Selecting VSDs and AC motor converters--part 3

Starting Requirements

Variable torque loads, such as centrifugal pumps and fans have a very low starting torque requirement and are easily pulled away and accelerated to the set speed by any VSD. The main area for concern is the high breakaway torque sometimes required on some pumps, such as slurry pumps, where some sediment can settle inside the pump during periods when the pump is stopped. The other limiting factor is the total absorbed power at full rated speed, which must be within the capacity of the drive. Constant torque loads, such as conveyors and positive displacement pumps, are slightly more difficult because they require full torque at starting, but this does not usually present a problem. However, for some types of load, such as wood-chip screw conveyors, an additional breakaway torque may also be required to pull them away from standstill. Other examples of this are extruder drives and positive displacement pumps, particularly when used with congealing fluids. This high torque is usually of a temporary nature but the drive must be selected to ensure that the VSD can provide the necessary breakaway torque without stalling.

There are two main factors that affect the starting and low speed torque capability of a squirrel cage motor controlled by an AC VVVF converter.

• To avoid over-fluxing the motor, the V/f ratio must be kept constant. At low frequencies, the voltage applied to the stator of the motor is low to keep this V/f ratio constant. Referring to the equivalent circuit of an induction motor , there is a volt drop in the stator winding and the air-gap flux is then significantly reduced. This affects the output torque of the drive. The problem can be relatively easily overcome by boosting the voltage at low speeds to compensate for the stator volt drop. Most modern converters provide a torque boost setting that may be adjusted by the user.

• Most VVVF converters have a current limiting control feature to protect the power electronic components against over-currents. So the maximum motor current is limited to the current limit setting on the converter. Since the motor torque is roughly proportional to the current, the output torque is limited to a value determined by the converter current limit setting.

Consequently, the starting torque is mainly limited by the current limit setting of the converter. It’s not economical, and usually not necessary, to design a converter with an excessively high current rating. So the starting torque capability is dependent on the extent to which the converter current rating exceeds the motor rated current. The converter is usually designed to run continuously at its rated current IN, with an over current rating of 150% the converter current rating, but for a limited time, usually of 60 sec. The current limit control is then set at the 150% level with a protection timer which times out after the period of 60 secs.

Starting torque of the variable speed drive system:

...where TN = Rated torque of the motor in Nm

Clearly, with an over-sized converter, there is a limit to how much torque the motor will produce above its rated torque. The motor will usually stall at 2.5 to 3 times its rated torque, depending on the design.

For very high starting torques, a larger motor and converter should be considered or the matter should be referred to the manufacturer.

Stopping Requirements

When a drive is operating in the first or third quadrants, the machine is operating as a motor and is driving a mechanical load respectively in the forward or the reverse direction. Energy conversion is taking place from electrical energy to mechanical energy.

Energy is stored in the rotating system as kinetic energy. When the drive changes its operation to the second or fourth quadrants, braking is required to retard the speed of the mechanical load. To reduce speed, the kinetic energy needs to be removed from the rotating system (AC induction motor plus mechanical load) and transformed into some other form of energy before the system can come to standstill.

This is usually a major problem with high inertia loads.

There are several methods of decelerating and stopping a variable speed drive system:

• Coast to stop, where the kinetic energy is dissipated in the load itself

• Mechanical braking, where the kinetic energy is converted to heat due to friction

• Electrical braking, where the kinetic energy is initially converted to electrical energy before being transferred back to the power supply system or dissipated as heat in the motor or a resistance

Most fixed and variable speed drives are stopped by removing the power and allowing the driven machine to coast to a stop. The rotating system, comprising the motor and load, would come to a stop after a 'natural' deceleration t_N time.

This type of stopping is adequate for most mechanical loads such as conveyors, screw conveyors, fans, etc. The actual stopping time depends on the load inertia, load losses and the type of process. However, there are some applications where additional braking is required to provide a shorter deceleration time.

++++ Braking times for rotating drives

The traditional approach was to use mechanical braking, but this requires considerable maintenance to both the mechanical parts and the brake pads. With mechanical braking, the braking energy is dissipated in the form of heat by the friction between the brake pads and the brake disk/drum.

For modern variable speed drive systems, electrical braking is the preferred method of braking. Electrical braking systems rely on temporarily using the motor as an induction generator with the mechanical load driving the generator.

It should be appreciated that, a motor always puts out a torque in a direction so as to cause the rotor to approach synchronous speed of the rotating air-gap magnetic field.

• In the motoring mode, the inverter output frequency will always be higher than the rotor speed

• In the generating (braking) mode, the inverter output frequency will be at a frequency lower than the rotor speed. The braking torque produced during deceleration is dependent on the slip in the motor.

