How to Troubleshoot 3 Phase AC Motors.

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Three phase induction motors are one of the most popular electric motors commonly found in processing plants or any manufacturing concern. They are used in situations where large power is required. The squirrel cage brand is the most popular and they perform various tasks wherever they are applied.

Because of the critical roles these motors play in any plant, a failure of the motor, inability to start, noisy operation and sundry other problems need to be remedied as soon as possible to avoid costly production downtime. The table below gives the commonly encountered problems in 3 phase squirrel cage induction motors, cause of problems and what remedy to apply to bring the motor back to production. This troubleshooting guide can also be applied to other types of three phase induction motors:

Motor Problem	Cause	Remedy
Motor fails to start	Blown fuses	Replace fuse with proper type and rating
	Overload Trips	Check and reset overload in starter
	Improper power supply	Check to see that power supplied agrees with nameplate specifications  and load factor
	Improper line connections	Check connections with wiring diagram supplied with motor
	Open circuit in winding or control switch	This is normally indicated by a humming sound when switch is closed. Check for loose wiring connections. Confirm that all control contacts are closing.
	Mechanical failure	Check to see that motor and drive turns freely. Check bearings and lubrication
	Short circuited stator	Indicated by blown fuses.  Motor must be rewound
	Poor stator coil connections	Remove end belts. Locate poor connections with test lamp.
	Rotor defective	Check for broken bars or end rings
	Motor may be overloaded	Reduce motor load
Motor stalls	One phase may be open	Check supply lines for open phase
	Wrong application	Change type or size. Consult motor manufacturer
	Overload	Reduce load
	Low voltage	Check that nameplate voltage is maintained. Check connection.
	Open circuit	Fuses blown. Check overload relay, stator and push buttons
Motor runs and then dies down	Power failure	Check for loose connections to line, to fuses and to control
Motor does not come up to speed	Motor is applied for the wrong application	Consult manufacturer for right application of motor
	Voltage too low at motor terminals because of line drop	Use higher voltage on transformer terminals or reduce load. Check connections. Check conductors for proper size.
	Starting load too high	Check load motor is supposed to carry at start.
	Broken rotor bars or loose rotor	Look for cracks near the rings. A new rotor may be required as repairs are usually temporary not permanent
	Open primary circuit	Locate fault with testing device and repair.
Motor takes too long to accelerate and/or draws high current (Amps)	Excessive load	Reduce load
	Low voltage during start up	Check for high resistance. Adequate wire size.
	Defective squirrel cage rotor	Replace with new rotor
	Applied voltage too low	Improve voltage at terminals of transformer by tap changing.
Wrong rotation	Wrong sequence of phases	Reverse connections at motor or at switchboard.
Motor overheats while running under load	Overload	Reduce load
	Frame or bracket vents may be clogged with dirt and prevent proper ventilation of motor.	Open vent holes and check for a continuous stream of air from the motor.
	Motor may have one phase open	Check to make sure that all leads are well connected.
	Grounded coil	Locate and repair
	Unbalanced terminal voltage	Check for faulty leads, connections and transformers.
Motor vibrates	Motor  misaligned	Realign
	Weak support	Strengthen base
	Coupling out of balance	Balance coupling
	Driven equipment unbalanced	Re-balance driven equipment
	Defective bearings	Replace bearing
	Bearings not in line	Line bearings up properly
	Balancing weights shifted	Re-balance motor
	Poly-phase motor running single phase	Check for open circuit
	Excessive end play	Adjust bearing
Unbalanced line current on poly-phase motors during normal operation	Unequal terminal volts	Check leads and connections
	Single phase operation	Check for open contacts
	Unbalanced voltage	Correct unbalanced power supply
Noisy Operation	Airgap not uniform	Check and correct bracket fits or bearing.
	Rotor unbalance	Rebalance
Hot bearings general	Bent or sprung shaft	Straighten or replace shaft
	Excessive belt pull	Decrease belt tension
	Pulley too far away	Move pulley closer to motor bearing
	Pulley diameter too small	Use larger pulleys
	Misalignment	Correct by realignment of drive
Hot bearings ball	Insufficient grease	Maintain proper quantity of grease in bearing
	Deterioration of grease or lubricant contaminated	Remove old grease, wash bearings thoroughly in kerosene and replace with new grease.
	Excessive lubricant	Reduce quantity of grease, bearing should not be more than 1/2 filled
	Overloaded bearing	Check alignment, side and end thrust.
	Broken ball or rough races	Replace bearing, first clean housing thoroughly

This troubleshooting guide is by no means exhaustive but can provide guidance for the resolution of commonly encountered problems as far as poly-phase induction motors are concerned.

