**2.1 Torque and power requirements for crane drives**

Speed control is an essential feature in crane drives. It is required for allowing soft starting and stopping of the travel motions for enabling its correct positioning of load. For the lifting drive the speed control in a wide speed range, from zero to nominal values, is required.

Electrical Drives for Crane Application 135

Fig. 2. Power and torque requirements for travel motion, a) and b) without wind influence,

Fig. 3. Power and torque requirements for hoist a) and b) without load, c) and d) with load.

The hoist torque-speed characteristic is shown in Fig.3a) for an unloaded hook. The characteristic resembles the one for the travel motion. However, it is always asymmetric with respect to the vertical axis, due to the gravitation force. This asymmetry becomes more pronounced when the hook is loaded (Fig.3c). For both unloaded and loaded situation, the speed, torque and power are given in Fig.3b) and Fig.4d). Again the amount of braking power is indicated. The worst braking case with a hoist motion, is when sinking a loaded hook. It should be noted that the weight of the hook may be considerable. The hook may be simple, or may consist of several parts to handle the load. For the hoist motion, the speed

c) and d) with wind influence.

Because of the precision when raising and lowering load, the possibility of working at a very low speed and hold a load in the standstill is required, without using the mechanical brakes.

The torque and power that have to be delivered by the drive may be obtained from the torque versus speed characteristic from the load (so-called mechanical characteristics) and the differential equation of motion, (Belmans et al., 1993; Mitrovic et al., 2011).

The differential equation of motion, describing the behavior of the drive is:

$$J\frac{d\phi}{dt} = T\_e - T\_l \tag{1}$$

where *Te* is the electromagnetic torque of the motor, *Tl* is the torque of the load, *J* is the inertia of the drive. If *Te* > *Tl*, the system accelerates (*d*/*dt* > 0), if *Tl*/*Te* < 0 it decelerates (*d*/*dt*>0). The stedy state operation is reached if *Tl* = *Te* and  *= const.* 

Multiplying equation (1) by the rotating speed , yields the power:

$$P\_e = \alpha \mathbf{T}\_e = \alpha \mathbf{T}\_l + \alpha \mathbf{J} \frac{d\alpha}{dt} = P\_m + P\_d \tag{2}$$

This equation shows that the mechanical power *Pe*=*Te*, obtained after the electromechanical conversion in the motor, is equal to the power absorbed by the load *Pm*=*Tl* only when the speed does not change. Otherwise, the amount corresponding to change in kinetic energy must be added (if the speed increases) or subtracted (if the speed decreases):

$$P\_d = \frac{dE\_{kin}}{dt} = \alpha \frac{Id\alpha}{dt} \tag{3}$$

In the following, the travel and hoist motion of the crane drives will be analyzed.

The mechanical characteristic is given in Fig.2a) for travel motion. Apart from the zone around zero, the torque is constant. The available torque is used for accelerating the system. For a travel in one direction, braking and reversing to full speed in other direction, the speed reference signal is given by top curve of Fig.2b). The torque reference signal is generated by converter (second curve), leading to the machine actual speed. Multiplying the actual and torque reference, yields the actual power (third curve). The peak power is found at the end of the acceleration period. If wind forces are taken into consideration, the torque versus speed curve is shifted horizontally as shown in Fig.2c). The torque and speed reference remain the same, as well as the actual speed. However, the torque reference and the actual power differ, as shown on Fig.2d).

During acceleration, kinetic energy is stored in the system. To stop the crane, this energy must be absorbed by the drive. In the indoor situation, this energy is well known and only present for a short period of time. For outdoor applications, the wind forces may become very important. When travelling in the same direction as the wind, the wind drives the crane and a situation may occur, where a continuous electrical braking is required. The drive must be capable of handling this inverse power direction either by consuming the power in a resistor or preferably by feeding it back to the supply.

