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AMAZON multi-meters discounts AMAZON oscilloscope discounts Introduction The protection of AC variable speed drives includes the protection of the following two major components of a VSD:
In modern digital AC variable speed drives, most of the protection functions are implemented electronically within the control system of the AC converter. However, to be effective, external sensors are necessary to monitor either the current or the temperature directly. The temperature rise in a motor and converter is the main cause of damage. Since temperature rise is usually the result of high current flow, the sensing of current is a common method of implementing overload and short circuit protection. AC frequency converter protection circuits Digital AC frequency converters usually include a considerable number of protection features to protect the AC converter itself, the output cable and the motor. However, the protection built into the AC converter control system does NOT protect the input side of the converter, which comprises the power supply cable and the rectifier. Short circuit and ground-fault protection must be provided upstream in the main distribution board (DB) or motor control center (MCC). Suitable protection can be provided by:
On the output side, a modern well designed VSD will protect itself from almost all the common faults on the motor side of the converter, such as short-circuit, ground fault, thermal overload, etc. VSDs also usually provide thermal overload protection for the motor. One of the few faults that will cause damage to the converter is the inadvertent connection of the mains supply to the motor terminals. The inrush through the reverse connected diodes in the inverter will result in inverter damage. The following protection features are usually available in most modern digital AC converters. These overall protection features are also summarized.
AC and DC under-voltage protection The under-voltage protection system monitors the voltage on the 3 incoming phases as well as the DC bus voltage and responds to various faults as outlined below. If the supply voltage falls to a low level as a result of some upstream power system fault, it’s unlikely that the converter will be damaged. The input diode rectifier of a PWM converter can safely operate at any voltage between zero and the over-voltage trip point. So, a power supply under-voltage event is not really a problem for the power circuit. Under-voltage protection is mainly required to ensure that all the various power supplies are operating within their required specification. If a power supply unit should lose output voltage regulation, the following could occur:
Under-voltage protection can be implemented in a number of ways within a VSD: • Loss of AC supply voltage Loss of AC power can be detected by monitoring the three AC line voltages and comparing these to a preset trip point. AC supply under-voltages can be caused by a complete loss of supply or alternatively a voltage sag (dip) of short duration. Since the power supply for the converter control circuits is taken from the DC bus via a switch mode power supply (SMPS), it’s not necessary to stop the converter immediately the supply voltage is lost. If required, the converter can continue to operate, initially taking energy from the large capacitor on the DC bus. As the DC bus voltage starts to fall, the output frequency can be reduced to allow the motor to behave as an induction generator, driven by the inertia of the mechanical load. This situation could be maintained for a period until the motor stops turning. Alternatively, the control circuit can be programmed to trip immediately the AC supply voltage is lost. The selection to trip (or not to trip) on loss of AC supply can usually be made by changing a bit in the control logic. • Loss of DC bus voltage The DC voltage can be monitored by a comparator circuit (hardware or software) that compares the DC bus voltage to a preset minimum voltage level. When the DC bus voltage falls below this level, the converter may be shut down (tripped). This trip level is typically set at the lowest rated input voltage, minus 15%. E.g., if the VSD is rated at 380 V-460 V ±10%, the lowest specified operating level would be 342 V AC, with an equivalent DC voltage of 485 V DC. The DC bus trip point would typically be set at 485 V DC -15%, that would be 411 V DC. In addition to this main DC bus trip point, some of the individual modules sometimes shut down independently. E.g., each driver module may have its own under-voltage sensing circuit to ensure that sufficient base or gate drive voltage is available before switching. If these trip, a signal would be returned to the main processor indicating local device failure. These local under-voltage trips are usually used only on critical modules, such as transistor driver circuits. AC and DC bus over-voltage protection Ultimately all the electrical components will fail if exposed to a sufficiently high over voltage. In an AC variable speed drive, over-voltages can occur for the following reasons: • High voltages in the mains power supply • High voltages generated by the connected motor behaving as an induction generator when trying to reduce the speed of a high inertia load (braking) too quickly In an AC converter, the DC bus capacitor bank, the DC bus connected power supply module and the main power electronic