Electric Vehicle Conversion Help - Electric Vehicle Controller Operation

Electric Vehicle Controller Operation

Understanding power, voltage and current in an EV motive system

By Mark E. Hazen

This article is intended to help EV owners understand motor controller operation in terms of power, voltage and current. Basic calculations reveal extreme differences between the average battery current, as viewed on the in-cab ammeter, and the motor current. This understanding is then applied to motor/controller operation under different load conditions to see how a controller might fail.

Basic Premise: P_B = P_M

To begin, our basic premise is that the power supplied by the battery bank (P_B) is, for the most part, equal to the power used by the motor (P_M), with the realization that a relatively small amount of power is wasted as heat in the controller. This premise helps simplify the discussion.

In reality, the power used by the motor is equal to the supplied battery power minus losses in the controller and cabling. The losses in the controller and cabling are very small, compared to what is being used by the motor, so we will ignore controller and cabling losses, along with other miner losses, in our calculations.

So, we are saying that supplied battery power (P_B) equals motor power (P_M): P_B = P_M

Transformation Ratio: t_on/T

Power is measured in Watts (W) where,

Power = Current X Voltage = Amps X Volts (100 A X 150 V = 15,000 W = 15 kW)

Figure 1 shows the battery side calculations, on the left side, that reveal the relationship between the motor current and the battery Current. The average battery current (I_B) is low compared to the motor current (I_M) and the ratio of the battery current to the motor current is equal to the ‘on’ time (t_on) to total cycle time (T) ratio (t_on/T).

The ‘on’ time (t_on) is the time during which the switches in the controller are turned on allowing current to flow. The total cycle time (T) includes the ‘off’ time when the controller switches are turned off.

The power delivered by the battery bank is the product of a high battery bank voltage and a battery current that is a fraction of the motor current (I_M) where I_B = I_average = I_M(t_on/T).

P_B = I_M(t_on/T) V_B

(t_on/T) can be referred to as a ‘transformation ratio’ that determines I_B from a given value of I_M, or can be used to determine a value of I_M based on a known value of I_B.

On the right side of Figure 1, we see that the average motor voltage is affected by transformation ratio ton/T where V_M= V_average = V_B(t_on/T).

The power used by the motor is the product of the motor current and the average motor voltage as determined by t_on/T.

P_M = I_MV_B(t_on/T)

Now, let’s work with these relationships in an actual example.

Example:

In this example, your foot on the accelerator pedal has positioned the throttle potentiometer to create a pulse width whose ratio of t_on/T is 0.1.

If t_on = 10us and T = 100us, then transformation ratio t_on/T = 0.1

If V_B = 150 V and average battery current I_B = 100 A, then

P_B = 100 A X 150 V = 15 kW

And,

V_M = 0.1V_B = 0.1 X 150V = 15 V = average motor voltage

Because P_B = P_M, P_M must equal 15 kW = I_MV_M = I_M X 15 V

So,

I_M = 15 kW/15 V = 1000 A

Also,

I_B/I_M = 100 A/1000 A = 0.1 = transformation ratio t_on/T

Note that the motor current is 10 times higher than the average battery current in this example when t_on/T = 0.1. However, the average motor voltage is only 15 V, so 15 V X 1000 A = 15 kW.

Power from the battery (P_B) = Power used by the motor (P_M) Power = Current (I) X Voltage (V) Power In = Power Out
Battery Side Calculations	Motor Side Calculations
P_B = I_BV_B	P_M = I_MV_M

Where, I_B = I_average = I_M(t_on/T)	Where, V_M= V_average = V_B(t_on/T)
So, P_B = I_M(t_on/T) V_B	So, P_M = I_MV_B(t_on/T)
Note: The battery current is low because of the transformation ratio (t_on/T), but V_B is the full voltage of the battery bank.	Note: The motor current is high, but the motor voltage is low because of the transformation ratio (t_on/T). The motor voltage reaches maximum when t_on/T = 1 where V_M = V_B.

Figure 1: Battery Side and Motor Side Calculations

Figure 2 is an overall system view showing the currents and voltages used in our calculations.

Figure 2: A System View of Voltages and Currents

Figure 3 is a graph that shows what the motor current would be at different values of t_on/T. In this graph, the average battery current is held constant for all values of t_on/T so that you can see the relative values of motor current and battery current for all throttle positions. You can see that when t_on/T is low, the motor and controller switch pulse currents are very high.

Figure 3: Motor Current as a Function of t_on/T at Constant Average Battery Current

Now that we understand that the controller transforms current and voltage according to the transformation ratio t_on/T, we can apply this to different conditions to further understand controller and motor operation.

Motor Acts as Generator Creating a Counter Voltage

When the throttle is first applied to the controller and the controller begins to feed current and voltage to the motor, the motor starts rotating and accelerating. Initially, the motor offers very little opposition to the current flow because the motor’s opposition to current is based on its RPMs. The faster the motor turns, the more opposition the motor develops to the current. This opposition that is developed by the motor is called counter electromotive force (CEMF), which is simply an opposing voltage that is being generated by the motor as it turns. In other words, the motor is also acting as a generator, generating a voltage that is in opposition to the voltage applied to the motor.

If the motor is free to accelerate rapidly, the generator action (CEMF) will increase quickly and will work to reduce the current being demanded by the motor. On the other hand, if the motor is caused to labor under mechanical load for an extended period of time, the current through the motor will be high for that period of time.