Because of the precision when raising and lowering load, the possibility of working at a very low speed and hold a load in the standstill is required, without using the mechanical brakes. The torque and power that have to be delivered by the drive may be obtained from the torque versus speed characteristic from the load (so-called mechanical characteristics) and

*e l*

where *Te* is the electromagnetic torque of the motor, *Tl* is the torque of the load, *J* is the inertia of the drive. If *Te* > *Tl*, the system accelerates (*d*/*dt* > 0), if *Tl*/*Te* < 0 it decelerates

> *eel m d <sup>d</sup> P T T J PP dt*

This equation shows that the mechanical power *Pe*=*Te*, obtained after the electromechanical conversion in the motor, is equal to the power absorbed by the load *Pm*=*Tl* only when the speed does not change. Otherwise, the amount corresponding to change in kinetic energy

*kin*

 

The mechanical characteristic is given in Fig.2a) for travel motion. Apart from the zone around zero, the torque is constant. The available torque is used for accelerating the system. For a travel in one direction, braking and reversing to full speed in other direction, the speed reference signal is given by top curve of Fig.2b). The torque reference signal is generated by converter (second curve), leading to the machine actual speed. Multiplying the actual and torque reference, yields the actual power (third curve). The peak power is found at the end of the acceleration period. If wind forces are taken into consideration, the torque versus speed curve is shifted horizontally as shown in Fig.2c). The torque and speed reference remain the same, as well as the actual speed. However, the torque reference and the actual

During acceleration, kinetic energy is stored in the system. To stop the crane, this energy must be absorbed by the drive. In the indoor situation, this energy is well known and only present for a short period of time. For outdoor applications, the wind forces may become very important. When travelling in the same direction as the wind, the wind drives the crane and a situation may occur, where a continuous electrical braking is required. The drive must be capable of handling this inverse power direction either by consuming the

power in a resistor or preferably by feeding it back to the supply.

*dE Jd <sup>P</sup> dt dt*

  (1)

 *= const.* 

(2)

(3)

the differential equation of motion, (Belmans et al., 1993; Mitrovic et al., 2011).

*d J TT dt* 

The differential equation of motion, describing the behavior of the drive is:

(*d*/*dt*>0). The stedy state operation is reached if *Tl* = *Te* and

power differ, as shown on Fig.2d).

Multiplying equation (1) by the rotating speed , yields the power:

must be added (if the speed increases) or subtracted (if the speed decreases):

*d*

In the following, the travel and hoist motion of the crane drives will be analyzed.

Fig. 2. Power and torque requirements for travel motion, a) and b) without wind influence, c) and d) with wind influence.

Fig. 3. Power and torque requirements for hoist a) and b) without load, c) and d) with load.

The hoist torque-speed characteristic is shown in Fig.3a) for an unloaded hook. The characteristic resembles the one for the travel motion. However, it is always asymmetric with respect to the vertical axis, due to the gravitation force. This asymmetry becomes more pronounced when the hook is loaded (Fig.3c). For both unloaded and loaded situation, the speed, torque and power are given in Fig.3b) and Fig.4d). Again the amount of braking power is indicated. The worst braking case with a hoist motion, is when sinking a loaded hook. It should be noted that the weight of the hook may be considerable. The hook may be simple, or may consist of several parts to handle the load. For the hoist motion, the speed

Electrical Drives for Crane Application 137

regenerative operating mode. For example, these regenerative conditions can occur when quickly decelerating a high inertia load and this can be considered as transient condition. The speed control of a load moving vertically downward (hoist) can be considered as

Fig. 5. Control scheme: a) V/Hz, b) vector control, c) DTC control.

winches and cranes (ABB, 2011).

Drive applications can be divided into three main categories according to speed and torque. The most common AC drive application is a single quadrant application where speed and torque always have the same direction, i.e. the power flow from inverter to process. The second category is two-quadrant applications where the direction of rotation remains unchanged but the direction of torque can change, i.e. the power flow may be from drive to motor or vice versa. The third category is fully four-quadrant applications where the direction of speed and torque can freely change. These applications are typically elevators,

In order for an AC drive to operate in quadrant II or IV in speed-torque plane, a means must exist to deal with the electrical energy returned to the drive by the motor. The typical pulse width modulated AC drive is not designed for regenerating power back into the three phase supply lines. Electrical energy returned by the motor can cause voltage in the DC link to become excessively high when added to existing supply voltage. Various drive components can be damaged by this excessive voltage. These regenerative conditions can occur when:

normal operating condition.

control is very important in the low speed range: avoiding damage to the load when putting it down and minimizing the stress on the mechanical brakes.