switching devices have the lowest tolerance to high voltages. The capacitor bank usually consists of individual capacitors in series and parallel. When capacitors are connected in series, the voltage sharing won’t be perfect, and so the maximum voltage will be less than the sum of the individual ratings. E.g., if two 400 Vdc capacitors are connected in series, the nominal rating would be 800 V DC. However, the actual safe operating voltage may only be 750 V DC, due to the unequal voltage sharing characteristics. This value will be a function of the capacitor leakage current and the value of the sharing resistor in parallel with each capacitor. The lower the value of the sharing resistor, the better the sharing will be but this will also increase drive losses. The peak voltage on the DC bus is v2 (1.414) times the mains phase voltage. If the maximum rated capacitor voltage is 750 V DC, and allowing for a plus 10% variation in the mains voltage, the practical limit for input voltage is 480 V AC. The power semiconductor switching devices, in the rectifier and inverter, are usually rated at maximum voltage of 1200 V DC. Although this seems well above the maximum capacitor rating, the voltage across a device during turn-off will be much higher than the DC bus voltage, particularly during fault conditions. This is due to stray inductances in the circuit. These voltage peaks can reach about 400 V, so the bus voltage prior to the fault must usually be limited to around 800 V DC maximum, depending on the drive design and the rating of the power devices. In analog converters, the over-voltage protection is usually a hardware protection system through a simple comparator circuit operating with a fixed set point. In modern digital converters, the over-voltage protection is usually provided by the microprocessor. This is possible because the DC bus voltage changes relatively slowly, due to the filtering effect of the capacitors. In microprocessor controlled VSDs, the processor can also provide some over-voltage control. Most DC bus over-voltages are caused by incorrect setting of the deceleration (ramp-down) times of high inertia motor loads. If the deceleration time is set too low compared to the natural run-down time of a rotating load, the motor will behave like an induction generator and power will be transferred from the motor to the DC bus. The DC bus voltage could rise until the over-voltage trip level is reached. Many VSDs have a selectable feature whereby the controller will override the set deceleration time and prevent the over-voltage trip. The DC bus voltage is allowed to rise to a 'safe' high voltage, typically 750 V DC, and rate of deceleration is controlled to keep the voltage below the trip level of 800 V DC. The under- and over-voltage protection is normally monitored at the DC bus because this is the source of DC power for both the inverter and the control circuits. Typical operating regions and the protection trip levels are summarized. ++++ Typical DC bus under- and over-voltage protection levels Output over-current protection The purpose of over-current protection is to avoid failure of the main power semiconductors (IGBTs, BJTs, MOSFETs, GTOs, etc) during phase-to-phase short circuits on the motor side of the converter. Unlike diodes and SCRs, fuses are not appropriate for the protection of most power switches due to their I^2 t characteristics. The most effective method of protection is to switch all the inverter switching devices off when the current rises above a given set point. The protection level is dependent on their safe operating area characteristic. This maximum fault current is usually what determines the maximum rating of the drive. Typically, the trip current is around 200% of the drive current rating, with current limiting at 150% or sometimes 180%. To maximize the effective rating of the VSD, it may be possible to operate closer to the trip current if the rate of rise (di/dt) of current is controlled. This can be achieved by introducing a choke between the power semiconductor device and the output terminals of the VSD. If a short circuit occurs on the VSD output, the rate of change of current (di/dt) will be equal to the bus voltage divided by the inductance: E.g., with a 600 V DC bus voltage and a 100 µH output choke, the current rise time will be limited to 6 amp/µsec. For a short circuit on the output of a 50 kW AC converter, with a trip current level of 200 amp, it will take 33.3 µsec to reach the trip point. This is significant when considering the propagation delay through the current feedback and protection circuits. The propagation delay is the amount of time between the actual current reaching the trip point and the turn off of the power devices. This delay exists in the current measuring device, the amplifiers through which the signal passes, the comparator itself, the power device driver circuit and the actual power device. If the propagation delay and the rate of change of current are known, then the actual device current when the power devices switch off can be estimated. E.g., if the total propagation delay is 3 µsec and the di/dt is 6 amp/µS, then the actual device current will be 18 amps higher than the current trip point when the devices actually turn off. This is called current overshoot. While larger output chokes will reduce this overshoot and have a few other advantages, they also introduce losses, are bulky and expensive. For