In our daily lives we are surrounded by electric motors. Sometimes they are obvious, sometimes we use them without realizing. They exist in different versions. In HVAC applications we distinguish AC motors and EC motors. What is the difference? What are the pros and cons? This article provides an overview of the main operating principles.
In this article we explain the differences between AC motors and EC motors in an understandable way. The options for controlling an AC motor and the advantages and disadvantages of the technologies mentioned are briefly discussed.
Magnetism - the basic principle of all electric motors
An electric motor is a machine that converts electrical energy into mechanical energy. Electrical energy is mainly converted by the motor into rotary motion. The electrical energy or power is expressed in kW, while the rotary motion is expressed in rpm. So the electrical power [kW] is converted by the motor into rotary motion [rpm]. However, a part of the electrical energy is lost through heat generation, mechanical friction and other losses in the motor. The efficiency of an electric motor tells you what part of the absorbed energy is available at the motor shaft. The efficiency is usually indicated on the nameplate by the symbol η expressed in %. η = 85% means that 25% of the absorbed electrical energy is lost. The higher the efficiency of the motor, the smaller the losses and the more energy is converted into torque.
The force with which the rotational movement is performed is called the torque and is expressed in Nm.
An electric motor functions as a dynamic interplay of magnetic forces. When an electrical current is applied, it generates a magnetic field that interacts with magnets situated on a rotating component. This interaction induces rotary motion, exemplifying the conversion of electrical energy into mechanical motion. The motor serves as a sophisticated mechanism wherein the orchestrated synergy between electricity and magnetism facilitates controlled and purposeful rotational movement, underpinning a wide array of applications in all industries, including HVAC industry.
A motor is made up of a stator and a rotor. The stator is the static part of the motor – the stationary part used to mount the motor to the air duct or installation. The rotor is the rotating part on which the motor shaft is mounted. In a fan, the fan blades are mounted on this motor shaft (on the rotor). The rotor usually has a cylindrical shape. In the stator, a magnetic field is generated through electromagnetism. The electric current flows through the motor winding in the stator and generates a magnetic field. Since it concerns alternating voltage and several windings are used, this magnetic field revolves around the rotor. The rotor follows this rotating magnetic field. You can compare it to magnets that attract each other.
AC motors - Asynchronous vs synchronous motor
AC motors are the standard for industrial applications. This type of motor is also regularly used in the HVAC sector, especially with larger capacities. AC motors are very reliable, robust and easy to maintain. We distinguish between synchronous and asynchronous AC motors. As mentioned above, a rotating magnetic field is created in the stator. A synchronous motor has a rotor made up of permanent magnets. Magnetic opposites attract each other. The magnets of the rotor will therefore follow the rotating stator field exactly (synchronously), regardless of the load.
The working principle of an asynchronous motor is a bit more difficult to explain in a simple way. The asynchronous motor does not have a rotor with permanent magnets; it's magnetic field is created by induction. To make this possible, the rotor is composed of electrical conductors. These conductive rods are usually made of aluminium or copper. They are mounted in the cylindrical rotor and are connected at both ends by short-circuit rings. The whole has a cage-like shape – hence the name squirrel cage rotor. Due to the principle of induction (Faraday's law), electric current flows through these conductors. Due to this reason, an asynchronous motor is also called an induction motor. This rotor current creates a magnetic field that interacts with the stator field, causing the motor to rotate.
Unlike a synchronous motor, an asynchronous motor will always rotate slower than the stator magnetic field. This difference is called the slip. Due to this difference, a reverse current is induced in the rotor of the asynchronous motor. The greater the load, the greater the difference (slip). The rotor accelerates until the magnitude of the induced rotor current and motor torque balances the load on the motor shaft. Since there is no induced rotor current (no torque) at synchronous speed, an induction motor always runs slower than synchronous speed.
Speed controllers for AC motors
Synchronous motors generally consume less energy than asynchronous motors, but can only be used in combination with a frequency converter. Asynchronous motors offer the choice of whether or not to be controlled by a speed controller. Speed controllers help to reduce mechanical shock during start-up. Thanks to speed controllers, many applications can be controlled more comfortably and precisely. Just think of demand-driven ventilation where speed controllers optimize the airflow and combine good indoor air quality with energy savings.
In HVAC applications, fans with asynchronous motors can be controlled with a frequency converter or with a fan speed controller. Both have their pros and cons. A frequency controller offers the most accurate control and is energy efficient. A fan speed controller is cheaper and much easier to install and use.
A frequency converter will optimize both the motor voltage and the frequency of the motor current via pulse width modulation. This requires IGBTs. Insulated Gate Bipolar Transistors are high-performance electronic components that can switch high-power electrical currents at very high frequencies. This technology enables optimum engine control, but it is not cheap. Usually a V/f or scalar frequency controller is chosen to control fans. A scalar frequency converter keeps the ratio V/f constant (constant torque) over the entire speed range. These are the simplest frequency converters given the small amount of motor data required by the drive. Only a limited configuration is necessary to control the motor. V/f is the only control method that allows multiple motors to be controlled by one frequency converter. In such applications, all motors start and stop at the same time and follow the same speed reference.
Unlike a frequency converter, a fan speed controller will only vary the motor voltage. This type of speed controller is only suitable for voltage controllable motors and can therefore be used in applications where the torque decreases with speed, for example controlling fans. The big advantage of this type of controller is the simple operation and the cost price. No configuration is needed; once everything is connected, the fan can be controlled immediately. The construction of a fan speed controller is much simpler than that of a variable speed drive. This also translates into the cost. A number of different technologies can be used for fan speed controllers – each with their own specific advantages and disadvantages. The most commonly used technologies are: Transformer speed controller (5 step controller) or Electronic fan speed controller (TRIAC phase angle control)
EC motors – motors with built-in speed controller
Brushless DC electric motors are also referred to as Electronically Commutated motors (EC motors). They are synchronous motors that are driven by direct current via a built-in (speed) controller. However, EC motors are connected to alternating current (mains voltage). This alternating current is internally converted into direct current with which the integrated controller controls the motor.
EC motors usually have a rotor made of permanent magnets that revolve around a stator. The built-in regulator contains a rectifier that converts the AC supply voltage into direct current (DC). The integrated regulator then sends the right amount of current, in the right direction, at the right time, through the windings in the stator. This creates a rotating magnetic field in the stator, which drives the rotor with permanent magnets. The position of each rotor magnet is determined using Hall sensors. The appropriate magnets are sequentially attracted to the magnetic poles in the stator. At the same time, the rest of the stator windings are charged with the reversed polarity. These attractive and repulsive forces combine to achieve smooth rotation and produce the optimum torque. Because this is all done electronically, precise engine monitoring and control is possible. An EC motor can therefore be regarded as the combination of motor and speed controller in one housing. EC motors can usually be controlled via an analogue signal (typically 0-10 Volt or PWM) or via Modbus communication. In this way fans with EC motor can be controlled via most HVAC sensors with analogue output or Modbus communication.
EC motors are usually more expensive compared to AC motors, but they offer some advantages. The main ones are: a high torque-to-weight ratio due to their more compact construction and lower energy consumption compared to AC motors. The permanent magnets and integrated electronics make this type of motor more expensive. The motor and the fan speed controller are combined in one housing. If the EC motor can be directly controlled via Modbus communication, all motor parameters such as temperature in the motor windings, power consumption, rotational speed, hour counter, etc. can be read remotely. The commissioning might be more complicated, but once installed, this solution offers more options - especially in terms of integration into BMS systems or smart ventilation systems.