### **2.2 Frequency converters for induction motor drives**

Today's adjustable speed drives (ASD) in the low and mid power range are normally based on the concept of variable voltage, variable frequency (VVVF). Fig.4 shows the basic concept of a single variable speed drive. The three-phase AC supply network is rectified. The DC capacitor, which links the supply rectifier to the inverter, assures that the inverter sees a constant DC voltage from which it generates the required supply voltage and frequency to the motor.

Fig. 4. Basic concept of a variable speed drive.

General classification divides induction motor control schemes into scalar and vector-based methods (Petronijevic et al., 2011). Opposite to scalar control, which allows control of only output voltage magnitude and frequency, the vector-based control methods enable control of instantaneous voltage, current and flux vectors. In numerous industrial applications, such as HVAC (heating, ventilation and air conditioning), fan or pump applications, good dynamic performances are not usually the main control objective. Fig. 5a) illustrates a V/Hz open loop control scheme, where pulse-width modulation (PWM) is realised applying the space vector technique (SVPWM) and output voltage fundamental component amplitude is modified with a voltage drop compensation *Us*0 at low output frequencies. Basic control structure of rotor field oriented control (RFO) is illustrated in Fig. 5b). Two inner PIcontrolled current loops for *d* and *q* stator current components are shown, as well as synchronous speed estimator (e, based on reference stator currents components). In its basic version, direct torque control (DTC) consists of a three-level hysteresis comparator for torque control and a two-level hysteresis comparator for flux control as shown in Fig.5.c).

Type of the front end converter, regardless of the control schemes, depends on the power and torque requirements of the drive. Adjustable speed drives in industrial applications are usually characterized by a power flow direction from the AC distribution system to the load (Rashid, 2001). This is, for example, the case of an ASD operating in the motoring mode. In this instance, the active power flows from the DC side to the AC side of the inverter. However, there are an important number of applications in which the motor begins to act as a generator and regenerates energy back into the DC bus of the drive. Moreover, this could be a transient condition as well a normal operating condition. This is known as the

control is very important in the low speed range: avoiding damage to the load when putting

Today's adjustable speed drives (ASD) in the low and mid power range are normally based on the concept of variable voltage, variable frequency (VVVF). Fig.4 shows the basic concept of a single variable speed drive. The three-phase AC supply network is rectified. The DC capacitor, which links the supply rectifier to the inverter, assures that the inverter sees a constant DC voltage from which it generates the required supply voltage and frequency to

General classification divides induction motor control schemes into scalar and vector-based methods (Petronijevic et al., 2011). Opposite to scalar control, which allows control of only output voltage magnitude and frequency, the vector-based control methods enable control of instantaneous voltage, current and flux vectors. In numerous industrial applications, such as HVAC (heating, ventilation and air conditioning), fan or pump applications, good dynamic performances are not usually the main control objective. Fig. 5a) illustrates a V/Hz open loop control scheme, where pulse-width modulation (PWM) is realised applying the space vector technique (SVPWM) and output voltage fundamental component amplitude is modified with a voltage drop compensation *Us*0 at low output frequencies. Basic control structure of rotor field oriented control (RFO) is illustrated in Fig. 5b). Two inner PIcontrolled current loops for *d* and *q* stator current components are shown, as well as synchronous speed estimator (e, based on reference stator currents components). In its basic version, direct torque control (DTC) consists of a three-level hysteresis comparator for torque control and a two-level hysteresis comparator for flux control as shown in Fig.5.c).

Type of the front end converter, regardless of the control schemes, depends on the power and torque requirements of the drive. Adjustable speed drives in industrial applications are usually characterized by a power flow direction from the AC distribution system to the load (Rashid, 2001). This is, for example, the case of an ASD operating in the motoring mode. In this instance, the active power flows from the DC side to the AC side of the inverter. However, there are an important number of applications in which the motor begins to act as a generator and regenerates energy back into the DC bus of the drive. Moreover, this could be a transient condition as well a normal operating condition. This is known as the

it down and minimizing the stress on the mechanical brakes.

