INTRODUCTION
In Chapter 1 a hydraulic system was defined as a means of transmitting power from a prime mover to the various functions of a machine where the power is required.
In most industrial applications, and some mobile applications also, the prime mover is an electric motor of some description. The electric motor provides input power to drive the pump, which converts the mechanical shaft rotation of the motor into hydraulic power in the form of flow and pressure.
For many years, electrically operated directional valves have been used to control and distribute the hydraulic power to where it is needed. In many cases, these are simple on/off devices, but increasingly there is a demand for proportional controls, where valves are capable of modulating their output, as opposed to simply switching it on or off. An analogy can be made with a simple on/off light switch and a dimmer switch. The first is only capable of switching the light on and off, while the second can vary the amount of light produced, from fully off to fully on and anywhere in between.
Once proportional valves had been developed to respond to analogue electronic signals, it was then a relatively small step to introduce digital electronic controls, thereby adding some ‘intelligence’ to the control function. As a result, hydraulic components are now available with built-in electronics that not only sense what the component is doing but also can make control decisions about how to react and then communicate their actions back to a centralised control station.
ELECTRIC MOTORS
Most electric motors used in industrial systems are three-phase induction motors.
These are sometimes referred to as squirrel-cage motors due to the configuration of their rotating component. They are typically powered by a three-phase AC supply at a relatively high voltage of around 400 V (480 V in the USA), which is the normal industrial supply voltage. By contrast, the domestic electricity supply is around 220 V (110 V in the USA) and single phase.
A single-phase supply involves just two wires, where the voltage in one is constantly changing relative to the other in a sinusoidal manner. The rate at which the voltage changes is known as the supply frequency (i.e. the number of complete sinusoidal cycles per second). In Europe, the supply frequency is a nominal 50 cycles per second (50 hertz (Hz)), whereas in North America and some other countries it is 60 Hz. A three-phase supply involves three wires, where the voltage in each varies sinusoidally relative to the other two (Fig. 5.1).
In an induction motor, the electrical supply is connected to windings in the stationary part of the motor known as the stator. The cyclical nature of the voltage and the
C H A P T E R
5 ELECTRIC POWER AND
resulting current in the windings generates a magnetic field that rotates within the stator. The speed of rotation of the magnetic field is determined by the frequency of the supply and the number of windings that can be arranged as, typically, two pole, four pole or six pole. The speed of rotation of the magnetic field (known as the synchronous speed of the motor) can be determined using the following formula:
N=120 × fP
where N is the rotational speed (rpm), f is the supply frequency (Hz) and P is the number of poles. For example, a four-pole motor operating with a 50 Hz supply voltage would have a synchronous speed of 1500 rpm.
As the magnetic field rotates within the stator, it induces a current within the bars of the ‘squirrel cage’ located within the rotor (Fig. 5.2). This induced current also
Fig. 5.1 Single-phase and three-phase AC supplies
COOLING FAN (AT REAR)
STATOR
WINDINGS ROTOR
ROTATING MAGNETIC
FIELD FRAME
Fig. 5.2 Squirrel-cage electric motor DEFINITION
pole – a set of windings in the stator of a motor. The greater the number of poles the slower the motor will rotate for a given frequency.
SINGLE PHASE
THREE PHASE LOWER
FREQUENCY HIGHER
FREQUENCY
VOLTAGE
TIME
TIME VOLTAGE
MEASURED LIVE TO NEUTRAL
VOLTAGE MEASURED
PHASE TO PHASE
creates a magnetic field, which interacts with the one created in the stator to cause the rotor and motor shaft to rotate.
In order to create a rotational force (torque), however, there must be a slight difference between the speed of rotation of the stator magnetic field and that of the rotor (known as slip). The difference is typically around 3–5% of the synchronous speed.
So, at its rated torque output, a 1500 rpm synchronous speed motor would actually rotate at around 1440 rpm.
Inevitably there will be losses internally in the motor due to friction, wire resistance, etc., so heat will be generated within the motor. Normally, a fan is attached to the motor shaft and incorporated in a cowling on the rear of the motor. The purpose of the fan is to force air across cooling fins on the motor frame in order to dissipate this heat. Most induction motors are able to rotate in either direction, and usually incorporate a bi-directional cooling fan to allow for this.
