Motor protection devices

Motor protection protects electric motors against thermal overload or failure of one or two phase conductors. On the one hand, the motor protection device must allow the motor to be operated economically with full utilization of the characteristic values, but on the other hand it must react sufficiently quickly to overloads.

The following devices are generally recommended for full motor protection:

  • Motor protection switch
  • Thermal monitoring
  • Overload cut-out for blockage situations
  • Primary motor protection

In addition to the main fuse for the power supply line, the motor protection switch is the primary device. The motor circuit-breaker monitors the currents in the individual phases (phase conductors), but these must be designed to cover peak currents when switching on and certain reserves. Failures of phase conductors are also detected as a result. The circuit breaker always has an overcurrent and a short-circuit current tripping device. In addition, they always switch off the main circuit directly.

Thermal monitoring

Electric motor drives are designed for a specific rated operation in which the critical parts of the motor remain within the permissible temperature range and short-term overloads do not have a hazardous effect.

Depending on the motor type and load at the operating point at the rated force of the electric actuator, the duty cycle must be observed. The duty cycle (ED) is a maximum permissible operating interval after which a rest phase for cooling down must take place in order not to damage or destroy the motor. The operating modes are defined in VDE 0520, among others.

For electric actuators, operating mode S3 - intermittent operation based on 10 min is generally used.

The duty cycle can be specified as a dimensionless percentage (ratio of service life to observation period). As a rule, the period of use is specified in addition to the percentage. If not, the period of use is 10 minutes.

Example DC motor: 20 % duty cycle S3 - e.g. 2 minutes of operation, then 8 minutes of cool-down

Insulation classes of the motor winding and their temperatures:

The insulation classes in accordance with DIN EN 60085 are identified by letters, each of which is associated with a specific maximum temperature. Insulation class F is used as standard for RACO's three-phase AC motors.

  • Insulation class E: Maximum permissible temperature of 120 °C (recommended ambient temperature max. 45 °C)
  • Insulation class F: Maximum permissible temperature of 155 °C (recommended ambient temperature max. 60 °C)
  • Insulation class H: Maximum permissible temperature of 180 °C (recommended ambient temperature max. 80 °C)

The temperature monitoring of the motor winding is designed according to the switch-off temperature for the corresponding ISO class. RACO three-phase motors are equipped with thermal contacts as standard. PTC thermistor sensors (PTC / PT100) are optional. The monitoring devices must be evaluated via the sensor systems EPS02 or EPS06 or by the customer controller.

Overload protection (UPM function)

Electric actuators or electric drives must always be operated in such a way that they switch off via limit switches and do not get into a blockage situation. Due to their torque behavior, three-phase motors in particular can reach up to 300% of the rated torque in the event of a blockage, which leads to the destruction of the motor winding within a short time and/or to mechanical damage to the electric cylinder or the customer's equipment due to the increased forces.

In addition to the motor protection switch, which can only provide rough monitoring with reserves for start-up operation and thermal protection, which only reacts in the long term to heating caused by the duty cycle, an overload protection should be used in applications where blockages may prevent the end position from being reached in order to switch off. This is a possible scenario, particularly in the conveying of bulk materials on slides and flaps.

In order to be able to react quickly here, the following supplementary measures, which do not replace the above, are recommended for full motor protection:

  • Speed monitoring using EPS06 sensor system
    This sensor determine the current speed (motor turns per minute - UPM). If this falls below a set value (preset at the factory to the nominal force), a switching contact is triggered which switches off the motor via the customer's control system. The function is effective on both sides and is equipped with a restart interlock in the blockade direction
  • Load monitor cos φ
    In order to be able to make a statement about the current motor load on the electrical side of the asynchronous motor, the evaluation of the cos φ is also suitable, especially for motors with a soft characteristic curve without a pronounced stall torque. Typical values are between 0.85 and 0.95 at rated power.  A motor at no load has a significantly worse cos φ than in the nominal load range. The phase shift between current and voltage approaches the theoretical maximum value of 90°. This means that the cos φ shifts towards 0. The motor therefore represents an almost purely inductive load. This results in a higher reactive power load on the grid. If the load on a motor is too high, the phase shift between current and voltage moves towards 0° and therefore the cos φ approaches 1. If the load is infinitely high, i.e. the drive is blocked, theoretically only the ohmic component of the motor winding is effective. The phase shift is therefore inverse to the load on the motor.
  • Mechanical force limit switch
    In special cases, type1 heavy duty actuators cylinder can be equipped with a mechanical force limit switch. For this purpose, the spindle system is mounted in the bearing housing in a spring assembly acting on push or pull. The cut-off force can be set separately for push and pull depending on the spring travel at the maximum force via microswitches (factory-set). These must then switch off the motor via the customer's control system when the contact is opened.