A motor as with any other device supplied with electricity must be protected from excessive currents which are normally referred to as overloads and short circuits either phase to phase or phase to earth. These faults are also referred to as symmetrical and asymmetrical faults respectively.
It is also vital that there is discrimination between the motor and starter protection and the upstream protection devices.
A motor, its starter and the machine or device driven by it will have a capital cost that may generally be considered to increase as the motor output rating increases. It is therefore logical to protect that investment with more complex protection systems as the motor output increases.
The use of protective devices that can be programmed remotely or have their contacts programmed is deprecated, for this simple reason, once the protection has been set there should be no reason for the either the settings or contact configuration to be changed so why use that type of device and why pay the premium for it. There is of course another very important reason to avoid these devices, when the protection relays have been set up it should not be possible for them to be changed either by computer malfunction, by operator error or by deliberate mischievous act. If there is a need to change the settings, which should be a rare occurrence, what is the problem with a properly authorised or competent person walking to the starter and modifying the settings manually? It should also be remembered that the authorised or competent person would have to physically access the starter within it’s controlled area to effect the changes.
If the current drawn by a motor is to be used for control purposes then that current should be detected by a separate current transformer and transducer and not via the protective relay circuit. Protection devices use a different type of current transformer to instrumentation devices with completely different characteristics and accuracies.
Any fault that is detected and causes the motor to be stopped, should via the motor’s control circuits, prevent the motor from restarting until the fault detecting device is replaced or reset and the control circuit is reset.
Phase to phase short circuits either at the motor or starter, or in its associated cables must be detected quickly and the circuit isolated automatically. This is achieved by using fuses or circuit breakers at the supply point to the motor starter.
Where fuses are utilised they must be of the high rupturing capacity (HRC) type their short circuit rating must be equal to or greater than the short circuit rating of the system at their point of use within the system. Conversely the short circuit level at the motor terminals must be high enough to ensure their operation if a short circuit occurs at the motor terminals. The selection of fuses is dependant on the motor’s full load current; it’s starting method, run up time and frequency of operation. The contactor main contacts and the cables from the starter to the motor must be able to withstand the fault current for the time it takes the fuse to rupture. It is important that the contactor(s) and overload relay are properly protected by the fuses so they need not be replaced after a short circuit downstream of the starter this is known as (IEC 60947-4-1) type 2 coordination. Contactors and overloads with (IEC 60947-4-1) type 1 coordination have to be replaced after a short circuit.
The pdf version of this article contains a drawing that shows the motor current curve, overload and fuse curves and the short circuit levels at the MCC and motor for star delta starting for a 55kW motor with a run up time of 15seconds located 150metres from the MCC where the fault level is 54.245kA. Using the tables in section 02.09.10.00 entitled motor load data a 55kW motor has a full load current of 103A.
It is a separate file entitled protection curves.pdf with an original paper size of A1 (841mm x 594mm) printing on to an A1 sheet will give a scale of 1:1, on an A2 sheet a scale of 0.707:1, on an A3 sheet a scale of 0.5:1 and on an A4 sheet a scale of 0.35:1. To be able to read the drawing a recommended minimum paper size of A3 should be used, at the stated scaling the text on the drawing has a minimum height of 0.9mm.
Where circuit breakers are used they must have at least a one second short circuit rating equal to or greater than the short circuit rating (Icw) of the system at their point of use. Again the short circuit level at the motor terminals must be high enough to ensure operation if a short circuit occurs at the motor terminals. The instantaneous (Ii) and short circuit (Isd) protection devices must be set to enable the motor to start and run normally but must detect and operate if a short circuit occurs at the motor terminals. The contactor main contacts, overload unit and the cables from the starter to the motor must be able to withstand the fault current for the time it takes the back up circuit breaker to open, with circuit breakers there is always a risk that a circuit breaker will fail to trip, it is therefore prudent to size equipment including cables to withstand the fault level for the time it takes the back-up circuit breaker to trip. Using the back up circuit breaker as the reference for the sizing of the contactors and cables will substantially increase the cost of the contactors and cables, especially if (IEC 60947-4-1) type 2 coordination is required, which it should be, therefore it is preferable and more cost effective to use fuses as the means of short circuit protection for motors. Consequently the detailed discussion within this article on the selection of equipment focuses on fuse protection only.
