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A Variable Frequency Drive (VFD) is an essential device for adjusting the operations of both industrial and non-industrial processes based on various variables.  

Industrial processes such as Pulp and Paper, Mining, Food & Beverage, and Textile, as well as non-industrial processes like HVAC, water treatment, draining, and irrigation, all require VFDs for efficient operations. 

In particular, HVAC systems play a crucial role in creating and maintaining a comfortable environment in our offices, public buildings, shopping malls, and homes by adjusting airflow based on temperature and humidity. VFDs are instrumental in achieving this airflow adjustment. 

The primary purpose of using VFDs is to match the speed or torque/power of the drive to the process requirements, depending on the specific application. By achieving these objectives, VFD users can reap numerous benefits, such as energy savings, improved system efficiency, reduced mechanical stress on machines, increased equipment lifetime, and reduced maintenance frequency. Additionally, VFDs can help lower noise levels on fans and pumps, among other things. 

In situations where downtime is not an option, such as in a hospital surgery room or a data center, or where service interruptions can lead to spoilage, such as in a food warehouse, it is crucial to ensure uninterrupted operation of the motor(s) even if the Variable Frequency Drive (VFD) fails. 

A common solution to this problem is to install a 2-contactor bypass. This mechanism enables the motor to bypass the VFD and continue running at full speed, ensuring uninterrupted delivery of the service despite the VFD failure.

To safely replace a drive in this simple system, the power supply must be shut down. However, a more efficient solution is to incorporate a 3-contactor bypass. 

A 3-contactor bypass includes an additional contactor before the VFD that disconnects the power supply to the VFD when it is in bypass mode. This allows for the motor to continue operating in the event of a VFD failure, while also enabling the VFD to be safely removed and replaced with a new unit without interrupting motor operation. 

The key distinction between a 2-contactor bypass and a 3-contactor bypass is as follows: 

• With a 2-contactor bypass, it is necessary to power off the system before replacing the VFD. 
• In contrast, a 3-contactor bypass enables the VFD to be replaced while the motor continues to run in bypass mode. 

During the deceleration phase, the motor speed exceeds the VFD output frequency, also known as the synchronous speed. As a result, the motor operates as a generator, and the energy is returned to the VFD, causing the DC voltage to increase. This phenomenon is known as “regeneration.” To prevent an overvoltage alarm, it is essential to set an appropriate deceleration time and to consume the regenerated energy in a braking resistor. 

However, Clean Power VFDs are equipped with an active front end (AFE) that feeds the power grid with the regenerated energy, eliminating the need for a braking resistor.  

The deceleration time can be calculated based on the mechanical attributes of the system, such as the motor and load inertia and the acceleration torque. Specifically: 

Deceleration time = (target speed – starting speed) x J / Ca 

Here, the target and starting speeds are measured in revolutions per second, J represents the total inertia of the motor and load (in kg.m2), and Ca is the acceleration torque, which is equal to 9.55 times the difference between the rated motor torque and the minimum load torque.

Electric motors are typically designed to operate at altitudes of 1000 meters or less above sea level.  

At higher altitudes, the atmospheric pressure decreases, which can compromise the motor’s heat dissipation and, consequently, its temperature, insulation, and grease life during operation.  

To ensure proper motor performance, specialized measures, such as derating, are required. 

The same issue applies to variable frequency drives (VFDs) that control the motor. 

 At higher altitudes, the VFD’s heat dissipation also worsens due to the low atmospheric pressure. In the absence of manufacturer recommendations, it is common practice to derate both the motor and VFD by 3% every 300 meters (1000 feet) when the altitude exceeds 1000 meters. 

The Clean Power VFDs design allow to operate without derating up to the altitude of 2000 m. 

Sink and source logic connections refer to the current flow in DC circuits and are commonly associated with PLC I/O signals. In a sinking input or output circuit, the electric load is provided a path to the ground. In contrast, a sourcing input or output provides the voltage source for the electric load. 

The digital inputs of the Clean Power VFD are compatible with both sink and source logic connections, and the selection can be made through software. The following images illustrates the two logics: sourcing (connection to PNP sensor) involves switching the +24V to the inputs, while sinking (connection to NPN sensor) involves switching the 0V to the inputs. 

To obtain an accurate understanding of efficiency, it’s crucial to consider the entire system instead of focusing on individual components. Efficiency can be defined as the ratio of output power to input power in any system. In our scenario, the mechanical power produced by the motor constitutes the output power, while the electrical power supplied to the VFD is the input power.

