IP, Motor Rating & Starting

Part 3

Ing Louis Aquilina

IP Explanation and Ratings EN 60529 outlines an international classification system for the sealing effectiveness of enclosures of electrical equipment against the intrusion into the equipment of foreign bodies (i.e. tools, dust, fingers) and moisture. This classification system utilizes the letters "IP" ("Ingress Protection") followed by two or three digits. (A third digit is sometimes used. An "x" is used for one of the digits if there is only one class of protection; i.e. IPX4 which addresses moisture resistance only.)

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0 No special protection

1 Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter.

2 Protection against fingers or other object not greater than 80mm in length and 12mm in diameter.

3 Protection from entry by tools, wires, etc., with a diameter of thickness greater than 1.0mm.

4 Protection from entry by solid objects with a diameter or thickness greater than 1.0mm

5 Protection from the amount of dust that would interfere with the operation of the equipment.

6 Dust tight.

Degrees of Protection - First Digit

The first digit of the IP code indicates the degree that persons are protected

against contact with moving parts (other than smooth rotating shafts, etc.) and

the degree that equipment is protected against solid foreign bodies intruding

into an enclosure.


0 No special protection

1 Protection from dripping water.

2 Protection from vertically dripping water.

3 Protection from sprayed water.

4 Protection from splashed water.

5 Protection from water projected from a nozzle

6 Protection against heavy seas, or powerful jets of water.

7 Protection against immersion.

8 Protection against complete, continuous submersion in water.

Degrees of Protection - Second Digit

The second digit indicates the degree of protection of the equipment inside the

enclosure against the harmful entry of various forms of moisture (e.g. dripping,

spraying, submersion, etc.). Submersion depth and time must be specified by

the end-user. The requirement must be more onerous than IP67


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Motor Starting

On starting an induction motor, a very large current flows in the stator (typically 5 8 times the full-load current FLC). The starting current surge reduces as the motor accelerates up to its running speed. The corresponding supply power factor at start-up is very low, typically at 0.2 lagging increasing to 0.5 on no load and maybe 0.85 at full load. Considerable resistive ( copper) losses occur. The only way to improve the situation is to reduce the supply voltage (soft starting) or...

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Soft Starting

Induction motors usually are Direct-On-Line (DOL) switch started. Inexpensive

Simple to operate and maintain

But When very large motors are started DOL, they

cause a large disturbance of voltage (voltage dip) on the supply lines that may result in malfunction of other electrical equipment.

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DOL Starting

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In the case of large Induction motors, the starting method can be either of: Star-Delta Starting

Autotransformer starting

Both are reduced voltage switch-starting methods to limit starting surge currents.

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Star-Delta Starting

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The current to the starting motor is lower due to the motor coils being in series across the line creating a higher impedance. When the starter connects to delta connection the motor is already turning and it does not have to go through the locked rotor phase.

To start a motor direct on line (DOL) in delta requires approximately 6 to 8 times the full load current of the motor and delivers full torque. Delta places 415v across each of the windings on the motor. A Star-Delta starter starts the motor in Star which places 240v across each winding on the motor before switching to Delta. This results in a lower starting current and also a lower starting torque.

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If a motor were simply started when connected in DELTA, the starting current would be huge and, just to be able to start the motor, not to run it normally, would require: large circuit breakers - big enough to allow the start-up surge current to pass without immediately shutting it off. (But the breakers would then be much too big to be able to protect the motor from over-current faults whilst it is running normally.) very thick 3-phase power service cables. (But the cable would then be much bigger than is necessary whilst the motor is running normally.) very large coils and contacts on the relays or contactors used to control the motor. (But they would then be much bigger than is necessary whilst the motor is running normally.)

One solution to this problem is to start the motor in STAR and then, when the motor has gained sufficient speed, change its connections to DELTA to allow the motor to run at its full speed and torque from then on.

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Technical explanation When the windings of a 3-phase motor are connected in STAR: - the voltage applied to each winding is reduced to only (1/3) of the voltage applied to the winding when it is connected directly across two incoming power service line phases in DELTA. - the current per winding is reduced to only (1/3) [1 divided by root three] of the normal running current taken when it is connected in DELTA. so, because of the Power Law V [in volts] x I [in amps] = P [in watts], the total output power when the motor is connected in STAR is: PS = [VL x (1/3)] x [ID x (1/3)] = PD x (1/3) [one third of the power in DELTA] where: VL is the line-to-line voltage of the incoming 3-phase power service ID is the line current drawn in DELTA PS is the total power the motor can produce when running in STAR PD is the total power it can produce when running in DELTA. a further disadvantage when the motor is connected in STAR is that the total output torque is only 1/3 of the total torque it can produce when running in DELTA.

