ELECTRIC MOTORS AND EVERYTHING ABOUT THEM

In some of their many forms, electric motors are probably the most widespread drive unit in all kinds of industrial and domestic applications. Below, we offer the most basic overview of the most common electric motors – squirrel-cage induction motors. 

This engine design is suitable for most standard applications, i.e. applications requiring standard drive dynamics. Examples include conveyors, mills, crushers, agitators, pumps, fans, grinders, simple positioners, roller and gate drives, and many other devices. What are the basic selection criteria?

Rotation speed

The rotation speed of an induction motor is determined by the number of poles and the frequency of the AC input voltage. For this reason, induction motors are offered at the following speeds:

• two-pole – synchronous speed 3,000 rpm, in reality approx. 2,740 - 2,990 rpm for induction motors
• four-pole – synchronous speed 1,500 rpm, in reality approx. 1,350 - 1,490 rpm for induction motors
• six-pole – synchronous speed 1,000 rpm, in reality approx. 830 - 990 rpm for induction motors
• eight-pole – synchronous speed 750 rpm, in reality approx. 630 - 740 rpm for induction motors

On request, it is also possible to design and make slower multi-pole motors (10-pole, 12-pole, etc.). Electric motors with switchable polarity are also available (most often two-speed, with customised three-speed ones also an option).

If none of these speeds suits you, there are a few basic options for changing the speed:

• belt drive – a simple and inexpensive solution; the main downsides are the high torque loss and high shaft and bearing loads
• gearbox with a constant gear ratio – in their design and cost, these are usually a more demanding solution with the advantage that the motor has a low radial shaft load and greater efficiency (more efficient torque transmission); for more details see the separate section on this
• continuously variable transmission – or a gearbox with variable gear ratio, features generally in keeping with the point above, with the possibility of changing the output speed
• frequency inverter – electronic component for a comfortable smooth or step change in motor speed by changing frequency and voltage; for more details see the separate section on this
  
  • for operations with a frequency inverter, extensions may need to be installed (most frequently temperature sensors, higher-level rotor balancing, a ventilation unit and, for larger motors, insulated bearing) – more here

Electric motor power

Power (or torque on the shaft) stands alongside speed as a basic quantity absolutely necessary when selecting an induction motor. Electric motors are made with a wide range of different powers, ranging from 0.04 kW to 200 kW, and even thousands of kW in special large motors.

Particular attention must be paid to this parameter, because if the drive power is too weak it may cause the driven equipment to malfunction and could even destroy (burn) the electric motor. When controlling the speed via a frequency inverter, it is necessary to take into account the decrease in rated output (more here).

Motor voltage

Motor voltage is another aspect absolutely essential for proper motor selection. In the Czech Republic, the public grid uses three-phase voltage 3AC 400 V with a 50 Hz frequency and single-phase voltage 1 AC 230 V 50 Hz (formerly 3AC 380 V 50 Hz and 1AC 220 V 50 Hz). In many other countries and, especially, in other continents, other voltage systems with different voltage and frequency levels are used. As for whether to choose a single-phase or three-phase electric motor, we strongly recommend the three-phase motor. The reasons are very simple: lower price, lower noise, lower engine heating, higher torque, etc.  

In the Czech Republic, the following voltages are typically used for three-phase motors:

• for motor outputs of up to 3 kW (inclusive): 3AC 400VY/230VD 50 Hz – this winding is designed for the direct starting of the motor and allows the motor to be powered by an inverter with single-phase input voltage (do not confuse this with a single-phase motor!)
• for motor outputs above 3 kW: 3AC 400VD/690VY 50 Hz – this winding allows for both direct motor start-up and double start-up via a star/delta switch

Electric motor shape

The shape of an electric motor is taken to mean the design of the motor clamping elements. Each motor needs to be fixed in the required position, whether this entails tightening via a flange to the driven device or to a base plate. The shape of the motor and its mounting position are expressed by the abbreviation IM... (e.g. IMB3 = horizontal position with feet).

Motors are supplied in the following basic shapes:

• foot shape – the motor is clamped to the base “on feet”
• flange shape – the motor is clamped at the front (on the shaft output side)
  
  • Please note: For motors up to 160 (up to 18.5 kW at 2,940 rpm in the standard range), three different flange sizes are available for each motor. The flange size must be defined when ordering a flange motor (total diameter, diameter of the pitch circle of the connecting holes or centre lock diameter).
• combined foot-flange shape – the motor is equipped both with feet and with a flange, i.e. put simply, this is a combination of both the above-mentioned shapes

The horizontal position is considered to be the basic mounting position. If the motor is to operate in a vertical position (regardless of whether the shaft is up or down), this must be made known when you make an enquiry or place an order. This is because a special motor configuration may be required (typically a case cover or locked bearings).

Working environment

An important and often rather neglected selection parameter is what characteristics are to be found in the working environment. When selecting a motor, attention needs to be paid to the following parameters: (We will gladly provide you with detailed information and assistance as you choose your motor.)