The direction of rotation is determined by how the three phases are connected (i.e.
reversing any two will reverse the direction of rotation). Unfortunately, it is not always easy to tell which way the motor will rotate when first connected, and this often has to be determined by trial and error. Ideally, in a hydraulic system the motor shaft should be disconnected from the pump shaft during this process, but in practice this is unlikely to be the case. When checking motor rotation, therefore, it must be ensured that there is sufficient hydraulic fluid available in the pump in order to lubricate it for the short period of time it takes to determine the rotation of the motor.
In particular, the cases of piston pumps must be filled with fluid before the pump is operated. A pump that is started up dry can be damaged in just the few revolutions it takes to determine the motor rotation.
The three-phase induction motor has been the standard method of driving hydraulic pumps in industrial applications for many years. In the majority of applications, fixed- speed motors have been used (1000 or 1500 rpm in 50 Hz supply regions, and 1200 and 1800 rpm in 60 Hz regions). Where a variable flow was required in a hydraulic system the designer had a choice of using either multiple fixed-displacement pumps or a variable-displacement pump.
Today, however, a third alternative exists, which is to use a variable-speed electric motor in conjunction with either a fixed- or a variable-displacement pump. Variable- speed DC motors have been available for many years but these were generally too expensive for use in hydraulic systems. However, the advent of variable-frequency drives for AC induction motors has made this alternative much more viable from a cost point of view.
As explained previously, the synchronous speed of an induction motor is dependent on the frequency of the AC supply. Therefore, if the frequency of the supply can be varied, it follows that the speed of the electric motor can be varied accordingly. In fact, in many situations few, if any, modifications may be necessary to the standard fixed-speed motor itself. All that is required is a variable-frequency controller to drive it (Fig. 5.3).
DEFINITION
The synchronous speed of a motor is the theoretical speed of rotation if there is no load on the motor shaft. In practice, there will always be some load (caused by bearing friction, fan resistance, etc.), so even with no external load the speed of the motor will be slightly lower than the synchronous speed.
The fixed-frequency AC supply is first rectified to convert it to DC, and then converted back to AC at a variable frequency, as determined by the control input signal. The output AC is in the form of a series of on/off pulses of varying duration, which approximate to a sine wave. This is achieved by simply switching the output on or off at the required frequency using high power capacity solid-state switching devices. This technique, known as pulse-width modulation (PWM), can also be used to control the current flow through proportional valve solenoids, as will be explained later.
The variable-speed drive described so far is a relatively low-cost method of achieving a variable-flow pump. However, a drive of this type has limitations if it is required to operate at very slow speeds or under high dynamic conditions (e.g. rapid speed changes). Although the motor itself will have a maximum drive speed, the maximum speed capability of the hydraulic pump will normally be much lower, and this will therefore be the factor that limits the capability of the arrangement from this point of view.
Where a more dynamic performance is required, or where speeds need to be accurately controlled (down to very slow or even zero speed), a closed-loop arrangement can be used (Fig. 5.4). In this case signals indicating the pump drive speed and pressure can be fed back to the motor controller in order to provide a more accurate and responsive control system. The type of electric motor used in such applications may also be uprated to one that incorporates permanent magnets in the rotor, which is more suited to this type of control arrangement.
FIXED- FREQUENCY
AC SUPPLY
VARIABLE- FREQUENCY
AC SUPPLY FIXED- OR
VARIABLE- DISPLACEMENT
PUMP
CONTROL INPUTS
VARIABLE-SPEED MOTOR VARIABLE-FREQUENCY
DRIVE (VFD)
CONVERTER DC BUS INVERTER
Fig. 5.3 Variable-frequency motor drive DEFINITION
rectification is the process that converts the bi-directional current flow of an AC supply to a single-direction current flow.
DEFINITION
An open-loop control system varies the output of a device or system by varying an input control signal. However, any disturbances or variations in the system may cause the output to change.
A closed-loop control system adds a feedback device so that any variations which tend to cause the output to change unintentionally are automatically corrected.