lighter vehicles that didn't use that feature. We built custom mechanical brakes as a safety, but the regen stopped the cart before the mechanical brakes made contact if you were easy on the stop pedal.


I remember the controller being economical, but the motor was quite expensive. It did bolt right into a Ford 9" differential with an adapter. We robbed that assembly from a broken down old golf cart.

I can't recall the brand of controller, but some of my contacts at my previous job may be able to find out. I googled but didn't stumble across anything familiar.
 
Originally posted by OkiePC:

Regen will increase DC bus voltage which would in turn flow back into the batteries but at what rate? Can that be limited by the drive or will you need another controller for that?

Assuming a common forward and reverse current limit, at any given motor speed the worst case current flow back to the batteries won't be any worse than the worst case current flow out of the batteries. The question is can a battery handle current flowing back at it at the same rate that it can produce current? I don't know enough about batteries to know the answer to that.


Originally posted by DickDV:

Ummmmm, hold it a minute here. A four-quadrant drive is a rare thing and involves a bi-directional front end. This provides so that excess DC voltage and energy on the DC bus can be inverted back to AC and injected backward into the power supply for reuse elsewhere. This is not what you want for a DC input configuration.

I think this is largely simantics. Any time I have heard of peolpe referring to the number of quadrants a motor can operate in it has always been relative to the motor, not the power source. Any of the AC drives I have dealt with will attempt to operate four quadrant until the bus voltage is exceeded. If the power structure were truly only two quadrant there would be motor direction/torque combinations where the drive simply could not produce motor torque. I'm not saying a 'normal' AC-line-supplied drive will operate indefinitely in all four quadrants without limit. I'm saying the drive-to-motor power structure is four quadrant. Bus energy management is an issue unto itself.

No matter how you look at it, as DickCDV said the drive will toss energy back at the batteries as the bus voltage climbs. It is a natural consequence of the system design. Energy will flow from the higher voltage to the lower voltage unless prevented from doing so by another component.

Keith
 
Ok so what happens if the buss voltage dos Not exceed the battery voltage? For instance on a heavy decell with near fully charged batteries? I dont want to take out the school kid at the 1st crosswalk in the morning, but I also dont want to lose all that precious juice to heat... My thought here is a super capacitor pack to absorb the regen, then assist in acceleration, thus discharging the capacitor pack for the next decell cycle.

It seems like I might need some type of voltage converter to boost the re-gen voltage above the capacitor and/or battery voltage.
 
I got lost

What is new?

Personally think you are going the wrong direction considering using AC, too many DC options already available.
 

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