电气技术专业英语 朱一纶 主编 中国电力出版社
Unit 9 Motor Controller 课件制作: 吴岱曦
Index • Text • 1. Introduction • 2. Relay • 3. Contactor
Reading materials • 1. Time Relay • 2. Novel controller • 3. Experiment material
Exercises • 1. Put the Phrases into English • 2. Put the Phrases into Chinese • 3. Sentence Translation • 4. Translation
1. Introduction • A motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor.
A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults.
A motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form of analog or digital input signals.
An electric current through a conductor will produce a magnetic field at right angles to the direction of electron flow. If that conductor is wrapped into a coil shape, the magnetic field produced will be oriented along the length of the coil.(Fig 9.1)
The greater the current, the greater the strength of the magnetic field, all other factors being equal.
If we place a magnetic object near such a coil for the purpose of making that object move when we energize the coil with electric current, we have what is called a solenoid.
The movable magnetic object is called an armature, and most armatures can be moved with either direct current (DC) or alternating current (AC) energizing the coil. The polarity of the magnetic field is irrelevant for the purpose of attracting an iron armature.
Solenoids can be used to electrically open door latches, open or shut valves, move robotic limbs, and even actuate electric switch mechanisms. However, if a solenoid is used to actuate a set of switch contacts, we have a device so useful it deserves its own name: the relay.
2. Relay • Relays are extremely useful when we have a need to control a large amount of current and/or voltage with a small electrical signal.
The relay coil which produces the magnetic field may only consume fractions of a watt of power, while the contacts closed or opened by that magnetic field may be able to conduct hundreds of times that amount of power to a load. In effect, a relay acts as a binary (on or off) amplifier.
In Fig 9.2, the relay's coil is energized by the low-voltage (12 VDC) source, while the single-pole, single-throw (SPST) contact interrupts the high-voltage (480 VAC) circuit.
It is quite likely that the current required to energize the relay coil will be hundreds of times less than the current rating of the contact. Typical relay coil currents are well below 1 amp, while typical contact ratings for industrial relays are at least 10 amps.
One relay coil/armature assembly may be used to actuate more than one set of contacts. Those contacts may be normally-open, normally-closed, or any combination of the two.
As with switches, the "normal" state of a relay's contacts is that state when the coil is de-energized, just as you would find the relay sitting on a shelf, not connected to any circuit.
Compare with transistors, relays have following advantages: • Relays can switch AC and DC, transistors can only switch DC. • Relays can switch high voltages, transistors cannot. • Relays are a better choice for switching large currents (>5A). • Relays can switch many contacts at once.
3. Contactor • When a relay is used to switch a large amount of electrical power through its contacts, it is designated by a special name: contactor. In other words, a contactor is a large relay, usually used to switch current to an electric motor or other high-power load.
Contactors typically have multiple contacts, and those contacts are usually (but not always) normally-open（Fig 9.3）, so that power to the load is shut off when the coil is de-energized. Perhaps the most common industrial use for contactors is the control of electric motors.
The top three contacts switch the respective phases of the incoming 3-phase AC power, typically at least 380 Volts for motors 1 horsepower or greater. The lowest contact is an "auxiliary" contact which has a current rating much lower than that of the large motor power contacts, but is actuated by the same armature as the power contacts.
The auxiliary contact is often used in a relay logic circuit, or for some other part of the motor control scheme, typically switching 120 Volt AC power instead of the motor voltage. One contactor may have several auxiliary contacts, either normally-open or normally-closed, if required.
Large electric motors can be protected from damage through the use of overload heaters and overload contacts of thermal overloads. If the series-connected heaters get too hot from excessive current, the normally-closed overload contact will open, de-energizing the contactor sending power to the motor.
1. Time Relay • Some relays are constructed with a kind of "shock absorber" mechanism attached to the armature which prevents immediate, full motion when the coil is either energized or de-energized.
This addition gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay armature motion on coil energization, de-energization, or both.
Time-delay relay contacts must be specified not only as either normally-open or normally-closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts.
First we have the normally-open, timed-closed (NOTC) contact（Fig 9.4）. This type of contact is normally open when the coil is unpowered (de-energized). The contact is closed by the application of power to the relay coil, but only after the coil has been continuously powered for the specified amount of time.
In other words, the direction of the contact's motion (either to close or to open) is identical to a regular NO contact, but there is a delay in closing direction. Because the delay occurs in the direction of coil energization, this type of contact is alternatively known as a normally-open, on-delay.
Next we have the normally-open, timed-open (NOTO) contact (Fig 9.5). Like the NOTC contact, this type of contact is normally open when the coil is unpowered (de-energized), and closed by the application of power to the relay coil.
However, unlike the NOTC contact, the timing action occurs upon de-energization of the coil rather than upon energization. Because the delay occurs in the direction of coil de-energization, this type of contact is alternatively known as a normally-open, off-delay:
Normally-closed, timed-open. Abbreviated "NCTO", these relays close immediately upon coil de-energization and open only if the coil is continuously energized for the time duration period. Also called normally-closed, on-delay relays.
Normally-closed, timed-closed. Abbreviated "NCTC", these relays open immediately upon coil energization and close after the coil has been de-energized for the time duration period. Also called normally-closed, off -delay relays.
2. Novel controller • Recent developments in drive electronics have allowed efficient and convenient control of these motors, where this has not traditionally been the case. The newest advancements allow for torque generation down to zero speed.
This allows the polyphase AC induction motor to compete in areas where DC motors have long dominated, and present an advantage in robustness of design, cost, and reduced maintenance. Here are some examples:
A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.
QJ Series auto-reduced voltage starter (Fig 9.6) is applicable to infrequent reduced-voltage start up and stop of AC 50Hz three-phase squirrel-cage induction motor with voltage of 380V and power of 10, 14, 28, 40, 55 and 75kW in general industry.
When the tapped autotransformer starts, it reduces the power supply voltage, so that the starting current reduces. This product also provides with thermal relay and under-voltage tripping gear.
When the motor is overloaded or the voltage of the line is at a certain value below the rated voltage, the motor will be switched off from the power supply to protect the motor.