Electrical input power Pin, depends on voltage (U), current (I), and the power factor (cosϕ). The power factor tells us what proportion of the total electric power is active power and how much is so-called reactive power. To produce the required mechanical power, active power is required. Reactive power is needed to produce magnetization in the motor. 

Mechanical output power Pout, depends on the required torque (T) and rotating speed (n). 

During the transfer, from electrical energy to mechanical energy, the main losses are thermal. Both in the drive and in the motor. 

These two terms are often used interchangeably and can cause confusion. The output power of a VFD is the input power minus any losses. This is expressed as Ploss=Pinput-Poutput 

When analyzing a complete system, from the grid connection to the motor, one can simply add up the losses of each individual component (such as contactors, fuses, VFD, filters, cables, and motor) to determine the total loss of the system. 

Efficiency, on the other hand, is more of a performance metric and is calculated as Efficiency = Poutput/Pinput. 

EMC compliance standards for Power Drive System (PDS) cover not just the VFD, but also the motor and all interconnecting elements such as control cables, motor cables, supply cables, contactors, junction boxes, etc., except for the driven load. The standards specify 2 different environment types (1st & 2nd Environment) based on the location of the point of common coupling (PCC). There are 4 different product categories (C1, C2, C3, and C4) that define varying limits for conducted RFI noise into the mains terminal, in the frequency range of 150KHz to 30MHz, depending on the rated voltage and current of the VFD, and whether or not it is installed by an EMC professional 

• 1st Environment includes domestic premises and buildings/facilities which are directly connected to a low voltage (e.g. 230/400V) mains supply which also supplies buildings used for domestic purposes. Directly connected means that there is no intermediate transformer between these buildings. 
• 2nd Environment includes all buildings/facilities which are not directly connected to a low voltage mains supply which also supplies buildings for domestic purposes. For example, a facility with a dedicated low voltage mains, supply transformer would be a 2nd Environment installation. 

• Category C1 – this category applies to VFDs connected to a mains supply <1000 volts for use in the 1st Environment. 
• Category C2 – this category applies to VFDs connected to a mains supply <1000 volts that when used in the 1st Environment will be installed and commissioned only by a professional. It defines a professional as a person or organization having the necessary skills in installing VFDs including their EMC aspects.
• Category C3 – this category applies to VFDs connected to a mains supply <1000 volts for use in the 2nd Environment. 
• Category C4 – this category applies to VFDs connected to a mains supply ≥1000 volts or rated ≥400 amps or for the use in complex systems in the 2nd Environment. 

While both terms are related to the current drawn by a motor, they have slightly different meanings and uses:

Motor FLA (Full Load Amperes):

• This is the actual current drawn by the motor when it is operating at its rated horsepower and rated voltage.

• You can find this value on the motor’s nameplate.

• It is used for various purposes, including:

  • Selecting circuit breakers and overload protection devices for the motor circuit.

  • Calculating the power consumption of the motor.

  • Comparing the performance of different motors.

Motor FLC (Full Load Current):

• This is a standard reference value for the current drawn by a motor of a particular size and type.

• You can find these values in tables provided by electrical standards like the National Electrical Code (NEC) in the US.

• It is typically used for initial circuit design and planning when the specific motor hasn’t been selected yet.

• It is not always identical to the actual FLA of a specific motor, as there can be slight variations between different manufacturers and models.

Key Differences:

• Source: FLA is based on the specific motor, while FLC is a general reference value.

• Accuracy: FLA is more accurate for the specific motor, while FLC is an estimate.

• Use: FLA is used for detailed calculations and selection, while FLC is used for initial planning.

In summary:

• Use FLA when you have a specific motor and need accurate information about its current draw.

• Use FLC for initial planning and design when the specific motor hasn’t been selected yet.

To ensure safety and protection, fuses are utilized to safeguard the input cable in the event of short-circuits and prevent damage to the VFD in case of a short-circuit within the drive. However, the recommendations for fuse types may vary depending on local regulations. For instance, in the IEC environment, high-speed aR fuse types are typically recommended, although gG fuses can also be utilized if they operate rapidly enough (max. 0.1 seconds). The operating time is influenced by the supply network impedance, as well as the cross-sectional area and length of the supply cable. 