**2.2 Frequency converters for induction motor drives** 

Fig. 4. Basic concept of a variable speed drive.

the motor.

regenerative operating mode. For example, these regenerative conditions can occur when quickly decelerating a high inertia load and this can be considered as transient condition. The speed control of a load moving vertically downward (hoist) can be considered as normal operating condition.

Fig. 5. Control scheme: a) V/Hz, b) vector control, c) DTC control.

Drive applications can be divided into three main categories according to speed and torque. The most common AC drive application is a single quadrant application where speed and torque always have the same direction, i.e. the power flow from inverter to process. The second category is two-quadrant applications where the direction of rotation remains unchanged but the direction of torque can change, i.e. the power flow may be from drive to motor or vice versa. The third category is fully four-quadrant applications where the direction of speed and torque can freely change. These applications are typically elevators, winches and cranes (ABB, 2011).

In order for an AC drive to operate in quadrant II or IV in speed-torque plane, a means must exist to deal with the electrical energy returned to the drive by the motor. The typical pulse width modulated AC drive is not designed for regenerating power back into the three phase supply lines. Electrical energy returned by the motor can cause voltage in the DC link to become excessively high when added to existing supply voltage. Various drive components can be damaged by this excessive voltage. These regenerative conditions can occur when:

Electrical Drives for Crane Application 139

As a general rule, dynamic braking can be used when the need to dissipate regenerative energy is on an occasional or periodic basis. In general, the motor power rating, speed, torque, and details regarding the regenerative mode of operation will be needed in order to estimate what dynamic braking resistor value is needed. The peak regenerative power of the drive must be calculated in order to determine the maximum resistance value of the

The peak breaking power required to decelerate the load, according to equation (4) is:

*b*

The peak power dynamic brake resistance value can be calculated as:

*<sup>J</sup> <sup>P</sup>*

<sup>0</sup> ( ) *b b*

(4)

*<sup>P</sup>* (5)

(6)

*b*

*t* 

where tb represents total time of deceleration, b and 0 initial and final speed in the

The value of *Pb* can now be compared to the drive rating to determine if external braking module is needed. If peak braking power is 10% greater than rated drive power external braking module is recommended. Compare the peak braking power to that of the rated motor power, if the peak braking power is greater than 1.5 time that of the motor, then the deceleration time, needs to be increased so that the drive does not go into current limit.

> 2 *dc*

> > *b*

*db*

*<sup>V</sup> <sup>R</sup>*

The choice of the dynamic brake resistance value should be less than the value calculated by equation (5). If a dynamic braking resistance value greater than the ones imposed by the choice of the peak regenerative power is made and applied, the drive can trip off due to transient DC bus overvoltage problems. Once the approximate resistance value of the dynamic braking resistor is determined, the necessary power rating of the dynamic braking resistor can be calculated. The power rating of the dynamic braking resistor is estimated by applying what is known about the drive's motoring and regenerating modes of operation. To calculate the average power dissipation the braking duty cycle must be determined. The percentage of time during an operating cycle (*tc*) when braking occurs (*tb*) is duty cycle

=*tb*/*tc*). Assuming the deceleration rate is linear, average power is calculated as follows:

2 *bb b av c b*

Steady state power dissipation capacity of dynamic brake resistors must be greater than that average. If the dynamic braking resistor has a large thermodynamic heat capacity, then the resistor element will be able to absorb a large amount of energy without the temperature of

Fig.7a) shows the experimental results (DC voltage and chopper current) for the variable frequency drive with braking module in DC link and external braking resistor, under a step

*t P <sup>P</sup> t*

the resistor element exceeding the operational temperature rating.

0

dynamic braking resistor.

process of braking.

(


In standard drives the rectifier is typically a 6-pulse diode rectifier only able to deliver power from the AC network to the DC bus but not vice versa. If the power flow changes as in two or four quadrant applications, the power fed by the process charges the DC capacitors and the DC bus voltage starts to rise. The capacitance is a relatively low value in an AC drive resulting in fast voltage rise, and the components of a frequency converter may only withstand voltage up to a certain specified level.

In order to prevent the DC bus voltage rising excessively, the inverter itself prevents the power flow from process to frequency converter. This is done by limiting the braking torque to keep a constant DC bus voltage level. This operation is called overvoltage control and it is a standard feature of most modern drives. However, this means that the braking profile of the machinery is not done according to the speed ramp specified by the user.

