ACDC: these guys still rock!
But for DIY EV converters they can be a real pain in the butt. That’s because AC or DC is one of the toughest questions to answer. It is tempting to simply follow automotive, because they all have AC motors. But it’s not that simple. It is important to realize that you don’t play in the same league as automotive.
EV Conversions have their own league, and their own rules. And this is why:
At the moment there is not a lot of choice in electric vehicles from automotive, and the Tesla S seems to be the superior one. Automotive also tends to standardize in their drivetrains, because they want to make as much margin possible out of their volume. But, if you’re considering an individual car for an EV conversion there are a lot of options.
The Awards at the Wave Trophy 2014, a famous E-rally with almost 100 teams in Central-Europe, put this into perspective. The WAVE trophy gave awards to the attendants in different categories: for range, efficiency, comfort, exclusivity, for sexiness, practicality, charging flexibility etc. The Tesla S won non of these awards. All these single awards went to converted cars! And the general competition award was won by the Tesla roadster.
So take in mind: it is possible to beat automotive, with whatever you want to focus on, within our own DIY league.
And for boats? The most common electric boats are mostly friendly, pretty dull, low speed vessels around 4-5 meters in length. But how about regenerative drive systems for bigger sailing boats, hybrid drive trains for big transport vessels, power trains for speed boats and passenger catamarans? This is something you will have to and be able to do yourself.
So, let’s help you with one of the major questions, AC or DC:
AC or (brushed) DC that’s the question. To make up our mind, we will have to consider the motors incl. the recommended controllers. What we see most in DIY conversions are the following motor controller/combinations:
- Brushed DC in combination with the Soliton controllers or Alltrax, Netgain , Zilla in combination with Kostov or Netgain motors;
- AC induction, for example the Siemens/DMOC combination or a Curtis controller combined with a motor from HPEV;
- AC permanent magnet, f.e. Bamocar/Emrax combination, or YASA motors.
Sometimes we hear of brushless DC, like the Scott Drive controller. It is pretty close to AC induction.
Let’s start with DC. Although it is generally considered as less advanced, or a lower standard than AC, for DIY conversions there are a lot of serious arguments in favor of DC.
Arguments for DC:
- nothing can beat the brushed DC/Soliton’s regarding the price/performance ratio in the power league;
- large flexibility of peak power (kW) possible, from 10kW all the way up to 300kW;
- large flexibility on different voltages from battery packs, 10V – 340V maximum. Ideal for a grow scenario if you want to increase your battery pack gradually;
- huge torque at low rpm, especially with the 11” Warp motors;
- DC motors can easily handle overloads for 10-30 seconds;
- To set the parameters, you’ll need only a laptop with internetbrowser. No additional costs in dongles, handhelds and software. Some basic settings are all you have to consider, whereas AC controllers require a thorough study of the manual;
- Forced air-cooling will give the motor 20-30% extra continuous power, and for a relatively low amount this is a luxury for electric motors;
- Maintenance of brushed DC motors is still close to zero. In most occasions, only after 100.000km or 2000 hours of intense use you’ll have to replace the brushes;
- A simple but well engineered RPM sensor is all you need to prevent your motor from over-revving;
For a twin motor setup, only 1 controller can do both motors.
Disadvantages of DC:
- regenerative braking is not supported for brushed DC;
- DC motors are more sensitive to over-revving, because of the construction with the brushes. It is vital for the drivetrain to have a working RPM sensor installed (not-integrated);
- relatively poor temperature monitoring: the thermal switches or thermistors do not work properly with sudden overloads, pushing the motor beyond it’s temperature limits in a short period. They do work when the motor gradually reaches it’s limits;
- a direct drive option requires a set up with a power switch relay, to make it possible to drive backwards. To make this set up safe (no switch activation while driving) requires additional electronics and/or skills;
- the specifications of most DC motors are optimistic, and only valid to a max. load of 1 hour, and with the right RPM, and reasonable to good airflow in the motor compartment.
Summary on DC:
- much driving fun for your money, but beware how to handle: additional temperature monitoring and RPM sensor are mandatory. Not to use in mountainous areas with long (steep) descends;
- Originally meant to replace the petrol motor, mount it on the gearbox with an adapter plate, add some batteries and there you go: and this is how simple it should be.
In the lower power ranges, there is a lot of choice, mostly in combination with a Curtis, Inpower or Sevcon controller. Some motors are completely sealed, which makes them suitable for off-road fun. But, for higher output, the mass-cooled AC motor becomes heavy, and the max. of this type is about 15kW continuous kW power IF you want to stay below 100kg. If you need more power, AC liquid cooled motors are the only option and they are expensive. A 37kW continuous power rated, liquid cooled AC motor will cost at least three times the price of it’s brushed DC equivalent.
The next 10 arguments are based our experiences with HPEV’s, Siemens DMOC and AC PM motors.
Advantages of AC systems:
- All AC controllers support regen. braking;
- AC systems have 3-4% better efficiency then DC;
- High IP rating for AC induction, suitable for off-road applications and boats;
Integrated RPM and temperature sensors which allows the controller to prevent damage from overloads and over-revving;
- In lower power ranges (up to 15kW continuous rated motors) there is lot’s of choice;
- Because of the easy reverse forward switching, AC systems are very suitable for boat conversions, or direct drive applications;
- Most controllers have many parameters which can be alternated;
- Most permanent magnet motors are extremely light weight. We have seen examples of 12kg liquid cooled PM AC motor rated at 30-50kW continuous power (!!), depending on RPM
- AC offers motor/controller options up to 700V suitable for DIY;
- World wide services options for strong brands like Curtis and Sevcon.
Disadvantages of AC:
- Most AC systems are 20-30% more expensive than DC alternatives In higher voltage ranges, prices quickly doubles or triples the price of a DC motor/controller combination. High KW AC motors are even more expensive;
- Input voltage ranges which AC controllers can handle are limited;
- The installation and setting the right parameters in the controller can be quite a hassle: Handhelds and dongles are extra investments. sometimes needed to do a proper installation;
Summary on AC:
AC systems can be regarded as more advanced regarding weight, efficiency and prevention from overheating and over-revving. But that comes with a price. Especially at high performance levels AC systems for DIY purposes are very expensive. Regen braking is nice to have but do you really need it? If you’re really aiming to meet the automotive standard then go for PM AC, and increase your budget!
For medium to heavy weight cars: make your dream come true and take brushed DC for a reasonable budget and a lots of fun scenario. If you know what you’re doing it’s worth saving the money.
For the rest, especially light weight cars and boats: go for the efficient and relatively safe AC options. The extra costs will compensate for the comfort.
For offroad-, mountainous-, and heavy city traffic the only option is AC.