During electrical braking, energy conversion takes place from mechanical energy to electrical energy. This energy can be disposed of in three ways:

• Dissipated as heat in the rotor of the motor, DC braking

• Dissipated as heat in the stator of the motor, flux braking

• Dissipated as heat in an external resistor, dynamic braking

• Returning electrical energy to the supply, regenerative braking

Electrical braking has several advantages over mechanical braking:

• Reduction in the wear of mechanical braking components

• Speed can be more accurately controlled during the braking process

• Energy can sometimes be recovered and returned to the supply

• Drive cycle times can be reduced without any additional mechanical braking

Current-source inverters (CSI) are capable of regenerative braking without modification and other braking techniques need not be considered.

Voltage-source inverters (VSI), which include PWM types, cannot regenerate without costly modifications to the rectifier module. The other electrical braking methods should always be considered first, provided that the cost of the lost energy is not critical.

DC Injection Braking

The basic principle of DC injection braking is to inject a DC current into the stator winding of the motor to set up a stationary magnetic field in the motor air-gap. This can be achieved by connecting two phases of the induction motor to a DC supply. The injected current should be roughly equal to the excitation current or no-load current of the motor.

In a PWM type VVVF converter, DC injection braking is relatively easy to achieve.

The inverter control sequence is modified so that the IGBTs in one phase are switched off while the other two phases provide a PWM (pulsed) output to control the magnitude and duration of the DC current. The configuration is shown.

++++ DC injection braking from a PWM converter

As the rotor bars cut through this field, a current will be developed in the rotor with a magnitude and frequency proportional to speed. This results in a braking torque that is proportional to speed. The braking energy is dissipated as losses in the rotor windings, which in turn, generate heat. The braking energy is limited by the temperature rise permitted in the motor. Precautions should be taken to check the motor heating time constant when using this method.

The braking torque won’t be high unless the rotor has been designed to give a high starting torque, i.e., has a high resistance or shows significant deep bar effect.

Another difficulty is that the braking torque will reduce as zero speed is approached and mechanical brakes might be necessary to bring the motor to rest sufficiently quickly or to hold a position at standstill. Nevertheless, the method can still give significant reductions in mechanical brake wear. All the braking power goes into heating up the rotor and this may limit the braking duty.

Motor over-flux braking

A technique that is gaining increasing popularity with modern PWM AC drives is the control of the motor flux. By increasing the inverter output V/f ratio during deceleration, the motor can be driven into an over-fluxed condition, thereby increasing the losses in the motor. The braking energy is then dissipated as heat in the stator winding of the motor. In many ways this is similar to DC injection braking because the braking energy is dissipated in the motor rather than the converter.

Braking torques of up to 50% of rated motor torque are possible with this technique.

Again the braking energy is limited by the temperature rise permitted in the motor.

Dynamic Braking

When the speed setting of the VVVF converter is reduced, the output frequency supplied to the connected motor is also reduced and the synchronous speed of the motor will decrease. However, this does not necessarily mean that the actual speed of the motor will change immediately. Any changes in the actual motor speed will depend on the external mechanical factors, particularly the inertia of the rotating system.

++++ illustrates the change in the motor torque when the converter output frequency is suddenly reduced from f0 to f1. The slip changes from being positive (motoring) to negative (generating) and the direction of energy flow is reversed, kinetic energy is converted to electrical energy in the motor, which is then transferred from the motor to the converter.

++++ Torque-speed characteristics of an induction motor when frequency is reduced from f0 to f1 In practice, the output frequency from the VVVF converter is reduced slowly to avoid the large braking currents that would otherwise flow. For maximum braking torque, the current can be controlled to remain at or below the current limit of the inverter bridge.

This procedure can be viewed as being the opposite of the normal startup sequence, where the motor accelerates from standstill to full speed at current limit.

During braking, the converter must have some means of dealing with the energy transferred from the motor. Since the polarity of the DC bus voltage does not change in the braking mode, the direction of the DC bus current reverses during braking. On PWM converters, which use a diode rectifier bridge, the braking current is blocked from returning to the mains power supply. Therefore, unless some mechanism is provided to absorb this energy during braking, the voltage on the DC bus will rise to destructive levels.

With dynamic braking, the braking energy is dissipated in a braking resistor connected across the DC bus of the converter. As described above, braking is achieved by reducing the inverter output frequency to be less than the actual rotor speed. The slip can be optimized to give as high a torque per ampere as for motoring.

Power flow is from the motor back through the inverter to the DC bus. The braking energy cannot be returned to the mains supply because the input rectifier can only transfer power in one direction. Instead, the energy is absorbed by the DC capacitor, whose voltage rises. To prevent the DC bus voltage rising to a dangerously high level, the capacitor needs to be periodically discharged. This is done by means of a dynamic brake module, consisting of a power electronic switch, usually an IGBT or BJT, and a discharge resistor connected across the DC link capacitor.