There are two technologies available to prevent the AC drive from reaching the trip level: Dynamic braking and active front end regeneration control. Each technology has its own advantages and disadvantages.

### **3. Dynamic braking**

A dynamic brake consists of a chopper and a dynamic brake resistor. Fig.6 shows a simplified dynamic braking schematic. The chopper is the dynamic braking circuitry that senses rising DC bus voltage and shunts the excess energy to the dynamic brake resistor. A chopper contains three significant power components: The chopper transistor is an IGBT. The chopper transistor is either ON or OFF, connecting the dynamic braking resistor to the DC bus and dissipating power, or isolating the resistor from the DC bus. The current rating of the chopper transistor determines the minimum resistance value used for the dynamic braking resistor. The chopper transistor voltage control regulates the voltage of the DC bus during regeneration. The dynamic braking resistor dissipates the regenerated energy in the form of heat.

Fig. 6. Voltage source inverter with diode front end rectifier and dynamic brake module.

controlling the speed of a load moving vertically downward (hoist, declining conveyor),

In standard drives the rectifier is typically a 6-pulse diode rectifier only able to deliver power from the AC network to the DC bus but not vice versa. If the power flow changes as in two or four quadrant applications, the power fed by the process charges the DC capacitors and the DC bus voltage starts to rise. The capacitance is a relatively low value in an AC drive resulting in fast voltage rise, and the components of a frequency converter may

In order to prevent the DC bus voltage rising excessively, the inverter itself prevents the power flow from process to frequency converter. This is done by limiting the braking torque to keep a constant DC bus voltage level. This operation is called overvoltage control and it is a standard feature of most modern drives. However, this means that the braking profile of

There are two technologies available to prevent the AC drive from reaching the trip level: Dynamic braking and active front end regeneration control. Each technology has its own

A dynamic brake consists of a chopper and a dynamic brake resistor. Fig.6 shows a simplified dynamic braking schematic. The chopper is the dynamic braking circuitry that senses rising DC bus voltage and shunts the excess energy to the dynamic brake resistor. A chopper contains three significant power components: The chopper transistor is an IGBT. The chopper transistor is either ON or OFF, connecting the dynamic braking resistor to the DC bus and dissipating power, or isolating the resistor from the DC bus. The current rating of the chopper transistor determines the minimum resistance value used for the dynamic braking resistor. The chopper transistor voltage control regulates the voltage of the DC bus during regeneration. The dynamic braking resistor dissipates the regenerated energy in the

Fig. 6. Voltage source inverter with diode front end rectifier and dynamic brake module.

the process requires repetitive acceleration and deceleration to a stop,

the machinery is not done according to the speed ramp specified by the user.

quickly decelerating a high inertia load,

controlling the speed of an unwind application.

only withstand voltage up to a certain specified level.

a sudden drop in load torque occurs,

advantages and disadvantages.

**3. Dynamic braking** 

form of heat.

As a general rule, dynamic braking can be used when the need to dissipate regenerative energy is on an occasional or periodic basis. In general, the motor power rating, speed, torque, and details regarding the regenerative mode of operation will be needed in order to estimate what dynamic braking resistor value is needed. The peak regenerative power of the drive must be calculated in order to determine the maximum resistance value of the dynamic braking resistor.

The peak breaking power required to decelerate the load, according to equation (4) is:

$$P\_b = \frac{\int a\_b (a\_b - a\_0)}{t\_b} \tag{4}$$

where tb represents total time of deceleration, b and 0 initial and final speed in the process of braking.

The value of *Pb* can now be compared to the drive rating to determine if external braking module is needed. If peak braking power is 10% greater than rated drive power external braking module is recommended. Compare the peak braking power to that of the rated motor power, if the peak braking power is greater than 1.5 time that of the motor, then the deceleration time, needs to be increased so that the drive does not go into current limit.