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Autotransformer Starting

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KM2 KM1

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There are basically three types of overcurrent (OC) relays:

Electronic

Thermal

Electromagnetic

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Electronic OCIT (overcurrent inverse time) have superseded electromagnetic types.

They have no moving parts except for their output trip relay

They have very reliable trip-characteristics

They are robust, smaller and lighter

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Thermal overcurrent relays are:

Less expensive

Work with bimetallic strips

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The minimum tripping current of a thermal OCR can be adjusted over a small range.

The adjustment alters the distance the strips have to bend before operating the trip contact.

For larger motors the heaters do not carry the full load current (FLC). They are supplied from current transformers (CTs) which step down the motor current so that smaller heater components may be used.

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To operate correctly, induction motors must be connected to a 3 phase ac supply.

Once started they may continue to run even if one of the 3 supply lines becomes disconnected.

This is called single phasing and can result in motor burn-out.

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Figure 4.23

Single phasing can happen when: One of the fuses blows up

One supply phase goes open-circuit

One of the contactor contacts goes open-circuit

The effect of single phasing is to increase the current in the two remaining lines and cause the motor to become very noisy due to uneven torque produced in the rotor.

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An increase in line current due to single phasing will be detected by the protective OCR.

The 3 thermal elements of an OCR are arranged in such a way that unequal heating of the bi-metal strips causes a differential movement which operates the OCR switch contacts to trip out the motor contactor.

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For large HV machines a separate device, called a negative phase sequence (NS) relay, is used to measure the amount of unbalance in the motor currents.

For star connected motor windings, the phase and line currents are equal so the line connected OCR is correctly sensing the winding current.

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With a delta connected motor, the situation is more complex as line currents divides phasorally between the two phases of the motor :

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Single phasing causes a large unbalance in a delta connected motor.

In this example IC is considerably higher than that in the other 2 windings.

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When the motor is at 60% of full load and single phasing occurs, the line currents are at 102% but the current in winding C is at 131%. The line current at 102% will probably not activate the OCR and the motor remains connected. Winding C will overheat and get damaged quickly.

Motors can be protected against this condition by using a differential type relay which trips out with unbalanced currents.

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If single phasing occurs when the motor is on light load, the motor keeps running unless protection trips the contactor.

If the motor is stopped it will not restart. When the contactor is closed, the motor will take a large starting current but develop no rotating torque. This can result in overheating and damage.

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Worse still, if the operator makes several attempts to restart the motor, it will burn out.

If the motor fails to start on 2 attempts, the cause must be investigated.

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Undervoltage protection is necessary in a distribution system that supplies motors.

If there is a total voltage loss (black-out) all the motors must be disconnected from the supply. This is to prevent the motors restarting together causing a huge current surge. Motors must be restarted in a controlled sequence.

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Undervoltage (UV) protection for LV motors is simply provided by the spring-loaded motor contactor because it will drop out when supply voltage is lost.

For large HV motor dedicated protection components are used for this purpose.

For essential loads, the restart may be performed automatically by a sequence restart system.

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The maintenance requirements for induction motors to ensure a trouble-free life for the motor are very simple:

Keep insulation resistance high and contact resistance low

Lubricate correctly and maintain a uniform gap

Ensure both the interior and exterior are always clean and dry

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Requirements:

General cleanliness

Clean external surfaces regularly

Use dry air at 1.75bar max to clean

Vacuum cleaning better than airblow cleaning

Use anti-condensation heaters especially if motors have a small duty-cycle

Check for contamination from oil or grease

Check bearing covers

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Check for stator damage caused by careless replacement of the rotor into the stator

Check for discoloured insulation indicating hot spots

Check the stator core for signs of rubbing with the rotor caused by worn bearings

Use insulation testers to check for the presence of moisture in the motor windings.

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Check the condition of bearings regularly

Keep clean and properly lubricated

Since bearing failure can be hard to predict, the best policy is to renew the bearings as part of the planned maintenance

Yet, bearings should be removed from the shaft as seldom as possible

The shafts of stationary motors should be periodically rotated a quarter of a turn to minimise vibration damage to the bearings.

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The motor starter and other control equipment should be regularly inspected to check and maintain the following items:

Enclosure

Dirt, rust, corrosion, vibration

Contactors and relays

Overheating, loose connections

Dirt, oil,

Excessive pitting (never file silver contacts)

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Connections

Check for tightness and signs of overheating

Overcurrent relays

Check for proper settings wrt motor FLC

Inspect for dirt, grease and corrosion

If possible perform an OCR performance test using calibrated current injection

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Observe the sequence of operation during a normal start-up, running, and shut-down of a motor. Look out for excessive contactor sparking (if possible).

Finally check the operation of the emergency stop and auto-restart functions.

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Documents

IP, Motor Rating & Starting