this reason it’s important to minimize the propagation delay in the over-current protection circuit. As a result, high bandwidth current feedback and amplifiers are usually used. To minimize propagation delays in the microprocessor, it’s common for over-current protection to be performed completely in hardware, even in a digital VSD. Over-current events can also occur as a result of sudden increases in the load torque on the motor or during periods of high motor acceleration. These increases in current occur relatively slowly, allowing the current to be monitored and controlled by the microprocessor. The increase in current can be limited to a preset value typically of up to 150% of the rated converter current. The current limit control system regulates the output frequency in such a way that it reduces the motor torque. If the over-current is due to a high rate of acceleration, current is reduced by reducing the rate of increase of current. If the over-current is due to a temporary motor overload, the output speed may be reduced. Typical over-current protection and current limit levels. ++++ Typical over-current protection levels and current limit settings Output ground fault protection Ground fault protection is designed to detect a short circuit between a phase and ground, on the output side of the VSD, and immediately shuts down the converter. This protection is generally not intended for protection of human life from electric shock, as the trip points are set much higher than acceptable human safety limits. This feature is mainly for the protection of the AC converter itself. ++++ Core balance current transformer for ground fault protection. The normal operating condition, no ground fault present Ground fault protection is usually implemented by means of a core balance current transformer. This is constructed from a toroidal magnetic core through which either the DC bus cables or the output motor phase cables are passed. A low current secondary winding is wound around the toroid and connected to the protection circuit. If the vector sum of all the currents passing through the core add up to zero, the flux in the core will be zero. A net zero flux is the normal operating situation. If an ground fault occurs and there is a path for current to ground, the sum of the currents through the core balance transformer will no longer be zero and there will be a flux in the core. ++++ Core balance current transformer for ground fault protection. Ground fault condition, the net current is not equal to zero This flux will result in a current being generated in the secondary protection winding, which is converted to a voltage via a burden resistor. A comparator circuit detects the fault and shuts down all the power device drives. Typically, the protection trip level is around 5 amp. Care must be taken in establishing the set point for the ground fault trip circuit. In all PWM VSDs, some leakage current will always take place to ground due to the high frequency components of the motor current waveform and the capacitance of the motor cables to ground. High leakage currents can sometimes cause some nuisance tripping of the ground fault protection. Heat-sink over-temperature protection Over-temperature protection is usually provided to prevent over heating of various components in the converter, particularly the junction temperature of the power semiconductors, which is limited to 150 deg. C. To ensure this limit is not reached, the heat sink temperatures are usually maintained at temperatures below 80 deg. C to 90 deg. C, depending on the actual design. Consequently most heat-sinks are fitted with temperature sensors or switches to detect when the maximum temperatures are reached. Other modules, such as the power supplies or device driver modules, may have their own individual over-temperature protection. It’s common to measure ambient air temperature close to the control electronics to ensure this does not exceed device ratings (usually ±70 deg. C). Low cost drives may rely on simple bimetallic temperature switches (microtherms), which operate at a specific temperature. However, most modern drives use silicon junction temperature sensors to feed back the actual temperature to the microprocessor. Using this method, the processor can provide a warning to the operator prior to actual shutdown. On more advanced VSDs, some corrective action might be taken automatically, such as reducing the motor speed or reducing the PWM switching frequency. Motor thermal overload protection Almost all modern VSDs include some provision for motor thermal overload protection. The simplest form of protection is to make provision for a digital input, which shuts down the drive when some external device, such as a thermal overload or thermistor relay is activated. Many manufacturers of VSD now make provision for a direct input from a thermistor sensor, so that only the thermistors need be placed in the motor windings and eliminates the need for a thermistor relay. The inputs are normally delivered with a resistor connected across the terminals, which should be removed during commissioning. This often creates some difficulties during commissioning for those who don’t read the installation manuals. The most common method used for motor thermal overload protection on digital VSDs is to use the current sensing method with a motor protection model as part of the microprocessor control program. The