• the risk of explosion of flammable vapours and gases or the risk of ignition of flammable dust – these environments clearly require specialised designs of explosion-proof or ignition-protected electric motors. The motor must have valid ATEX certification available
• ambient temperature – most basic motor designs are designed to operate in an environment where the air temperature is -20 °C to +40 °C
  
  • for temperatures outside that range, motors have to be configured accordingly
• elevation – most basic engine designs are designed to work in an environment at up to 1,000 metres above sea level
  
  • at higher elevations, the motor power needs to be reduced
• relative humidity – most basic engine designs are designed to work in an environment of up to 30 g of water per 1 cubic metre of air (equivalent to 65% humidity at 30 °C)
  
  • more demanding conditions tend to prevail in the tropics and certain operations, and again require a specialised configuration for the electric motor
• water and dust – conventional electric motors are manufactured with an IP55 degree of protection (dust and splash protection) and can therefore work without problem even in an outdoor environment
  
  • for very humid environments (dishwashers, dryers, some air-conditioning systems, etc.) we recommend special motor configurations
  • it is generally necessary to keep the motor ribbing as clean as possible in order to maintain the necessary cooling capacity
• motors in contact with the gearbox oil charge – in this case, a shaft radial shaft oil seal needs to be configured
• aggressive environments – aggressive environments (lyes, softeners and other aggressive chemicals) can cause very rapid damage to motor components
  
  • in a situation like this, the motor must always be configured individually

Electric motor equipment – extensions

Depending on your needs, it is possible to retrofit several basic extensions expanding the functionality or adding protective and measuring functions. The extensions most commonly used are:

• electromagnetic spring brake – used to quickly stop and lock the drive
  
  • it is a brake with a DC electromagnet powered either directly by DC voltage (usually 24 V DC), or by single-phase or multi-phase AC with an integrated rectifier
  • the basic design is taken to be a brake with single-phase input voltage of 1AC230 V (DC205 V magnet voltage)
  • the braking force is defined by the brake torque
• ventilation unit – used for the efficient cooling of the motor, especially in operating modes where the actual cooling is insufficient, e.g. when the motor speed is significantly reduced by the frequency inverter, if there is frequent switching, etc.
• IRC or pulse rotation sensor – used to measure the speed of an electric motor, it is applies primarily in connection with a frequency inverter with speed feedback
  
  • it is suited to dynamically more demanding applications and applications requiring higher rotational speed accuracy, or when positioning
• anti-condensation winding heating – this is a heater in the winding, preventing the condensation of water in the winding
  
  • though seldom used in the Czech Republic, it is a popular export to countries with more arduous climatic conditions
• motor temperature protection – besides the essentially mandatory motor current protection (circuit breaker, relay), protecting the motor from overload or phase failure, it is also necessary to run direct checks on the temperature of the winding and, where appropriate, the bearings
  
  • the reason for this is that the winding can overheating not only because of overloading, but also in dynamically more demanding operating modes (typically in connection with the frequency inverter)
  • the basic types of thermal protection of the winding are as follows:
    
    • PTC thermistors – the most widespread type of protection, a resistance sensor with a sharp rise in resistance at a critical temperature
      
      • requires an evaluation device (inverter, thermistor relay 3RN0); unsuitable for temperature measurement because, apart from the critical limits, it reacts non-linearly
      • compulsory equipment for inverter-powered motors in potentially explosive environments (must be evaluated by a certified thermistor relay)
    • thermo-time switches – bimetallic temperature sensor, the control circuit opens at critical temperature
      
      • KTY84 – linear resistance temperature sensor, suitable for winding temperature measurements, requires an external evaluation device
      • PT100 – linear resistance temperature sensor, used for winding and bearing temperature measurements, requires an external evaluation device
        
      • PT1000 – linear resistance temperature sensor, suitable for winding and bearing temperature measurements, requires an external evaluation device

Electric motor equipment – mechanical properties

The motor design allows for changes to the mechanical properties of the drive in keeping with the needs of the application. The most common adjustments include:

• reinforced bearings on the drive side or on both sides
  
  • this adjustment is suitable for drives with an increased radial shaft load (typically when connecting the belt drive, for fans and agitators with a propeller directly on the motor shaft, etc.)
  • depending on the size of the motor, either larger bearings are installed (usually series 63xx instead of 62xx) or a roller bearing is used, or both solutions are combined
• secured bearings
  
  • used when the axial force acting on the motor shaft is increased
• insulated bearings
  • used for motors with larger axle heights when powered by an inverter, as protection against bearing currents
• bearings with top-up lubrication
  
  • bearings for smaller motors (usually up to 250) have a permanent grease cartridge as standard
  • optionally, it is possible to change the configuration to bearings with top-up lubrication, thus prolonging the life of the bearings (if lubrication is carried out at the set intervals and the right lubricant type is used)
• alternative positioning of the screw terminal and cable outlets
  
  • used where standard positions are inappropriate
  • most Siemens motors have a screw terminal at the top, but it can optionally be supplied on the left or right side, and the outlets can be turned 90°
• rotor balancing to a higher accuracy class
  
  • used when high operating speeds are required or to minimise vibrations at normal speeds (typically in conjunction with a frequency inverter)
• other types of mechanical adjustments
  
  • in addition, there are no end of customised mechanical adjustments accommodating the needs of a wide variety of applications, such as different types of shaft adjustments, noise reduction fans, etc.

Summary

1. clarify the power and speed you require and check your network voltage

2. clarify the shape of electric motor you require

3. make sure that the electric motor is sufficiently equipped and has the features required for your working conditions

4. check whether you need to install any extensions or make mechanical adjustments for the application

5. please let us know the above requirements by making an enquiry, or select the appropriate version of the basic electric motor in our e-shop and configure it on the product details page