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Controllers
From SanFranciscoEVA
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CONTROLLERS
What is a controller?
A controller is a device that controls the the flow of electricity from the batteries to the motor. This is important in an EV in order to have smooth control of vehicle speed.
A controller is much like a household light dimmer switch, only a lot bigger. By adjusting the knob of the dimmer switch, or moving the potentiometer attached the accelerator in an electric car, it signals a larger electronic switch to vary the size of electrical pulses to the load. The load in a house would be the light bulb, in a car it would be the motor.
Another appropriate analogy is that the controller takes the place of the carburetor or fuel injection system in an "ICE" aka: Infernal Combustion Engine:-).
There is usually a device called a Pot Box which converts the movement of the accelerator pedal or throttle input to a low level electrical signal which the controller can use. The most common Pot box contains a potentiometer with a resistance of 0-5K Ohms with 0 indicating an off position. There are variations to this, such as the inductive throttle sensor, and pot boxes with different resistance ranges, but the 5K pot is the most common for the ====Controllers come in many sizes===
The common DC size specifications are battery voltage range and maximum motor current.
As an example, a typical electric bicycle controller may be rated for 24 to 36 Volts and 50 Amps. This would indicate that it should run on a battery pack with either a 24 or 36 Volt nominal rating. In actual use a 24 V battery can drop as low as 14 Volts under load and a 36 V battery will rise over 42 volts when being charged, so the controller needs to be able to operate over a wider range than the nominal voltage rating. The motor current rating is usually a peak rating. The controller can supply this much current to the motor for a short period of time ranging from 20 seconds to 5 minutes. If the controller is asked to provide more power than is is rated for, it can overheat. Most modern controllers have built in current limits and temperature cutbacks to protect the electronics in a controller from failing in such a situation.
The peak power rating of a controller, as specified in Watts, is the battery voltage multiplied by the peak current. A 36 Volt 50 Amp bicycle controller would be rated at 1800 Watts. Since 746 Watts = 1 HP, this would be about 2.4 HP. But that's electrical power. Since most electric bicycle motors are no more than 60% efficient the actual shaft HP would be under 1.5 HP. It is important to remember that these rating are also only peaks, and the motor will draw the peak current and voltage at only one particular motor speed.
Some typical controller sizes:
- Electric bicycle: 24 to 36 Volts and 50 Amps.
- Golf car or go cart: 36 to 48 Volts and 300 Amps.
- Slow electric car conversion: 72 to 108 Volts and 400 Amps.
- Average electric car conversion: 120 to 144 Volts and 500 Amps.
- Fast electric car conversion: 156 to 192 Volts and 600 to 1000 Amps.
- Fast electric race car: 200+ Volts and 1000+ amps.
- Typical production electric car: 312 Volts and 400 Amps.
The controller is one of the most expensive parts of an EV. Choosing the correct one for your application is a very important task. Many makes and models have a history of blowing up when undersized or due to poor design or manufacturing. When they do blow up, it is often not cost effective to rebuild them.
And while we're speaking of failed controllers: Properly fusing the system with semiconductor fuses (ultrafast response time) is important to reduce the risk of fire in case of a controller failure. This can also improve the odds that a failed controller might be rebuilt.
What controllers are available?
The DC controllers that are available (check the controllers section of the evparts.com website for more detail on the most popular) are from Curtis and DCP. The EVCL "Godzilla" controller is also available by special order from the manufacturer. These controllers span the range of system voltage and current from 72V - 336V and from 400A - 1200A. This is enough of a spread to cover virtually every sort of DC powered EV.
AC controllers are currently not very available, but there is one notable exception. Metric mind (www.metricmind.com) sells surplus inverters and motors manufactured by Siemens. These are very high quality components, with a reasonable price tag.
How does a DC motor controller work?
The DC motor controllers used in EV's almost always use some form of pulse width modulation (PWM) to vary the power delivered to the motor. This includes SCR, MOSFET, and IGBT based controllers. The controller has a timer which cycles every few milliseconds. The controller functions like a switch that turns on with every cycle of the timer. Based on the throttle setting at the time, the switch is turned off somewhere in the cycle. How much of the time cycle the switch remains on determines how much power is delivered to the motor. The motor sees an "average" voltage, and operates based on that voltage level.