measurement of motor current is necessary for other purposes, so it’s a small step to provide motor thermal modeling. The model can continuously estimate the thermal conditions in the motor and shuts down the VSD if limits are exceeded. The simplest motor model is to simulate a eutectic thermal overload relay by integrating motor current over time. This simplistic method does not provide good motor protection because the cooling and heating time constants of the motor change at different speeds. Over a period of time, the motor protection features in VSDs have become more sophisticated by using motor frequency as an input so that shaft fan cooling performance, at various speeds, can also be modeled. The most advanced VSDs require motor parameters such as rated speed, current, voltage, power factor and power to be entered to enable a comprehensive motor thermal model to be implemented in software, affording excellent motor protection without any direct temperature measurement devices. For these motor models to be accurate and effective, previous conditions need to be stored in a non-volatile memory in case the power is interrupted. This can be achieved through simple devices such as capacitors or non-volatile memory chips, such as EEPROMs and NVPROMs. Overall protection and diagnostics The following block diagram is a summary of the protection features commonly used in modern digital PWM AC converters. As outlined above, many of these protection functions are implemented in software, using suitable algorithms. The main exceptions are the over-current protection and the ground fault protection, which are implemented in hardware to ensure that they be sufficiently fast to adequately protect the power semiconductor devices. ++++ Example of VSD protection block diagram Operator information and fault diagnostics Modern digital variable speed drives (VSDs) all have some form of operator interface module which provides access to internal data about control and status parameters during normal operation and diagnostic information during fault conditions. This module is sometimes called the human interface module (HIM), or something similar. The HIM usually provides an LED or LCD display and some buttons to interrogate the control circuit. This operator interface can also be used to install and change VSD settings parameters. In addition, modern VSDs also permit the transfer of these parameters to remote locations via serial digital data communications. Some details about the serial communication are covered in the section on installation. The communications interface permits control from a remote programmable logic controller (PLC) as part of an overall automated control system. The diagnostic information can be transferred over the serial interface to a central control center so that an operator can take full advantage of the information available. When an internal or external fault occurs, the control circuit registers the type of fault, which helps to identify the cause of the fault and the subsequent rectification of the problem. Modern microprocessor controlled converters employ a diagnostic system that monitors both the internal and external operating conditions and responds to any faults in the way programmed by the user. The control system retains the fault information in a non-volatile memory for later analysis of the events that occurred. This feature is known as fault diagnostics. There are three main levels of operator information and fault diagnostics:
The following is a brief list of typical internal parameters and fault conditions. ==== Module Parameters and fault diagnostics Power supply Power supply voltage, current and frequency DC bus DC link voltage and current Motor Output voltage, current, frequency, speed, torque, temperature Control signals Setpoint, process variable, error, ramp times Status Protection circuits, module failures, internal temps, fans running, switching frequency, current limit, motor protection, etc Fault conditions Power device fault, power supply failed, driver circuit failed, current feedback failed, voltage feedback failed, main controller failed ==== ++++ Typical list of variable speed drive parameters At the first level, most modern digital VSDs provide information about the status of:
At the second level, when a fault occurs and the VSD stops, diagnostic information is provided to assist in the rectification of the fault, thereby reducing downtime. There is always some overlap between these levels of diagnostics. E.g. a persistent over current trip with no motor connected can indicate a failed power electronic switching device inside the converter. The table shows the most common external fault indications provided by the VSD diagnostics system and the possible internal or external problems that may have caused them. === Protection Internal Fault External Fault Over-voltage Deceleration rate set too fast Mains voltage too high transient over-voltage spike Under-voltage Internal power supply failed Mains voltage too low Voltage sag present Over-current Power electronic switch failed driver circuit failed Short circuit in motor or cable Thermal overload Control circuit failed Motor over-loaded or stalled Ground fault Internal ground fault Ground fault in motor or cable Over-temperature Cooling fan failed heat-sink blocked Ambient too high enclosure cooling blocked Thermistor trip Motor thermistor protection === ++++ Variable speed