How to Test a 3 Phase Motor Windings With an Ohmmeter

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Every 3 phase motor has six (6) terminals with the supply voltage connected to three (3) of those terminals. The most common configuration of a three-phase  motor is the Delta (∆) – Star (Wye) configuration with the Delta side connected to supply voltage. The terminal configuration of a 3 phase motor is shown below:

Terminals Configuration of a 3 Phase Motor

The W2U2V2 terminal set is the star side of the 3 phase motor while the U1VIW1 is the Delta side of the motor connected to the supply voltage.

The 3 phase motor is a rugged piece of equipment but as with everything man made, there comes a time when this beautiful piece of machinery fails either due to old age, misapplication, mal-operation or any other adverse cause.

The most common failure mode of a 3 phase AC motor is burnt winding or shorted winding leading to the damage of the motor. Often it is required to test the winding of the 3 phase windings with the aid of a multimeter or ohmmeter to determine whether the motor is still good or burnt or shorted.

How to Test the Winding of a 3 phase Motor

To determine whether a 3 phase motor is still good or has gone bad, a simple ohmmeter test across the windings of the motor will reveal its true state of health. As shown below, the indicated terminal matrix (blue lines) shows the way the windings of a 3 phase motor should be tested with an Ohmmeter:

How to test the windings of a 3 Phase Motor with an Ohmmeter

The first thing to do before testing the windings of the motor is to remove the links linking terminals W2U2V2 and the disconnect the motor from supply (L1, L2, L3). A multimeter terminals placed across this matrix of terminals will indicate the following readings for a good 3 phase motor:

(a) Terminals W1W2, U1U2, V1V2 will indicate continuity for a good motor

(b) Every other terminal combinations should indicate Open for a good motor

(c) Readings between any of the six (6) terminals and the motor frame signifying earth     

    (E) should indicate open for a good motor.

Ohmmeter Readings for a Bad 3 phase Motor

In the case of a burnt or bad 3 phase motor, this matrix of terminals should indicate  the opposite readings for a bad motor:

(a) If any of the terminal combinations W1W2, U1U2, V1V2 should indicate open then 

     the motor is bad.

(b) If any other terminal combinations should indicate continuity instead of open, then            

     the motor is bad.

(c) If the reading between any of the six (6) terminals and motor frame (E) should  

     indicate continuity, then the motor is dead.

Electric Motor Specifications for Hazardous Locations

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A hazardous environment or location is some combustible materials that could easily ignite and create a fire hazard. Electric motors operated in such an environment need to be carefully specified and selected in order not to pose an explosion threat to an installation.

Arcs, sparks and high temperatures on the surfaces of electric motors are common sources of ignition of hazardous substances. Therefore the right motors need to be specified and selected for use in this kind of environments otherwise the consequences of an explosion and eventual fire resulting from using the wrong kind of motor could range from minor injuries, production down time and destruction of valuable investment as well as death.

Information Required to Specify an Electric Motor for Use in Hazardous Locations

Four basic information are required to specify and select the right motor for use in a hazardous environment:

1. Class of materials in the hazardous environment

2. Division classification of the hazardous environment

3. Group classification of the hazardous environment

4.     The autoignition temperature (AIT) of the hazardous material in the environment

How to Select Electric Motors for Use in Hazardous Locations

Class of Materials in the Hazardous Environment.

There are three classes of hazardous locations according to the NEC and CEC:

Class Location	Hazardous Materials Present
I	Contains flammable gases or vapors e.g Hydrocarbon gases
II	Contains dusts either electrically conductive or explosive in air
III	Contains filings and flyings that are easily ignitable e.g texile industry

*NEC -  National Electric Code

*CEC -  Canadian Electrical Code

Division of the Hazardous Environment

Division describes the condition under which the hazardous material is present in the particular environment. There are two basic divisions: Division 1 and Division 2 locations

Division 1 Location
A Division 1 location is one where an explosive or ignitable material is present under normal operating condition. Division 1 locations include environment where explosive materials are routinely exposed to the environment during regular operation and scheduled maintenance.

Division 2 Location
A Division 2 location is one where hazardous substances are stored or handled under abnormal conditions.

Group Classification of Hazardous Environment.

Class I and Class II are divided into groups according to the behaviour of the hazardous material after it has been ignited. These groupings are shown below:

Class Location	Groups
I	A, B, C, D
II	E, F, G

Autoignition Temperature of Hazardous Materials

A key information required to accurately specify an electric motor in a hazardous environment is the autoignition temperature of the hazardous material, AIT for short.