The peak power dynamic brake resistance value can be calculated as:

$$R\_{db} = \frac{V\_{dc}^2}{P\_b} \tag{5}$$

The choice of the dynamic brake resistance value should be less than the value calculated by equation (5). If a dynamic braking resistance value greater than the ones imposed by the choice of the peak regenerative power is made and applied, the drive can trip off due to transient DC bus overvoltage problems. Once the approximate resistance value of the dynamic braking resistor is determined, the necessary power rating of the dynamic braking resistor can be calculated. The power rating of the dynamic braking resistor is estimated by applying what is known about the drive's motoring and regenerating modes of operation.

To calculate the average power dissipation the braking duty cycle must be determined. The percentage of time during an operating cycle (*tc*) when braking occurs (*tb*) is duty cycle (=*tb*/*tc*). Assuming the deceleration rate is linear, average power is calculated as follows:

$$P\_{av} = \frac{t\_b}{t\_c} \frac{P\_b}{2} \frac{o\_b + o\_b}{o\_b} \tag{6}$$

Steady state power dissipation capacity of dynamic brake resistors must be greater than that average. If the dynamic braking resistor has a large thermodynamic heat capacity, then the resistor element will be able to absorb a large amount of energy without the temperature of the resistor element exceeding the operational temperature rating.

Fig.7a) shows the experimental results (DC voltage and chopper current) for the variable frequency drive with braking module in DC link and external braking resistor, under a step

Electrical Drives for Crane Application 141

The PWM rectifier basically operates as a boost chopper with AC voltage at the input, but DC voltage at the output. The intermediate DC-link voltage should be higher than the peak of the supply voltage. The required DC-link voltage needs be maintained constant during rectifier as well as inverter operation of the line side converter. The ripple in DC link voltage can be reduced using an appropriately sized capacitor bank. The AFE inverter topology for a motor drive application, as shown in Fig.8, has two three-phase, two-level PWM converters, one on the line side, and another on the load side. The configuration uses 12 controllable switches. The line-side converter is connected to the utility through inductor.

For a constant dc-link voltage, the IGBTs in the line-side converter are switched to produce three-phase PWM voltages at *a*, *b*, and *c* input terminals. The line-side PWM voltages, generated in this way, control the line currents to the desired value. When DC link voltage drops below the reference value, the feed-back diodes carry the capacitor charging currents,

The steady state characteristics as well as differential equations describing the dynamics of the front-end rectifier can be obtained independent of an inverter and motor load. This is because the DC-link voltage can be viewed as a voltage source, if *Vdc* is maintained constant for the full operating range. The inverter is thus connected to the voltage source, whose terminal voltage *Vdc*, remains unaffected by any normal inverter and motor operation (Jiuhe

Furthermore, as shown in Fig.8, the rectifier can also be viewed as connected to the voltage source *Vdc*. Thus, the rectifier is able to control magnitude and phase of PWM voltages *Vabc*

> 1 1 10 2 2 20 3 3 30

In synchronous rotating *d-q* reference frame Equations 8 and 9 represent the dynamic *d-q*  model of an active front end inverter in a reference frame rotating at an angular speed of .

*E i iV <sup>d</sup> E L i Ri V dt E i iV* 

*a b c*

(7)

The inductor is needed for boost operation of the line-side converter.

Fig. 8. Active front end inverter topology.

et al., 2006).

irrespective of line voltages *E123*.

and bring the DC-link voltage back to reference value.

The dynamic equations for each phase can be written as,

change of induction motor load in regenerative regime. Danfoss frequency (series VLT 5000) converter is used in experimental set-up. For the supply voltage of 400 V, DC link voltage is about 540 V. When negative load torque is applied, DC link voltage rises. The chopper transistor voltage control regulates the voltage of the DC bus during regeneration to near 800 V allowing current flow in the resistor. Regenerative energy is then realised into heat. After the end of the regenerative period, DC voltage returns to a value that corresponds to a motor regime. The Fig.7b) shows the line voltage and current at the input of the diode rectifier.

Fig. 7. a) DC voltage and chopper current, b) line voltage and current.

A voltage source PWM inverter with diode front-end rectifier is one of the most common power configurations used in modem variable speed AC drives, (Fig. 6). An uncontrolled diode rectifier has the advantage of being simple, robust, and low cost. However, it allows only unidirectional power flow. Therefore, energy returned from the motor must be dissipated on a power resistor controlled by a chopper connected across the dc link. A further restriction is that the maximum motor output voltage is always less than the supply voltage.