In the UL environment, NEC Article 430.52(C)(5) recommends the use of high-speed fuses for motor branch circuit protection in motor control systems that use solid-state devices such as VFDs and soft-starters. For instance, class J or class T fuse can be selected by the user. Class J fuses are fast-acting and time delay versions rated for 600 volts AC and can interrupt a minimum of 200,000 amps. The ampere ratings for the J-type fuses range from 1 to 600 amps. Class T fuses are also fast-acting and current limiting, rated for 300 volts AC and 600 volts AC versions, with ampere ratings ranging from 1 to 800 amps. 

To define the fuse rating, the VFD nominal current rating (e.g. 34A for the 18.5kW clean power VFD) must be considered. The calculated value can be determined by multiplying the VFD rated current by 1.25 (for normal operation, continuously available without overload). For instance, a 34A current rating VFD, 1.25 x 34A = 42.25A. The manufacturer catalog of fuses should be consulted to select a fuse with a standard size just above the calculated value, which in this case would be a 50A fuse. The fuse voltage must also be greater than line voltage, and a 600 VAC fuse should be selected. 

It is essential to always use UL compliant cables (copper and minimum temperature 75C / 167F) and fuse holders specified for the selected fuses to ensure safety and protection. 

Grounded or grounding refers to the act of connecting an electrical circuit or conductive object to the ground, or to a conductive material that is connected to the ground. 

On the other hand, bonded or bonding refers to the act of connecting two or more conductive objects to establish electrical continuity and conductivity. It is important to note that bonding does not necessarily mean grounding, as the connected objects may not be connected to the ground. However, bonding ensures that the metallic parts of the objects can form an electrically conductive path for electrical continuity. 

When measuring the input and output current of a VFD, you may observe that the output current is higher than the input current. This contradicts the belief that the output current should be lower than the input current due to a VFD’s efficiency being less than 100%. 

The input current from the electric grid to the VFD is the real current that produces useful work or torque. The VFD’s output current to the motor has two components: the real current that generates useful work or torque (watts) and the reactive current that generates the magnetic field in the motor (vars). 

The total output current from the VFD is the square root of the sum of squares of both the real current and reactive current. The real current is supplied by the electric grid while the reactive current is supplied by the DC bus capacitor inside the VFD. Therefore, it is possible for the output current of the VFD to be higher than the input current. 

Whether in industrial fields or non-industrial fields, most of the moving processes and machines are driven by electric motors. 
Cutting, crushing, forming, rolling, mixing, extruding, hoisting, conveying, pumping, blowing, compressing …there’s always an electric motor behind these actions. 
Some say that the electric motor is the workhorse of processes. 

Electric motors can be divided into AC and DC motors. AC motors, particularly squirrel cage motors, are the most used motors. We focus here on AC motors. 

A VSD delivers 3 essential controls to the motor: its speed, torque, its power. 
Without forgetting many other benefits, such as protection, and energy savings. 

We must always consider the system made of the motor and the drive together! 

Induction motors 

Motors in which the magnetic field is induced (hence the name). They run slower than the magnetic field (slip), hence their designation as asynchronous. They generally run at a constant speed which changes slightly when mechanical loads are applied to the motor shaft. Due to its simplicity, robustness, and low cost, this type of motor is the most widely used and, in practical terms, is quite suitable for almost all types of machines.  
It is possible to control the speed of induction motors by frequency inverters. 

Synchronous motors:  

Synchronous motors are three-phase AC motors that run at a fixed speed, without slip, and are generally applied for large outputs. 

Motor lifetime: 

The useful lifetime of the motor depends almost exclusively on the life of the winding insulation. The lifetime of a motor is affected by many factors, such as moisture, vibration, corrosive environments, and others. Among all of these factors, the most important is the working temperature of the employed insulation materials. An increase from 8 to 10 degrees above the rated temperature class of the insulation system can reduce the motor lifetime by half. 

Constant torque applications, such as extruders, conveyors, hoisting equipment, compressors, and crushers, typically require a VFD with a Heavy-Duty (HD) rating. These applications demand a high level of overload tolerance while maintaining constant load torque, making Heavy-Duty VFDs essential for handling the strain during start-up or sudden load increases.

In contrast, variable torque applications, including centrifugal fans and pumps, generally only require a Normal Duty (ND) VFD. In these applications, the torque decreases as the speed reduces, meaning that the drive does not need a high overload capacity, as the load remains proportional to the speed.

Heavy-Duty drives are designed to handle 150% of their rated current for up to one minute, while also being able to run continuously at 100% of their rated current. Normal Duty drives, commonly used for fans and pumps, offer a lower overload capacity, typically 110% of the rated current for one minute, while also being able to run continuously at their full rated current.

