How to Protect Electric Motors ? | Electrical Engineering

Protective devices must be incorporated in the control circuit (or circuits) to interrupt the flow of current to the motor to guard against overloads or other conditions dangerous to the motor or the driven machine, and also to guard against faulty or careless operation of the equipment itself.

The most frequent causes of abnormal operation of electric motors are overloads, open-field, open-phase, reversed phase, over-speed, overheating of bearings or other mechanical parts, low voltage, or voltage failure.

1. Overload Protection:

An overload is an increase in motor current above its safe limit and may be caused by an overload on driven machinery, by a low line voltage, or by an open line polyphase system that results in single phase operation. Overload protection is an essential feature of any controller that is designed to properly protect a motor and still obtain its maximum available power under a range of overload and temperature conditions.

An excess current lasting for only a second or two will not normally cause any serious damage hence protective device should not operate on overloads of short duration and limited magnitude. On the other hand, the protective device must respond to overloads of long duration and of high magnitude which are dangerous to the motor in that they may burn the windings and insulation. So whatever overload protective device is employed, it should possess a time lag to ensure that it operates only for a sustained overload.

Apparatus having an inverse time lag will trip the motor out after a very short interval on a heavy overload but after a relatively long interval on a small overload. Such protection is obviously more satisfactory for most purposes, because the motor temperature will rise more quickly on a heavy overload. This inverse-time characteristic is possessed by certain special types of relays, by a fuse, and also by an ordinary solenoid relay provided with an oil dashpot.

Overload or overcurrent protection may be affected by fuses, electromagnetic and thermal overload relays. Overload relays or fuses need only be placed in two of the three phases of a 3-phase machine if the neutral of the system is not earthed, but should be placed in all three phases if it is earthed. Although an overload relay is more expensive than a fuse but it has the advantage that it automatically resets when the switch is reclosed. The selection of a particular protective device depends on the kW rating, type and purpose of the motor, starting duty and nature of overload.

2. Open-Field Protection:

DC motors of large size may attain dangerously high speed with the loss of field excitation while those of small size may not race due to friction etc. Field loss relays are available to protect dc shunt or compound wound motors against loss of field excitation. Different arrangements exist with starting equipment for dc and ac synchronous motors.

3. Open-Phase Protection:

This condition may be caused by a blown fuse, an open connection, or a broken line. If the motor is at standstill at the time of occurring phase failure, stator current will rise to and remain at a very high value, but the motor will remain stationary and thus the windings may get damaged due to poor ventilation. Dangerous conditions may exist also while the motor is running.

4. Reversed Phase Protection:

In case two phases of the supply of a 3-phase induction motor are interchanged, the direction of rotation of the motor will reverse. This is known as phase reversal. In elevator operation and industrial appli­cations, this might result in serious damage. Phase failure and phase reversal relays are available. These protect the motor, machines, and personnel from the hazards of open- phase or reversed phase sequence conditions.

5. Over-Speed Protection:

Excessive speeds are possible with certain motors that might damage a driven machine, materials in the industrial process, or the motor. Over-speed protection may involve a fine selection and use of control equipment for such applications as paper and printing plants, steel mills, processing plants and the textile industry.

A centrifugal device operates at a certain predetermined speed, usually about 20 per cent above normal, and causes the motor to be shut down.

6. Protection against Overheating of Bearings:

Protection against overheating of bearings or other mechanical parts may be provided by placing thermocouples in the desired situations and arranging so that when the temperature reaches a certain value, relays are operated and the motor is caused to shut down or alarm bells are caused to be rung.

7. Low-Voltage Protection:

This protection is necessary partly to avoid damage to the motor and its driven machine due to heavy currents on switching on full rated voltage after a temporary shutdown, and partly to protect the operator of the equipment. The low-voltage release consists usually of a simple solenoid held up by the voltage of the supply and which causes the starting equipment to return to the “off” position on the failure of the supply.

In addition to the features mentioned above to guard against faults on the motor or driven machine, protection must also be provided against careless manipulation of hand- operated gear or faulty operation of automatic equipment owing to sticking of relays or contactors or to dirty contacts.

The main danger with hand-operated gear is the too fast movement of the starting arm and it can be avoided by providing a mechanical ratchet to prevent the arm being moved more than one step at a time. The other possibility is starting of dc machines with a weak field resulting in an unnecessarily heavy current to provide a given torque. For protection against such possibility mechanical or electrical interlocks may be provided.

The mechanical interlock may consist of a metal plunger, operated by the field rheostat arm that prevents the movement of the starting arm except when the rheostat is in the full-field position. In an electrical interlock, the plunger is controlled by a solenoid, which is energized only when the field is fully on. Similar interlocks are to be provided in the control equipment for ac commutator motors used in speed control by brush shifting; the main switch should be prevented from closing unless the brushes are in the low-speed position.

With fully automatic equipment, there is always possibility that one or more of the accelerating contactors remain closed after being de-energised, due to dirt having clogged their movement. However, this can be avoided by placing auxiliary contacts on the moving arm which are closed when the contactor is open. These contacts are connected in series with the start button, so that the latter is inoperative if any of the contactors have remain closed.