++++ PWM AC converter with a DC link dynamic brake

The IGBT or BJT is controlled by a hysteresis circuit to turn on when the capacitor voltage is too high and turn off when the voltage drops below a certain level. Alternatively, the IGBT may be switched on and off at constant frequency, with duty cycle varying linearly between 0% and 100% as bus voltage changes over specific range.

The switching level of the braking IGBT should be chosen to be higher than the mains supply when it’s operating at highest voltage tolerance, but below the maximum safe switching voltage of the inverter components. In practice, for a converter connected to a 3-phase 415 volt supply, with a nominal DC peak voltage of 586 V, the switching level would have to be set at least 10% above this at 650V, but below 800V, which is the maximum safe operating voltage of the DC bus. A practical switch-on level is typically 750V, with the hysteresis between the upper and lower level being 20V to 30V lower.

The range of allowable voltage swing is determined by the IGBT and capacitor voltage ratings and the tolerance on the supply voltage.

When the motor speed is very low, the inverter can apply a frequency which is slightly negative to maintain the necessary slip for good braking torque, allowing the motor to be electrically braked right down to zero speed. However, this operation requires a high quality of control and braking is often only available to about 2% of rated speed with standard drives.

The resistor value is selected to allow a DC bus current which corresponds to 100% rated torque at maximum motor speed and its power rating must reflect the required braking duty with respect to duration, magnitude and frequency of braking. The braking IGBT must be selected to switch the maximum braking current, determined by the value of braking resistor and the maximum bus voltage. The IGBT is usually of the same type and size as that used in the inverter stage.

++++ DC bus voltage with hysteresis control on a dynamic brake Example: A 22 kW VSD and motor combination must provide 100% rated motor torque while braking. The maximum braking duty is 3 seconds in every 10 seconds.

Assume that the bus voltage during braking is 650 V DC and the DC bus over-voltage trip level is set at 700 V DC. What braking resistor should be used for the application?

To achieve rated torque while braking, the resistor must absorb a full 22 kW when the motor is at full speed. Therefore, the maximum DC bus current will be roughly ...

To absorb 34 amps when the bus voltage is at 650 V DC, the braking resistor will need to have a resistance of 650/34 = 19 ohms. If braking only occurs for 3 sec in every 10 sec, then the duty will be 30%. The power rating of the braking resistor will then be 30% of 22 kW, which is approximately 7 kW continuous. Care must be taken to allow adequate excess power rating when selecting a braking resistor, as the instantaneous power is very high and hot spots can cause premature failure.

The maximum transistor current will occur at the maximum DC bus voltage ...

Allowing a safety margin, a braking transistor rated at 50 amps would be selected.

Regenerative braking

From the point of view of the inverter, regenerative braking is achieved in a similar way as for dynamic braking. When braking is required, the output frequency of the inverter is reduced to a level below the actual rotor speed. The path for the braking power is from

the motor through the reverse connected diodes of the inverter, into the DC bus capacitor,

... which rises in voltage. Since the normal diode rectifier cannot return power to the mains supply, a thyristor converter must be used.

The two alternative methods are illustrated below:

• If a thyristor rectifier bridge is used in place of the diode bridge to supply power for normal motoring, the current flow in a thyristor rectifier cannot change direction for braking. Regeneration is only possible by changing the polarity of the DC bus voltage. This can be achieved by fitting a reversing switch between the rectifier and the capacitor and switching it according to the required power flow direction. Such a system is useful in drives where braking is occasional rather than continuous and the changeover does not need to be fast (e.g. small electric locomotives).

++++ VSD with reversing switch on DC bus for regenerative braking

• For faster transfer to braking, the system can be used with a diode rectifier to supply the motoring power and a thyristor rectifier to extract the braking power.

++++ VSD with separate thyristor bridge for regenerative braking Note that it’s not possible to operate both rectifiers from the same level of AC voltage. Suppose the supply voltage is 415 V phase-to-phase. During motoring, from the formula, the DC bus voltage across the capacitor can be estimated as ...

The capacitor voltage will rise during braking with 700VDC being a typical value.

The thyristor rectifier will need to operate as an inverter with a firing angle greater than 90 deg. and with a negative DC voltage, which is the reason for its connection in reverse polarity compared to the diode rectifier.

At firing angles near 180 deg., slight noise on the supply can prevent a thyristor from fully recovering its forward blocking ability. The firing of the next thyristor gives a short circuit path across the DC side, a condition called inversion failure and which is difficult to clear. To prevent this, it’s usual not to operate a thyristor rectifier with a firing angle greater than 150 deg. From the motor point of view, there is essentially no difference between dynamic braking and regenerative braking. The main trade-off is between the initial cost of the regenerative system compared to the economy of returning the braking energy to the mains. This depends on the type of application, the braking effort and the duration of the required braking.