drive diagnostics table The internal diagnostics system can provide an operator with information about faults that have occurred inside the drive. This can be further broken down into fault conditions, such as a failed output device, commutation failures, etc. Fault conditions are indications that a particular module or device has failed or is not operating normally. To provide fault condition monitoring, the drive must be specifically designed to include internal fault diagnostic circuits. E.g., power semiconductor drivers may include circuits that measure the saturation voltage, which is the voltage across the device when it’s on, for each power semiconductor. This can identify a short circuit in the power switch and the VSD can be shut down before the external over-current trip or fuses can operate. Considerable cost and effort is required to implement internal fault condition monitoring, and only a few high performance VSDs provide extensive internal diagnostics. This feature can be very useful for trouble-shooting, but this is usually only warranted when down time represents a major cost to the user. Electric motor protection The useful life of an electric motor is dependent on the following main components:
Using modern materials, most of these components can be designed and constructed to have a high level of reliability. Experience has shown that mechanical failure is rare and the most likely causes of failure are:
During the above abnormal operating conditions, the temperatures in the stator and/or the rotor windings can rise to excessive levels, which causes the degradation of the insulation materials used to isolate the windings from each other and the grounded frame of the motor. The temperature rise in a motor winding is mainly due to the I^2 R losses, or copper losses, where the heat is generated by the load current (I) flowing through the resistance (R) of the stator windings. During design, the cross-sectional area of the stator windings is selected with a particular maximum load current in mind. The design objective is to balance the I^2 R losses, at maximum rated load, with adequate ventilation or cooling so that the resulting temperature rise in the winding will be below the critical temperature of the insulation materials chosen. In AC motors, the stator current is proportional to the mechanical load torque. In DC motors, the armature current is proportional to the mechanical load torque. Consequently, each standard motor size is rated for a maximum stator or armature current. Excessive winding temperature most commonly occurs when the load current exceeds the maximum rated value. This condition is called thermal overloading. When the temperature in a winding rises above a certain critical level, the insulation is permanently damaged. The critical temperature, above which permanent damage takes place, is dependent on the type of insulation material used. In the standards, the different types of insulation are classified into Classes, such as Class-B, Class-F, Class-H, etc. E.g., the temperature in a winding with Class-F insulation is permitted to safely rise to a maximum of 140 deg. C, or 100 deg. C above the commonly specified maximum ambient temperature of 40 deg. C, without permanent damage to the insulation. If the working temperature of the winding increases above 140 deg. C, the characteristics of the insulation material start to degrade. Above 155o C, the insulation will be permanently damaged and its useful life sharply reduced. Insulation failure results in short circuits or ground faults, which would require the replacement or repair of the faulted winding. Long insulation life is particularly important for electric motors, which operate in strategic locations in industry under continuously changing load conditions. A constantly applied temperature rise of just 10 deg. C above the maximum rated temperature can reduce the useful life of a motor to 50% of its original value, as illustrated in the curve below. ++++ The effect of temperature rise above maximum rated temperature on the useful life of an electric motor To protect a motor from insulation damage due to excessive temperatures, any potentially damaging operating condition should be detected by a sensing device and the motor should be disconnected from the power supply before insulation damage can occur. The most common devices used for the protection of electric motors are: • Current sensing devices, such as thermal overload protection relays, which continuously monitor the primary current flowing into the motor windings and initiate a trip when a preset current level is exceeded. • Direct temperature sensing devices, such as thermostats, thermistors, thermocouples and RTDs, which continuously monitor the actual temperature in the motor windings and initiate a trip when a preset temperature level is exceeded. Thermal overload protection -- current sensors Current sensing thermal overload (TOL) protection relays, whether of the indirectly heated bimetallic type or the electronic type, monitor the stator current in AC motors or armature current in DC motors, and use this information to determine if the motor has become over-loaded. TOL relays should be designed to match the thermal characteristics of the motor. On smaller motors, a bimetallic type of TOL relay is normally mounted in conjunction with the motor contactor, which opens when an overload condition is detected. Additional