Most VFDs are dual-rated, meaning they can be used for both Normal Duty and Heavy-Duty applications, albeit with different current capacities. For example, a VFD capable of powering a 20 HP motor in Normal Duty mode could also drive a 15 HP motor in Heavy-Duty mode. The difference lies in the drive’s ability to handle the required current and overload conditions.

During configuration, the user must select the appropriate mode—either Normal Duty or Heavy-Duty. The VFD will then adjust its overload protection settings and trip if the current or duration exceeds the set thresholds, ensuring safe operation within the defined limits.

The power Factor (PF) is a critical measure of the efficiency and design quality of an electrical system. It is derived from the relationship between active power (P) and apparent power (S).

• Active Power (P): The real power in kilowatts (kW) used by essential equipment like motors, lighting, and computers, which is converted into mechanical energy, heat, or light.

• Apparent Power (S): Measured in kilovolt-amperes (kVA), this is the product of the root mean square (RMS) voltage (Vrms) and current (Irms). It determines the rating of electrical equipment.

The Power Factor is calculated as the ratio of active power to apparent power, with values ranging from 0 to 1, indicating the system’s efficiency in utilizing electrical power. A higher power factor signifies a more efficient electrical installation, while a low Power Factor suggests inefficiencies.

Why Improve Power Factor?

1. Cost Reduction: Enhancing the Power Factor can lead to significant savings on electricity bills. Utility companies often charge extra for low Power Factor, so improving it can reduce or eliminate these penalties.

2. System Optimization: Improving the Power Factor can optimize the use of electrical infrastructure, allowing for the use of smaller transformers, switchgear, and cables. This reduces power losses, voltage drops, and enhances overall system performance.

3. Increased Capacity: By reducing the current load on transformers and other equipment, improving the Power Factor can free up capacity, allowing for additional load without the need for expensive upgrades.

Within IEC 61508 there are basically two types of safety functions: high demand, and low demand.  

A high-demand safety function is for a demand which occurs more often than once per year (e.g. once per day) and a low demand is for something which has an expected demand rate of less than once per year (e.g. once every 10 years). 

There is no definition of “demand” in IEC 61508, but IEC TR 631161 defines a demand as an “event that causes the safety control system to perform the safety control function.” Within the process industry, a demand may also be referred to as a process update, or a process deviation. 

IEC 61508 defines a third mode of operation called continuous mode, but the requirements are similar to the high-demand mode.   

In low and high-demand modes two things need to happen for someone to get hurt.  
1) The safety system needs to fail, and  
2) a demand must occur while the safety system is in the failed state.  

In continuous mode, an incident happens as soon as the safety system fails dangerously, as it is the safety system that is maintaining safety.   

While IEC 61508 as a basic standard needs to cover both low and high-demand modes, this is not the case for sector-specific standards. For instance, machinery only has high demand and process control has mostly low demand. 

STO (Safe Torque Off) is a functional safety feature, built-in into the drive, to disable the output so that the drive cannot generate torque in the motor. 
If STO1 or STO2 input does not have a +24V enable input signal, the drive is disabled to a high degree of integrity, where no single component failure, and only very unlikely combinations of multiple component failures could result in it being enabled. 

SIL Classifications: There are four discrete integrity levels associated with SIL. SIL1, SIL2, SIL3 & SIL4; which is basically differentiated by the range of Risk Reduction Factor (RRF) and Mean Time to Failure (MTTF). Hence, the higher the RRF and MTTF, the higher the rating of SIL will be for the respective demand mode of failure. This is known as the Probability of Failure on Demand (PFD), which is a value that indicates the probability of a functional system failing to respond to a demand (for a particular consequence).

Choosing the right Clean Power Variable Frequency Drive (VFD) and motor for your application is essential for ensuring optimal performance, longevity, and system reliability.

Understanding the Application

Before selecting a Clean Power VFD and motor, it is crucial to understand the operating environment and the specific tasks to be performed. The selection process involves:

1. Motor Selection:

– Choose a motor based on the load’s driving force and movement cycles.

– Basic movement cycle: start > acceleration > constant speed > deceleration > stop.

2. VFD Selection:

– Select a Clean Power VFD capable of driving the chosen motor effectively.

Key Considerations in VFD Selection

1. Motor Current Rating

The primary consideration when selecting a VFD is the motor’s current rating.

The VFD must provide a current equal to or greater than the motor’s full load amperage (FLA). This ensures the motor can operate at its maximum capacity without the risk of underpowering.

