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Solid state gives better protection

This article was originally written in the period 1995-2000

Allen-Bradley compares solid state motor protective devices and electromechanical overload relays. Enhanced protection and improved features are claimed.

Electric motors fail for a number of reasons, including excessive heat, moisture and contamination; short circuits; mechanical problems; and simple old age!

The primary cause of motor failure is excessive heat, which is caused by:

  • Excess current (current greater than the normal motor full load current)
  • High ambient temperatures
  • Poor ventilation of the motor.

In general, a single motor protective device alone cannot protect the motor from excessive heat due to all three of these causes. Currents greater than normal motor full load current can be caused by high inertia loads, such as loaded conveyors, locked rotor conditions, low voltage, phase failure and phase imbalance. If a motor is continuously overheated by only 10Cdeg, its life can be reduced by as much as 50%.

All of these problems can be accounted for and measured to provide the very highest level of protection to the motor, so that the motor achieves the longest possible electrical and mechanical life in the application. Protecting motors against these various problems can be achieved with the functionality that electronics now provides in solid state motor protective devices.

By measuring parameters such as current, temperature and phase imbalance, damage to the motor’s stator and rotor can be prevented, as well as providing an early warning that there may be trouble with another part of the mechanical system, such as conveyors, belts, gears and bearings.

There are significant differences in the construction and performance of electromechanical overload relays and solid state motor protective devices. They include the principles of operation of the two types of devices, the reduced heat dissipation and the energy savings that can be realised with solid state systems.

Traditional electromechanical overload relays (eutectic alloy or bimetallic) do not measure current directly. These devices operate by passing current through a heater element, which simulates the actual heating effect that is taking place in the motor. During overload conditions, the heat generated within the heater element reaches a level that causes a mechanism to operate and an auxiliary contact to open. In typical installations, the contactor coil is wired in series with this contact. When the auxiliary contact opens, the contactor is de-energised, removing current from the motor.

Solid state motor protective devices, on the other hand, actually do measure current directly. Current can be measured in a variety of ways, but the most typical method of measuring current is using current transformers. Once an overload condition is reached, the electronic circuit of the motor protective device operates due to the increased level of current, causing a contact to open (similar to the electromechanical overload relay), de-energising the contactor and removing current from the motor.

Solid state electronics in motor protective devices not only provide traditional overload protection against the overcurrents that the motor is subjected to, but they also provide more information and protection against other fault conditions. These include: phase loss, phase imbalance, jam protection, ground fault (earth fault) protection, underload protection and over-temperature protection.

Under a phase loss condition, the motor current in the remaining two phases of a fully loaded motor increases to 1.73 times the normal motor full load current. A phase loss can occur due to a blown fuse or a poor electrical connection. With solid state electronics, in the motor protective device, the device can be designed so that it will trip, dropping out the starter within a 2 second period, thereby providing improved protection to the motor. Traditional electromechanical overload relays may take 40 seconds or longer after a phase loss, before the heat generated is sufficient to cause a trip.

Phase imbalances also cause an increase in current in the motor. A phase imbalance of only 5% requires a reduction in the permissible motor output of 25%, derating, for example, a 10HP motor to 7.5HP. Therefore, very small phase imbalances cannot be tolerated, because damage to the motor may occur, or the required output of the motor may not be achieved.

A high overload – or jam condition – can put unnecessary mechanical and thermal loading on the motor and the transmission elements. This might occur when a bearing seizes or parts are jammed in a conveyor. By sensing this jam current, the motor can be taken offline to prevent damage to components in the mechanical system. Jam protection is particularly useful in applications such as conveying systems, mills, mixers, crushers and saws.

Motors (for example, fans or submersible pumps) that are cooled by the medium being handled can become overheated in spite of being underloaded, due to the absence of sufficient medium. Causes might be clogged filters or closed valves. Often, such machines are installed in inaccessible places, making repair time consuming and expensive. An underload, represented by the consumption of less than normal motor full load current, may indicate that there is a mechanical defect, such as a torn conveyor belt, damaged fan blades, broken shaft or worn tools. Such conditions may not harm the motor, but they do lead to loss of production or an interrupted production process. Rapid detection by the motor protective device helps to minimise both the damage and the downtime.

For increased safety of personnel in the area of motors and equipment, solid state motor protective devices can also provide ground fault protection. Ground faults in the motor occur when the insulation of the motor windings is damaged, perhaps by high voltage surges, age, sustained overloading or overheating, mechanical vibration or foreign objects, such as dust particles. In these instances, leakage to ground can occur. In grounded systems, the fault current can rise rapidly to a very high level, ultimately turning into a short circuit current. Solid state motor protective devices can sense ground fault currents before they rise to dangerous short circuit fault levels, and switch the motor offline before severe damage or severe injury to personnel occurs.

Motor failure is most often caused by excessive heat. However, excessive heat is not only caused by increased in current, but it can also occur for other reasons, such as the ambient environment around the motor. Severe ambient conditions where there may not be sufficient ventilation to the motor, or where severe changes occur in the ambient temperature, would be applications where sensing current only would not be sufficient to protect the motor properly.

In these types of applications a temperature input from a positive temperature coefficient sensor or a resistance temperature detector (both of which are embedded in the motor windings) can be utilised to provide temperature input to the motor protective device. This additional information, utilised in conjunction with the motor current, will properly protect the motor, even when there may not be sufficient cooling.

A new white paper fromAllen-Bradleyexamines the use of solid state motor protective devices. Get a free copy.

Added on 7 January 2009 at 2:55 pm | Read more Drives articles