The AIT refers to the minimum temperature at which there is sufficient energy for a hazardous material (chemical) to ignite spontaneously in the absence of a spark, flame or other source of ignition. The AITs of various Class I and Class II hazardous materials are shown in the table below:

*Class	Group	Hazardous Material	Autoignition Temperature
			°C	°F
I	A	Acetylene	305	581
	B	Butadiene	420	788
		Ethlylene Oxide	570	1,058
		Hydrogen	500	932
	C	Acetaldehyde	175	347
		Cyclopropane	498	928
		Diethyl Ether	180	356
		Ethylene	450	842
		Isoprene	398	743
	D	Acetone	465	869
		Amonia	651	1,204
		Benzene	498	928
		Butane	287	550
		Ethane	472	882
		Ethanol	363	685
		Gasoline	246 - 280	475 - 536
		Methane	537	999
		Propane	450	842
		Styrene	490	914
II	E	Aluminium	650	1,202
		Bronze	370	698
		Chromium	580	1,078
		Magnesium	620	1,148
		Titanium	330	626
		Zinc	630	1,166
	F	Coal	610	1,130
	G	Corn	400	752
		Nylon	500	932
		Polyethylene	450	842
		Sugar	350	662
		Wheat	480	896
		Wheat Flower	380	716

*Source : American Institute of Chemical Engineers (AIChE) - www.aiche.org

Characteristics of Motors in Class I, Division 1 & 2 Hazardous Locations

To be able to apply an electric motor successfully in a hazardous environment, these motors must possess certain critical characteristics that make them suitable to operate in these environment without creating problems.

1. These motors must be built and labelled as explosion-proof
2. An explosion-proof motor must contain an internal explosion without rupturing
3. An explosion-proof motor must have flame paths for exhausting hazardous gases during an explosion and for cooling the hazardous material as they leave the motor to prevent further explosion.
4. The more severe the explosion hazards, the stronger the enclosures of the motor and the longer the flame paths. For example, motors for use in group A environment which has acetylene gas require the highest enclosure strength and longest flame paths compared with those used in group D environment with only propane gas.
5. Motors for use in hazardous environment are assigned a temperature code (T-Code), an identification number which describes the maximum temperature of surfaces subject to contact with hazardous materials. The temperature value defined by the T-code applies under all conditions of motor operation including burnt out, overload and locked rotor current.
6. The T-code for a given motor must be less than the AIT of the hazardous gas or mixture in the environment where the motor operates. This is to ensure that the hazardous materials do not spontaneously ignite when it contacts the motor surfaces and enclosure during operation.

Temperature Codes of Motors Operating in Hazardous Locations According to NEC, CEC & IEC Designations:

The table below gives the temperature codes for motors operating in hazardous environment according to NEC/CEC  & IEC letter designations are given in the table below:

**NEC/CEC Designation T - Code	IEC Designation T - Code	Maximum Surface Temperature
		°C	°F
T1	T1	450	842
T2	T2	300	572
T2A		280	536
T2B		260	500
T2C		230	446
T2D		215	419
T3		200	392
T3A	T3	180	356
T3B		165	329
T3C		160	320
T4	T4	135	275
T4A		120	248
T5	T5	100	212
T6	T6	85	185

   
 **Source : American Institute of Chemical Engineers (AIChE) - www.aiche.org

Class I, Division 2 Motors

The minimum requirements for motors for use in Class I, Division 2 locations are less stringent than those for use in Class I, Division 1 locations. Some basic characteristics and requirement for motors for use in Class I, Division 2 locations are enumerated below:

1. An explosion-proof motor certified for use in a Class I, Division 1 location may also be operated in a Class I, Division 2 area provided the motor meets the group and T-Code requirements.
2. TEFC (Totally Enclosed, Fan Cooled) motors and ODP (Open Drip -proof) motors can be used in Division 2 locations provided they do not have ignition sources such as arc-producing brushes or switching mechanisms.
3. Three-phase induction motors with low surface temperatures and no sparking parts can be used in Division 2 locations.

Characteristics of Motors for use in Class II locations

1. As class II locations contain ignitable dusts, electric motors operating in this kind of environment must be dust-ignition-proof.
2. The enclosures of dust-ignition-proof motors are designed to exclude hazardous materials from accessing the internals of the motors unlike explosion-proof motors.
3. The T-code of a dust-ignition-proof motor must correspond to a maximum surface temperature below the AIT of the hazardous dust materials.

Nameplate Requirements for Motors Operating in Hazardous Locations

Selecting the right kind of motor that meets the requirements for use in hazardous environment is not an easy task. However, manufacturers of electric motors for use in hazardous locations have standard nameplates that contain the following information:

(a) Type of Enclosure of Motor

(b) Class of hazardous materials for which the motor is suitable

(c) Group of the hazardous materials 

(d) T-code of the electric motor

The above information makes it less tedious in selecting and specifying a motor for use in a hazardous environment.

