Induction motors

In the world of machines and devices, where durability and operating economy are paramount, the induction motor is unparalleled. This type of motor is the heart of most industrial installations worldwide. Whether we are talking about large production lines, ventilation systems in office buildings, or a domestic water pump – it is highly probable that an asynchronous motor is responsible for the movement.

Why have induction motors gained such immense popularity? The answer lies in their construction. Simple design, lack of complicated contact elements, and high resistance to harsh operating conditions make it an almost maintenance-free component. At the PLE Service store, you will find a wide selection of drive units that convert electrical energy into mechanical energy with high efficiency and reliability.

What is an induction motor and how does it work?

To understand the phenomenon of this device, it is worth looking at its physics. Every electric motor of this type consists of two main parts: a stationary stator and a rotating rotor. The operating principle is based on the phenomenon of electromagnetic induction – hence the name induction motor.

When power is supplied to the windings, a rotating magnetic field is created inside the stator. This field sweeps the rotor, which is located inside. Unlike commutator motors, here we do not supply current to the rotating element physically. The changing magnetic field penetrates the rotor, causing an electromotive force to be induced in it. This, in turn, forces current to flow in the rotor bars. According to the laws of physics, the induced current in a magnetic field generates a force – a torque is created, which begins to rotate the shaft.

The principle of slip in asynchronous motors

A characteristic feature that distinguishes an induction motor from constructions such as a synchronous motor is the presence of slip. For the motor to operate, the rotor speed must always be slightly less than the speed of the rotating magnetic field (the so-called synchronous speed). If the rotor were to equalize with the field, the magnetic flux would stop changing relative to the rotor, the current would stop flowing, and the torque would disappear. This difference in speed is precisely the slip. The name asynchronous motor is often used because the shaft movement is not perfectly synchronized with the grid frequency.

Induction motor construction

Analyzing the construction of an induction motor, we see a device that has evolved over decades. Each element here has its task and affects the final quality of the motor's operation.

Stator as a field generator

The stator is the external, stationary part of the motor. It is built from a packet of steel sheets (often called dynamo sheets), which are insulated from each other to minimize energy losses. The stator slots house the winding made of copper wire. It is the stator windings, powered by alternating current, that are responsible for creating the appropriate magnetic field. Depending on the winding method, the motor can have one or many pole pairs, which directly affects its rotational speed at a given grid frequency.

Rotor - the moving element

This is the heart of the mechanical system. The rotor is mounted on a shaft and rotates inside the stator. The most common solution is the squirrel-cage motor. Its rotor consists of bars (aluminum or copper) connected at the ends by end rings. The rotor is practically indestructible because it does not have wire windings that could break under centrifugal force. The second type is the rotor in a slip-ring motor, where we have a classic winding brought out to the outside, but more on that later.

Shaft and bearings

The entire torque is transmitted to a shaft made of high-grade steel. The shaft must be supported in a way that allows free rotation with minimal resistance. Here, the bearing plays the most important role. Induction motors typically use rolling bearings, which must cope with radial forces (from belt tension or rotor weight) and axial forces. A good quality bearing guarantees quiet operation and long life.

Housing and cooling

The motor housing (often cast iron or aluminum) acts as protection and dissipates heat. Most industrial motors have a fan mounted on the shaft, which forces air flow along the housing fins. This prevents the motor from overheating even during continuous operation.

Types of induction motors based on power supply

Depending on the available electrical installation and machine requirements, different drive variants are used. The most important division concerns the number of supply phases.

Three-phase motors are the industrial standard

A three-phase induction motor is the king of factories. It is powered from a 400V (formerly 380V) network, where three phases are shifted by 120 degrees relative to each other. This configuration ensures that a rotating magnetic field is created naturally and automatically. Three-phase motors are characterized by:

• High starting torque - the motor starts reliably even under load.
  
• Smooth operation - the torque is stable over time.
• Higher efficiency than single-phase models.

If you have the option of supplying three-phase power, choosing three-phase electric motors will be the best decision. Every three-phase motor in our offer is tested and adapted for heavy-duty work.

Squirrel-cage or slip-ring rotor?

Another division concerns the construction of the rotor itself, which is crucial for the starting characteristics.

Squirrel-cage motor

This is the most common type of motor. Its rotor is a solid bar construction, also called a cage.

Advantages: Very simple construction, low cost, high durability, no abrasive elements like brushes.
Disadvantages: High starting current (the current surge at start-up can be 5-8 times greater than the rated current), relatively low starting torque compared to slip-ring motors. Thanks to modern electronics (soft starters, inverters), the problem of high starting current in squirrel-cage motors has been largely solved, making them a universal drive.