Another type of controller is a contactor controller, which uses a number of contactors to switch the batteries among various combinations of series and parallel to acheive different discrete voltage settings.
It is also possible to use a (very) large potentiometer set up as a variable resistor as a controller. The resistor funcions as a voltage divider. This setup uses all the energy that comes from the batteries, but it wastes everything that passes into the resistor, so is very inefficient except at full throttle.
How does an AC motor controller work?
For the most common types of AC motors, changing the AC frequency changes the motor speed. An AC motor controller uses the same technology as used in industrial variable speed drives for AC motors (typically called VFD's for variable frequency drive). Most factories have several machines that use these types of drives. Usually, there is a box on the wall with a small display and a couple of buttons. This is the variable frequency drive. These have been around for a long time, and the technology is well developed. Most modern drives of this type use computer-controlled pulse width modulation techniques to generate an AC waveform at the appropriate frequency. All that is needed is to add an inverter to convert the DC from the batteries into the AC waveform that comes into the drives. Some VFD's can take DC directly as an input. The inverter and adjustable frequency drive stages can be combined to some degree, as they both use PWM technology to do their respective jobs.
AC drives for EV's can be very sophisticated, incorporating software into their controllers that helps to increase low-end torque and to enable the motor to function as a generator to help stop the EV (regenerative braking). One company, AC Propulsion, has incorporated software into their AC controller that allows the controller to double as a charger
Can I build my own controller?
This question has not yet been answered.
Why does my controller whine?
Whining is a characteristic of Curtis controllers, specifically the 1221C and 1231C models. What happens is that at low speeds (and high motor current), the commonly used large motors (ADC 9") did not have the inductance or resistance to let the current fall back into line during the "off" time of the 15 kHz pulse width (see "how does a DC controller work" elsewhere in this FAQ for more definition). To fix this problem, Curtis introduced the "C" models to replace the previous "B" models. The "B" models operated at a constant 15 kHz. The "C" models change to a lower pulse frequency (1.5 kHz) at lower throttle settings. Curtis calls this "frequency shifting." This gives the current time to fall off, so that the next pulse doesn't just increase the current more.
The whine comes from the fact that the 1.5 kHz frequency is right in the middle of the audible range (15 kHz is at the high end). The motor windings "sing" in tune with the PWM'ing of the controller.
Something similar happens when a large AC industrial motor is ramped up to speed using a typical AC drive. As the drive steps up the speed, the motor can be heard to actually play a scale (albeit a little off key) as the frequency changes in small steps.
So controller whining is a part of normal operation for Curtis "C" model controllers. Other brands, like the Auburn and the DCP, operate at a continuous 15-18 kHz, and are designed to properly handle the modern low-inductance, low-resistance EV motors
Can I add regen to my DC EV?
Most DC EV's use series wound motors, which are difficult to add regen to, although the difficult to obtain Zapi controller is available with regen. A few EV's use shunt or compound wound motors, and the controllers for these usually include regen. The drawback is that they are usually one-off or custom items, and are fairly rare. Design of a controller to add electronic regenerative braking to your DC EV (whether series or shunt wound) is beyond the scope of this FAQ. If you want to learn how to do so, then please pose the question to the EVDL (see questions regarding the EVDL in this FAQ for instructions on how to join). Typically, regen is most easily added to an EV with a series wound motor by adding a separate system. This is usually a purpose built motor or generator actuated via the brake light switch.
Do I really need a controller?
That depends on what you call a controller. A bunch of contactors arranged to step the voltage applied to the motor up and down by switching the batteries around in various combinations of series and parallel is sometimes labeled a contactor controller. The next step up is electronic controllers. First there were SCR controllers, and now they use FET's and IGBT's.
So to truly get by without a controller, you would by definition not have any means of changing the voltage applied to the motor, except a large ON-OFF switch. This approach has been used by some drag racers, who just use a (very, very) large contactor as an on-off switch.
Obviously, this results in (very, very) high currents until the motor gets up to speed. Basically you have no means to control your speed or acceleration. While this can be good on the drag strip, it is obviously downright dangerous in, say, a parking lot.
So simply put, yes, you need a controller for a street EV. But that doesn't mean it has to be complicated. As noted above, a contactor controller is fairly simple and has been used effectively.
