7325 Acceleration Uncontrollable Mode UNCM

When a vehicle is rolling down a steep slope, it is possible for the propulsion motor to attain a large value of back-emf. In such a case, if E>Vbat, current cannot be forced into the motor, and the use of Q1 becomes meaningless. The supply voltage saturation limit prevents the driver from supplying more power into the motor to move faster than the velocity attained due to gravity. Therefore, the driver cannot control the vehicle using the acceleration pedal; he can only slow the vehicle by using the brake pedal. If the brake pedal is not used in this situation, then the vehicle enters the uncontrollable mode. When E>Vbat, ia starts to decrease, and once it reaches zero, diode D1 becomes forward biased and turns on. ia continues to increase in the negative direction until it reaches its steady state value of

The mode of operation in this stage is in Quadrant II. The current and switch conditions are shown in Figure 7.20. Depressing the acceleration pedal to increase d1 does not control the vehicle in any way, and the vehicle is, in fact, rolling downwards while regenerating into the source in an uncontrolled way. The protection mechanism must kick in at this stage to prevent overcharging of the batteries. Of course, the driver can regain control by switching to the brake pedal from the acceleration pedal and forcing controlled regeneration through the use of Q2. This will help slow the vehicle on a downhill slope.

FIGURE 7.20 Voltage and current waveforms during acceleration in the UNCM.

7.3.2.6 Braking Operation (CCM in Steady State)

The most efficient way of recovering energy during vehicle braking is through regeneration in the motor drive system. Let us assume that E<Vbat, and the brake pedal is depressed. Q1 is kept off during this period, while braking is controlled through the gate signal ib2. For regeneration, the power flow must be from the motor to the energy source storage, requiring armature current ia to be negative. Turning Q2 on helps ia become negative (from a previously positive value), and an average negative current can be established in a relatively short time for vehicle braking and regeneration. Voltage and current waveforms during braking operation in CCM are shown in Figure 7.21.

An analysis similar to the CCM during acceleration will yield the Ia1 and Ia2 values in the steady state CCM during braking as i

H

H

[v-\

[¿«-i j

A

tbl = 0

A

¿2

r

W

/

W

A

Vbatt

Qi off D, off Q; on »i ^ff

Oioff D | on Qitiff Djoff

FIGURE 7.21 Voltage and current waveforms during braking operation in CCM.

The current ripple during braking is

During braking CCM, Ia1<0 and E<VbatAlso, note that

Therefore, the condition for continuous conduction during braking is

The average voltage equation in the braking CCM is

The average motor torque is

The average motor terminal voltage is

Substituting in from Equation 7.15 and iO from Equation 7.14 into Equation 7.13,

The average speed-torque characteristics are as follows (Figure 7.22):

0 0

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