Slip-ring motor

This is an induction motor in which the rotor has traditional three-phase windings, and its ends are brought out to slip rings. Brushes slide on the slip rings, allowing an additional, external resistive starter to be connected to the rotor.

It is used where very heavy starting is required - e.g., a large crane, crusher, conveyor belt starting with a full load.

By increasing the resistance in the rotor circuit, we obtain powerful starting torque while limiting the current drawn from the grid. However, this is a more expensive solution and requires more frequent maintenance (brush replacement), which is why in many places it is being replaced by sets: squirrel-cage motor + inverter.

Parameters to pay attention to

When choosing an electric drive from our offer, it is worth analyzing the nameplate and technical documentation. What is important?

Rated power (kW): Specifies how much mechanical work the motor can perform continuously without the risk of overheating. It is important to choose power with a slight reserve, but not excessive, because an underloaded induction motor has a worse power factor and efficiency.

Rotational speed (rpm): Depends on the number of pole pairs and the grid frequency (Hz).

2 poles ~ 2800-2900 rpm.
4 poles ~ 1400-1450 rpm (most popular).
6 poles ~ 900-960 rpm.

It is worth remembering that the given speed applies to operation under load. At no-load, the rotor spins faster, almost at synchronous speed.

Efficiency (IE Classes): Currently, energy-efficient motors IE3 or IE4 are the standard. Although they are more expensive to purchase, they consume less energy, which pays off quickly at current electricity prices. High efficiency also means lower heat losses, i.e., cooler motor operation.

Mechanical size (Shaft height): This parameter (e.g., 80, 90, 100, 132) defines the distance from the base of the feet to the center of the shaft. This is a standardized standard, which facilitates replacing the motor with another model from a different manufacturer.

Applications - where will you find induction motors?

The versatility offered by the induction motor means that the list of applications is very long:

Transport: Every belt, roller, or chain conveyor in a warehouse is driven by such a motor.

Ventilation and air conditioning: Industrial fans, ventilation units, smoke extraction systems.

Pumps: From small circulation pumps to powerful pumps in waterworks. Asynchronous motors cope well with this constant type of load.

Machining: Saws, grinders, conventional lathes.

Cranes and elevators: Older elevator constructions used two-speed motors, today inverter systems dominate.

Maintenance and operation of induction motors?

Although the induction motor is considered bulletproof, it requires periodic attention. The basis is bearing maintenance. Larger motors have grease nipples, in smaller ones the bearing is usually enclosed. A worn bearing generates noise, vibrations, and heat, which can lead to mechanism damage.

The second enemy is temperature. It is easy to overheat the motor if the fan gets clogged with dust or sawdust. Blocked airflow drastically reduces cooling capacity. It is worth regularly cleaning the housing fins. The condition of electrical connections in the junction box should also be checked. A loose wire can cause sparking, increased resistance, and a voltage drop on one of the phases. Operating a three-phase motor with one phase missing (so-called two-phase operation) usually results in the windings burning out in a very short time. Therefore, it is always worth using external motor circuit breakers to protect against overload and phase failure. Often, a thermistor (PTC sensor) is mounted inside the windings, which, when connected to a controller, will switch off the drive if the winding temperature exceeds a safe threshold.

Induction motor vs other solutions

Customers often ask whether an induction motor or a synchronous motor would be better? Or perhaps a brushed DC motor?

• Asynchronous motors win on price, simplicity, and the lack of need for complicated control in simple applications (you connect them to the grid and they work). They are resistant to overloads.
  
• Synchronous motors (e.g., servomotors) are better where ideal position precision and dynamics are required (robotics), but they are significantly more expensive.
  
  
• Commutator motors (brushed) offer easy speed control by voltage, but the brushes wear out, and sparking excludes them from many hazardous areas.

For 90% of general industrial applications, where the machine simply needs to operate stably and continuously, the induction motor remains unrivalled.

At the PLE Service store, we have gathered devices that meet rigorous quality standards. We offer three-phase motors of various powers, speeds, and mounting methods (foot-mounted, flange-mounted). We have models in aluminum housings (light, good heat dissipation) and cast iron housings (rigid, corrosion-resistant, for heavy industry) in stock.

If your machine is down due to a malfunction or you are planning to build a new device, our industrial assortment will surely satisfy you. Remember that a well-chosen motor is an investment in peace of mind. Pay attention to the shaft diameter, foot spacing, and supply voltage. If you have doubts about how to connect the motor or which capacitor to choose - our advisors are here to help.

Asynchronous motors from our offer guarantee that the rotor of your business will spin without interruptions. Choose a proven drive that will not fail under load. We encourage you to use the PLE Service offer!