Earth faults are much more common than phase to phase faults. Any level of current flowing to earth is a fault. An earth fault can have sufficient energy release to cause the ignition of flammable materials in the vicinity of the fault which can give rise to ‘danger’. If fuses are used to protect against earth faults it follows that the earth fault current has to reach a level in excess of the motor’s full load current before they rupture. If circuit breakers are used to protect against earth faults using their instantaneous and short circuit protection it again follows that the earth fault current has to reach a level in excess of the motor’s full load current before they trip. To minimise the risk of danger, motors should be protected against earth faults by a separate earth fault relay (EFR) which has variable current and trip time settings. The settings should allow for the imbalances and transients that occur when a motor stops and starts and for the leakage that is always present due to capacitive and inductive coupling. The settings should also be selected to ensure compliance with the maximum permitted earth fault current and disconnect times required by the IET Wiring Regulations.
A complete power supply failure will, of course, cause the motor to stop; the control system must recognise this and prevent the motor restarting if the power returns. If the power to a site fails, and then the motors’ control systems allow all the motors to start simultaneously on the restoration of power there is a good chance that the supply will trip on overload it will almost certainly breach the 1% volt drop limit at the point of common coupling with the PES. The starting of a motor should be under proper control and must not occur solely on the restoration of a power supply. A more serious condition is either the loss of one of the phases or an excessive imbalance of the phase voltages this can cause serious overheating of a motor. BS EN 60034-26 gives details of the permitted current imbalances for specified voltage imbalances. A supply failure relay (SFR) should be used to protect against all of these conditions. The settings should allow for the maximum permissive volt drop of the supply and for phase voltage imbalance.
The overload of a motor is where the driven load exceeds the output rating of a motor, where this occurs the motor slows down and the current increases, the cooling fan where it is attached to the motor drive shaft also slows down thereby decreasing the cooling air flowing across the motor. The increase of the motor current and where the cooling fan is driven by the motor the lowering of the rate of cooling can lead to excessive heating of the motor which can in turn lead to winding damage and possibly fire. The fuses or circuit breakers protecting the motor would not normally detect this condition which is why specialist overload relays are fitted within the starter.
At their simplest an overload relay (OLR) is a bi-metal strip heated by the current flowing in the motor supply cables, when the current exceeds a predetermined amount for a set time the overload trips and cuts the electricity supply to the motor. The tripping current and time are determined according to the time/current curve of the relay, which is expressed in the multiples of the setting current against time.
The modern equivalent of the bi-metal strip is the electronic overload relay which accurately measures the motor current, that current is then used in the mathematical modelling of the motor to accurately maintain the time/current status of the motor thermal capacity utilisation value. The electronic overload relay usually has a wider current setting range and a larger trip class setting range when compared to the bi-metal strip and other mechanical devices.
The overload’s trip class is the trip time at six times the current setting when the motor is cold, thus at class 10 the trip time at six times the current setting is ten seconds. The overload units must when in combination with the contactors and fuses be rated for IEC 60947-4-1 type 2 coordination.
For multi-speed motors overload units for each speed will be required. The overload unit for each speed will need to be rated for the motor class and full load current for that speed.
Larger motors, that is, those with outputs greater than 45kWo should be additionally equipped with motor over-temperature detection using positive temperature co-efficient resistors (PTCR usually called thermistors) embedded in the motor windings. The thermistors must be field replaceable.
Stall and jam protection should be considered for motors with outputs greater than 250kWo or for motors that operate on voltages above low voltage.
Differential protection should be considered for motors with outputs greater than 500kWo or for motors that operate on voltages above low voltage.
Where motor starters have a sequential starting system, that is, star delta, Korndorffer and rotor resistance a starting time check timer should be incorporated to ensure that the starting sequence completes within a preset time. The failure to complete the starting sequence within a preset time should stop the motor.