Steps to Consider:

– Check the motor’s nameplate for its FLA.

– Ensure the VFD’s continuous current rating matches or exceeds this value.

2. Motor Operation Cycles

Understanding the motor’s operation cycles is crucial for choosing a VFD. This involves knowing how often the motor will start and stop, and the duration of these cycles.

Key Considerations:

– Frequent start/stop cycles can cause thermal stress.

– The VFD should be capable of handling the motor’s duty cycle.

3. Motor Potential Temporary Overload

Motors often encounter temporary overload conditions that the VFD must handle without tripping.

Temporary Overload Capacity:

– VFDs are typically rated for overload conditions, such as 110% of the VFD’s rated current for 60 seconds.

– Ensure the VFD’s overload rating aligns with the motor’s overload requirements.

4. Ambient Temperature

The ambient temperature where the VFD operates significantly affects its performance and lifespan. Higher temperatures can reduce the VFD’s capacity and efficiency.

Temperature Adjustments:

– Check the VFD’s de-rating, which shows how current capacity decreases with rising ambient temperatures.

– Select a VFD with sufficient capacity to handle expected temperature ranges.

5. Duty Cycle (IEC Definition)

The duty cycle, as defined by IEC standards, represents the operating time and rest time of the motor.

Duty Cycle Categories:

– Continuous operation (S1): Running at constant load for a long period.

– Short-time duty (S2): Running at a constant load for a limited time.

– Intermittent periodic duty (S3-S9): Cycles of running and stopping.

VFD Selection:

– Match the VFD to the specific duty cycle category of your motor application.

– Ensure the VFD can handle the motor’s operating pattern.

6. Service Factor (NEMA Definition

The service factor defined by NEMA indicates the motor’s ability to operate over its rated load under specific conditions.

Service Factor Implications:

– A motor with a service factor of 1.15 can operate at 115% of its rated load without damage.

– Select a VFD that can handle the motor’s rated load and service factor. This might mean choosing a VFD with a higher current capacity.

Additional Considerations for Specific Operations

1. Start

– The motor torque must be larger than the load starting torque.

2.  Acceleration

– The motor torque must be larger than the load torque required for acceleration:

– Motor torque > Load torque + Acceleration torque.

– Acceleration torque = (total inertia of the system * (target speed – starting speed)) / (9.55 * acceleration time).

3. Deceleration

– During deceleration, the motor acts as a generator, and energy flows back into the Clean Power VFD.

– The deceleration capability is determined by the Clean Power VFD’s ability to consume or regenerate this energy.

– Ensure the deceleration time is correctly set to avoid triggering alarms or entering a “self-protective” lock state.

4. Constant Speed

– Check that the motor temperature rise specification is compatible with both the movement cycles and the operating environment.

– During acceleration or constant speed operation, the Clean Power VFD’s capacity is limited by the peak current it can deliver to the motor.

– Ensure that the rated current output and overload capacity are the correct size for the application.

5. Clean Power VFD Current Ratings

Clean Power VFD current ratings are provided for Normal-duty operation = continuous operation (S1) / Running at constant load for a long period. It’s an RMS value.

– It’s the current rating suitable for variable torque applications where torque drops with speed. Overload capacity: 110% of the VFD nominal output current for 60 seconds every 10 minutes at 40°C.

6. Driving Multiple Motors

While it is possible to drive multiple motors with a single Clean Power VFD, additional precautions are necessary:

6.1. VFD Current Rating:

– Select a Clean Power VFD with a rated output current greater than the combined total of the individual motor currents.

– V/F control must be selected to drive multiple motors with one VFD.

6.2. Considerations for Multiple Motors:

– For a single motor: VFD rated output current > motor’s rated current.

– For multiple motors: VFD rated output current > total rated motor current.

– If several motors are sequentially started: VFD rated output current > total rated current of started motors plus the starting current of the next motor.

6.3. Thermal Protection**:

– Add a thermal relay for each motor on the Clean Power VFD output side.

– Auxiliary thermal relay contacts should stop or disable the drive.

6.4. Continuous Drive at Low Speed:

– Install a temperature detection device on the motors since they cannot be protected by the VFD’s thermal function.

7. Acceleration and Deceleration Time

When acceleration or deceleration time is restricted, the selection of the VFD cannot be fully based only on matching the VFD current capacity with the motor’s rated current.

