Here we use cookies for the website to work in a good way for you. By continuing to browse, you agree that we use cookies. What are cookies?

Home / About BEVI / Knowledge bank / General technical information on electric motors

General technical information on electric motors


Three phase single speed motors can normally be connected for two different voltage ranges.

This is because the three phases of the stator winding can be connected in two ways: connection in star (higher voltage), or delta (lower voltage) with a ratio of √3. The lowest voltage is used when the motor is connected in D and the highest voltage when the motor is connected in Y. The voltage at Y = √3 × the voltage at D.


a) 220-240 VD/380-420 VY - may be labelled 230/400 V (standard for motors 3 kW and smaller). Suitable for direct o line (DOL) starting on 380-420 V supplies.


b) 380-420 VD/660-720 VY - may be labelled 400 VD (standard for motors 4 kW and larger). Suitable for Star/Delta starting on 380-420 V supplies or direct on line starting on 660-720 V supplies.

The mains voltage can vary ± 10% at 400 V or ± 5% for wide voltage range rated motors without needing the motor's nominal power to change. Note that the efficiency is set at 230 V and 400 V values, respectively.


The motors are balanced with a half key. Special degrees of balancing are available on request.

Fuses and motor protection

Fuses do not provide protection for a motor, but are only protection against short circuits in the circuit.

Motor protective switches

Raised motor temperature, due to overload or failure of a phase, is prevented by a motor protective switch. The current, to which the thermal overload protection is to be set, is indicated on the motor's nameplate. In some cases, an ordinary motor protective switch is not sufficient protection. This applies especially to more difficult operating conditions, e.g. starting of equipment with high moment of inertia, when using frequency inverters and operating conditions with large differences in cooling temperature. In these cases, thermal protectors (eg, clixon) or thermistors in the windings can be used.

Thermal protectors

Thermal protectors are usually mounted in the motor winding. When a certain temperature is reached, the thermal protectors break an electrical circuit, e.g. the supply voltage to a contactor that switches off the motor. The breaking contact is a temperature sensitive bimetallic spring. BEVI can retrofit thermal contacts in all motor sizes.


Thermistors are used for temperature monitoring.

The protection unit consists of the thermistors that can be mounted in the windings and a trigger device. The thermistors are temperature sensitive resistors which at a certain temperature change the resistance considerably. This is sensed by the trigger device which in turn e.g. switches the supply voltage off to the main contactor. BEVI's IE3 motors have thermistors as standard. BEVI can also retrofit thermistors in all motor sizes.


As standard, the fan and cowl is fitted at the non-drive end (cooling form IC 411). Other cooling methods can be supplied e.g. a separately driven cooling fan which is often used with inverter drives.

Heaters to prevent condensation

Motors used in conditions of wide temperature variations or extreme climatic conditions can be damaged by condensation and dampness in the windings. In motors fitted with heaters, the windings are heated to a few degrees above the ambient temperature when the engine is switched off. This is enough to prevent condensation. The standby heater must be switched off when the engine is running.

Smaller motors can also be heated by applying a low voltage across the motor winding. The voltage should be 5-10% of the rated voltage over two phases.

BEVI can fit heaters to all motor sizes on request.

Insulation class

The motors are manufactured with different quality on the insulation material. The insulation materials are divided into different classes which are indicated by a letter, e.g. B or F. The insulation class indicates the upper temperature limit that the insulation material can withstand. Ambient temperature, permissible temperature rise and the temperature reserve are factors that determine how much a motor can be loaded.

The rated power of a motor is normally specified for an ambient temperature of + 40°C. If the ambient temperature is higher, the output power must be reduced.

BEVI's motors are normally wound with class F material but can be ordered with other materials, e.g. our wood dryer motors are wound with class H material.

Insulation classAEBFH
Ambient temperature (°C)4040404040
Permissible temperature rise (°C)607580105125
Temperature reserve  (°C)55101015
Max temperature (°C)105120130155180

Motor duty types

The motor duty type is indicated by one of the designations S1 – S9. S1 is the normal operation after which the rated power of the motor is indicated. However, in certain operations the rated power of the motor can be increased. Depending on how the load and thus the output power of the motor vary with time, different duty types are given below. The rated power for each type of operation is determined by a load test, which the motor must pass without exceeding the temperature limits set in IEC 60034-1: 2017.

For duty type S2, the designation must be followed by the length of the load period. In the operating modes S3 and S6, the designation must be followed by an intermittent factor. Example: S2 60 min, S3 25%, S6 40% . At duties S4, S5, S7, S8, S9, designations must be followed by the moment of inertia etc.

S1 - Continuous duty
The motor operates at a constant load for enough time to reach temperature equilibrium.

S2 - Short-time duty
The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature.

S3 - Intermittent periodic duty
Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise.

S4 - Intermittent periodic duty with starting
Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise. (Similar to S3, but there is a significant starting time within the periodic operation.)

S5 - Intermittent periodic duty with electric braking
Sequence of identical duty cycles - starting, operation, braking and rest. Again thermal equilibrium is not reached.

S6 - Continuous operation periodic duty
Sequential, identical duty cycles with a period at load followed by a period at no load. Difference between S1 is that the motor runs at no-load, without actual stopping.

S7 - Continuous operation periodic duty with electric braking
Sequential identical cycles of starting, running at constant load and electric braking. No rest periods.
As per S6, but with a significant starting and electric breaking periods. Again motor operates at no-load for period instead of stopped.

S8 - Continuous operation periodic duty with related load/speed changes
Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods and thermal equilibrium is not reached.

S9 - Duty with non-periodic load and speed variations
Load and speed vary periodically within the permissible operating range. Frequent overloading may occur.

Enclosure (Degree of Protection)

Correct protection class is a prerequisite for a motor to be able to work safely for a long time in tough duty and challenging atmosphere. Motors are produced in degree of protection IP55 as standard, but are also available to other standards.


The motor construction, power rating and mounting dimensions meet the requirements of the international standards listed below.


  • IEC 6034-1:2017
  • IEC 60072-1:1994 

Standard on efficiency measurement methods

  • IEC 60034-30-1:2014
  • IEC 60034-2-1-2014
Jörgen Danielsson

Product manager electric motors

Direct: +46 499-271 26
Send e-mail

BEVI Knowledge bank

BEVI's Knowledge bank collects information about our areas of expertise, electric drive systems and power generation.


BEVI AB (headquarters)

Bevivägen 1
384 30, Blomstermåla, Sweden

Tel | +46 499-271 00
E-mail |

Quick links

About BEVI