Motor Loading and Consequences

So, what can cause this high-current condition over an extended period of time? If you start out in first or second gear, the high gear ratio allows the motor to increase its RPMs quickly, so the motor and controller experience moderately high currents for only a relatively short period of time. On the other hand, if you start out in third, forth or fifth gear, or even direct drive, the motor will take much longer to accelerate and to generate an opposing voltage to limit current. During the labored acceleration period, both the average battery current and the motor currents are very high, much higher than when starting out in first gear.

As the throttle is pressed, the ratio of t_on/T increases toward 1, which is 100% duty cycle or full ON. At any given t_on/T ratio, the actual battery current and the motor current will be determined by the mechanical load on the motor, where the mechanical load is determined by the gear ratio and the mass of the vehicle, and a few other things.

When you start out in first gear, the motor has the leverage of the gear ratio to work against the load. Therefore, the motor doesn’t demand as much current, and power, as it would if starting out in a higher gear.

So, if you start out in a high gear or direct drive, the motor will demand very high starting and running currents because of the heavy motor loading and because the motor cannot accelerate quickly. You are using much more power and energy when forcing the motor to operate under heavy load. What is more, if you go to full throttle right away from start, the motor receives full voltage when its opposition to current (CEMF) is very low. This causes a worst-case condition in which the motor current is extremely high, stressing all system components: motor, batteries and controller.

Heavy loading is hard on the motor because the very high sustained currents heat up the windings and the brushes. If the winding insulation breaks down because of high temperatures, the motor is toast. The brushes and commutator also wear out more quickly.

Heavy loading is also hard on the controller because, especially at duty cycles lower than 50%, where t_on/T is less than 0.5, the motor currents can be very high, threatening the MOSFET, or IGBT, switches inside the controller. (Refer again to Figure 3.) Electronic devices used in controllers have a continuous current rating, specified usually at two temperatures (25^oC and 125^oC), and a peak pulse current rating. If either of these ratings is exceeded, the switches can and will fail.

The current limiting circuit inside the controller cannot protect the electronic switches from all scenarios. At low duty cycles where t_on/T is less than 0.5, the high average battery current is transformed into even higher motor current pulses that may not be totally captured in time by the current limiting circuit. Slight time delays in current limiting activation open windows of opportunity for unbridled current surges to destroy the switches.

Even assuming that the current limiting circuit is ideal, there are cases in which the current rating of the controller switches is exceeded. Where a motor is connected directly to the vehicles differential, average battery current may already be 400 A at 10% throttle (t_on/T = 0.1), which means that the motor current and the peak pulse current through the controller switches starts at 4000 A. This may, or may not, be within the capability of the switches at a temperature of 25^oC, but, to make things worse, as the switches heat up, their current handling abilities decrease.

How about at full throttle? If the switches are turned full ON, the battery current and the motor currents must be equal. This is only true if the motor current is less than the current limit of the controller. If the current limit is 500 A and the motor is demanding more, the current limit circuit will kick in and actually reduce the duty cycle to an amount less than 1, where t_on/T is less than 1. This forced duty cycle reduction keeps the average battery current at 500 A while the motor current is higher during t_on. While you are reading 500 A on your ammeter, the switches in the controller are experiencing much higher pulse motor currents.

As an example, under direct drive conditions the throttle may be applied full ON to cause the vehicle to accelerate rapidly under this heavy mechanical load. In so doing, the controller goes into current limiting right away, which means that the controller is not really full ON. The current limiting circuit reduces the t_on/T ratio well below 1 to keep the average motor current and battery current at the limit. If the limit is 500 A and t_on/T is held back to initially 0.2 (20%), the motor current and peak pulse current through the switches will be 2500 A. This high pulse current may be a threat to the switches, especially as temperature rises.

As the direct-drive motor increases its RPMs, the ratio of ton/T increases toward 1 and the motor and peak controller currents decrease while the average battery current is held at 500 A. Also, the average continuous current through the switches in the controller stays at 500 A for an extended period of time. This period of acceleration causes rapid heating of the switches in the controller, which may cause them to fail if the heat sinking cannot take the heat away quickly enough.

Many good controllers are destroyed because some EV owners don’t understand this. A controller that is designed for reliable everyday transportation use can easily be destroyed when employed in heavy-load and racing applications.

EV’ers that race their vehicles experience this with every run. That’s why those who race buy 1,000 A and 2,000 A controllers that can survive the heavy loading of racing.

Racers know that there is a big difference in the price of a racing controller and a street controller. Novices get into trouble when they buy a street controller to save money and use it for racing, or abuse it by using high gears or direct drive all of the time.

The bottom line is, if you want to go direct drive, start out in high gears or want to race, buy a higher-priced controller that is designed for heavy loading applications.

Summary of Key Points

Motor current is a lot higher than average battery current (ammeter current) when the controller switches’ duty cycle is less than 50%, where t_on/T is less than 0.5.
Starting out in a high gear or direct drive or full throttle represents heavy loading, which may exceed the ratings of the controller switches.
As the temperature of the controller switches increases, the continuous and peak current handling capability of the switches inside the controller decreases.
Street controllers are designed to be low-cost and reliable for everyday transportation applications.
Racing controllers are much more costly than street controllers because they are made with more costly heavy-duty components to withstand the high currents caused by heavy mechanical loading.
Don’t use a low-cost street controller for heavy-loading applications such as high-gear starts, direct drive or racing.

________________

Power from the battery (PB) = Power used by the motor (PM)

Power In = Power Out

Power from the battery (P_B) = Power used by the motor (P_M)