Aggressive acceleration/deceleration rates will lead to higher currents, potentially causing overload or overcurrent faults. The system’s capacity (VFD + motor) must be selected to satisfy the predetermined acceleration/deceleration time.

Ensure that the Clean Power VFD capacity is fully considered in advance for operations requiring frequent acceleration/deceleration, such as vertical lift operations.

Putting It All Together

1. Calculate the Required VFD Current Rating:

– Start with the motor’s FLA.

– Adjust for duty cycle and service factor.

– Consider potential overloads and ambient temperature.

2. Match the VFD Specifications:

– Verify that the chosen VFD meets or exceeds all calculated requirements.

– Ensure the VFD can handle the expected operational conditions.

Example Calculation:

– Motor FLA: 27 A

– Service Factor: 1.15

– Duty Cycle: S1 (Continuous)

– Potential Overload: 110% for 60 seconds

– Ambient Temperature: 40°C

Step-by-Step:

1. Adjust for Service Factor:

– Motor operational current = 27A x 1.15 = 31 A

2. Consider Overload Capacity:

– Overload current = 31 A x 1.1 = 34.1 A (for temporary conditions)

3. Temperature Adjustment:

– Check VFD de-rating: The Clean Power VFD has no derating up to 50°C

4. Choose a VFD:

– Select a VFD with a continuous current rating of at least 34 A, and ensure it supports the overload and duty cycle requirements.

Actual full-load speed (the speed at which an induction motor will operate at nameplate rated load) will be less than Synchronous Speed. The difference between Synchronous Speed and Full-Load Speed is called Slip. 
Slip is expressed as rpm, or fraction of the Synchronous Speed, or as a percentage of the Synchronous Speed. 
Slip (rpm) = Synchronous Speed – Motor Speed , usually we write S = ns – n 

S(%) = (ns – n) x 100 / ns 

Therefore, for a given Slip (%), the Motor Speed is: 

n = ns x ( 1- S (%) / 100) 

Consider the following: What is the slip of a 6-pole motor when the speed is 960 rpm for f = 50Hz? 
S(%) = (1000-960) / 1000 = 4% 

As the motor runs at a speed different from the Synchronous Speed, the rotor winding will “cut” the magnetic force lines of the field and so, according to the laws of electromagnetism, induced currents will flow through the winding rotor.  
The heavier the load, the higher the torque required to move it. 
To obtain higher torque, the speed difference must be greater so that the induced current and the generated field become higher. Therefore, as the load increases, the motor speed decreases. When the load is at zero (motor at no load) the rotor practically rotates at its synchronous speed. 

To move a load, a motor must deliver torque that overcomes the load torque. This load torque consists of friction, inertia of the moving parts and the load itself. 
The load and its torque depend on the application. It could be the weight of a mass in a hoisting application, air pressure in HVAC application, filters cleanness, and pipes in pumping applications … 
Depending on the application, the motor must be defined accordingly to load torque and required speed. 
Even obvious, it’s worth reminding that the motor must be chosen prior to selecting the drive! 

A motor’s torque / speed curve must be calculated for every motor type separately. A typical torque/speed curve is shown in the graph here below: 

Locked rotor torque 
The amount of torque the motor produces when it is energized at full voltage and with the shaft locked 
in place is called locked rotor torque. This value is also frequently named “Starting Torque”. It is the 
amount of torque available when power is applied to break the load away and start accelerating it up to 
speed. 

Pull up torque 
The lowest point on the torque-speed curve for a motor that is accelerating a load up to full speed is called 
Pull up torque.  

Breakdown torque 
This is the maximum amount of torque that is available from the motor shaft when the motor is operating at full voltage and is running at full speed. The load is then increased until the maximum point is reached. 

Full load torque 
Full load torque is the rated continuous torque that the motor can support without overheating. 

The maximum load torque is reached just below nominal speed. 

Constant torque: defines a load where the amount of torque needed to drive the machine remains constant whatever the speed at which it is driven. Types of constant torque loads include: 

• conveyors 
• cranes 
• compressors 
• crushers 

• mixers 

Variable torque: requires low torque at low speeds, and increasing torque as speed increases. Variable torque loads make up a large percentage of motor applications. Types of variable torque loads include: 

• Fans 
• Blowers 
• AHU (Air Handling Unit) 

• Chillers 
• Most water pumps (irrigation, downhole, submersibles …) 
• Centrifugal pumps (often used in sewage treatment) 

Fans and blowers are primarily moving air. Centrifugal pumps, however, can move different types of fluids such as oil, coolants, water, etc.