features usually include phase failure (single phasing) detection. The main advantage of the bimetallic TOL relay is its low cost and simplicity. Bimetallic TOL relays don’t provide adequate protection for repeated starting, jogging and other periodic duties. The reason is that the heating and cooling time-constant of the bimetal are equal, whereas the cooling time-constant of a typical squirrel cage motor is approximately twice its heating time-constant, mainly because the cooling fan stops when the motor is stationary. During repeated starting and jogging, the bimetal cools down faster than the motor and, consequently, does not provide adequate thermal protection. For larger motors and those with an intermittent duty, it’s necessary to use an electronic motor protection relay, whose thermal characteristics and settings are designed to more closely match those of the motor. In this case, overload protection is usually part of an overall motor protection relay, which also provides protection against short circuits, ground faults, stalling, single phasing, multiple starts, etc. Several adjustable settings enable the motor protection relay to be matched to the type, size and application of the motor. Modern microprocessor relays can also store and display data such as line currents, unbalance currents, thermal capacity of the motor, etc, and transfer this information to a remote host computer over a serial communications link. Although current sensing TOL protection devices, which monitor stator or armature current, are cost effective and have a reasonably good response time, they seldom take into account other environmental conditions, such as reduced cooling, restricted or total loss of ventilation or excessively high ambient temperatures. E.g., in AC variable speed drives, the shaft mounted fan cooling on a standard AC motor is reduced as the motor speed is reduced, which changes the heating time constant of the motor when it’s running at speeds below 50 Hz. Most modern digital AC converters have built-in thermal overload protection, which is designed to compensate for the changes in the heating and cooling time constants as the speed is adjusted. But, monitoring the stator current is not always a reliable method for protecting the motor winding insulation from damage due to over-temperature. Thermal overload protection - direct temperature sensing For the more difficult applications, direct temperature sensing of the winding temperature at hot spots or other strategic points is preferable. There are several types of devices that can be used for direct temperature sensing. Some of the most common techniques are summarized. The following are some of the applications where direct temperature sensing is considered to be more reliable than stator or armature current sensing: • AC squirrel cage induction motors supplied from AC frequency converters • AC motors which have frequent transient overloads • AC motors which are frequently stopped or started • AC motors in high inertia applications with long starting times • AC motors in applications where the rotor can lock or stall • DC motors controlled from DC converters • Thermal protection in mechanical applications, such as large bearings, gear housings, oil baths, heat sinks, etc. === Type Operating Principle Operating Curve Protection Provided Number Required Microtherm (Thermostat) Bimetallic strip with contacts normally open or normally closed Temperature monitoring for non-transient overloads 2 or 3 connected in parallel for N/O in series for N/C Positive temperature coefficient thermistor Variable non linear resistance of thermistor sensor Temperature monitoring for transient overloads 2 or 3 connected in series Thermocouple Peltier effect J type (T < 750 C) K-type (T < 1250 C) T-type (T < 350 C) Continuous temperature monitoring at hot spots 1 per hot spot RTD resistance temperature detector Variable linear resistance of platinum sensor Pt-100? High accuracy continuous temperature monitoring at hot spots 1 per hot spot === ++++ Protection devices used for direct temperature measurement In practical applications, one or more direct measuring thermal sensors are usually used to monitor the temperature at several strategic points in an electric motor. These sensors are used in conjunction with an associated relay or controller, which is connected in the motor control circuit to provide the following: • Alarm: Draws the attention of the operator to the high temperature condition, using audible and/or visual alarms, without tripping the motor • Trip: Stops the motor by tripping the power supply circuit to the motor To achieve the objectives of separate alarm and trip setpoints, microtherms and thermistors require a group of two sensors at each strategic point. The first, with a lower temperature setpoint, is used to provide the alarm function and the second, with a higher temperature setpoint, is used to provide a trip function. With thermocouples and RTDs, which can continuously measure the actual temperature at each strategic point, the electronic controller normally has two preset temperature levels for alarm and trip. Two separate contact outputs can then be used to initiate and alarm or trip the motor. A detailed description of the various direct temperature sensing methods of motor protection is given in -- A: Motor protection - direct temperature sensing. |
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