How to Size a Portable Generator for Home Use

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Portable Generators are a reliable source of power in the absence of utility power. They provide electrical power to supply our critical power needs when the utility company is unable to supply us electrical power due to fault in their transmission system or during maintenance interventions in their power infrastructure or in the worst case of a natural disaster such as earthquake or a hurricane that has destroyed section of the power grid.

What Size of Portable Generator Do I need ?

The size of portable generator you need depends on your power requirement when the need arises to use the generator. Do you require the generator to power all of your electrical appliances at once? Or do you require the generator to power some critical electrical load during power outage? The bigger your power needs, the bigger the size of your generator and the more expensive your portable generator will be!

Running Watts of an Electrical Appliance

The running watts of an electrical appliance is the power it can draw continuously with rated voltage and current. It is usually calculated as:

Running Watts = Rated Voltage x Rated Current.

Note that the above formula will give power in volts-amps or VA but assuming a power factor = 1 which is rarely the case, we get power in watts. This approximation is done to enable easy sizing of a portable generator for home use.

The running watt can easily be calculated by using the rated voltage and current on the name plate of the appliance. Generators are also rated for their running watts. It is the power the generator can deliver continuously at rated voltage, current and frequency. A generator must not be made to continuously carry load beyond its running watts for a very long time otherwise the generator’s life will be shortened and the device becomes damaged in a short time.

Surge Watts or Start up Power of an Electrical Appliance

Certain devices and appliances have an electric motor or compressor in them. They require additional watts to start them. This additional watt may also be referred to as the surge watt of the device. The surge watts required by these devices may sometimes be two or three times the watts required to run the device. Heat producing devices also called resistive loads such as light bulbs, toasters or coffee makers do not require surge watts at start up. A generator must have enough surge watts capacity to handle devices that require surge watts at start up to prevent a nuisance tripping of the main power breaker in the generator.

As shown above, a generator must have sufficient surge capacity to carry loads requiring additional power during start up. Consider a refrigerator that works for one third of the time within a given time cycle. Each time the refrigerator compressor starts, a generator powering the refrigerator must have sufficient surge power for the compressor each time it comes on!

How to Calculate the Size of Portable Generator Required

To properly size a generator, care should be taken to analyse the load the generator is to power so that both running watts and surge watts can be correctly calculated. To calculate the size of generator:

Add up the total running and surge watts for each appliance. Multiply the total sum gotten by a contingency of 15 – 20 % to get the capacity of your generator.

As a guide during the sizing calculation for domestic application;

Surge watts for refrigerators and air conditioners = 2 x running watts 

Surge watts for motors (surface or submersible pumps) = 3 x running watts

Microwave Oven = 1.5 x running watts

Sample Sizing Calculation

Suppose the following loads are to be powered by a portable generator:

Electrical Load	Number	Running Watts (W)
Refrigerator	1	800
Submersible pump	1	800
Lighting loads	lot	150
Air Conditioner (1hp)	1	800
Deep Freezer	1	500
Microwave	1	600
Computer	1	300
TV	1	400

Determine power rating for generator  as shown below:

Electrical Load	Number	Running Watts (W)	Surge Watts (W)
Refrigerator	1	800	2 x 800 = 1,600
Submersible pump	1	800	3 x 800  = 2,400
Lighting loads	lot	150	0
Air Conditioner (1hp)	1	800	2 x 800 = 1,600
Deep Freezer	1	500	2 x 500 = 1,000
Microwave	1	800	1.5 x 800 = 1200
Computer	1	300	0
TV	1	400	0
Total		4,350	7,800
Total Power Required	= 4,350 + 7,800 = 12,150W
Add 15% contingency	= 12,150 x 1.15 =13,972.5W
Size of Generator needed			= 15,000W or 15KVA   standard Size

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How to Calculate Inverter Power Rating and Inverter Battery Backup Time

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Inverter systems are a common feature in our homes and workplace where they play a prominent role in the ensuring uninterruptible power to sensitive loads and devices. For home applications, there is the need to adequately size your inverter to be able to meet the expected load demand. 

Inverters convert DC voltage to AC voltage. They have a battery system which provide adequate backup time to provide continuous power in the home. The inverter system then converts the battery voltage to AC voltage through electronic circuitry. The inverter system also has some charging system that charges the battery during utility power. During utility power, the battery of the inverter is charged and at the same time power is supplied to the loads in the house. When utility power fails, the battery system begins to supply power via the inverter to the loads in the home as shown below:

How to Size and Calculate the Inverter Power Requirement

Inverter power is rated in VA or KVA.

Power in VA = AC Voltage x AC Current in Amps

Power in KVA = AC Voltage x AC Current in Amps/1000

Power in Watts = AC Voltage x AC Current in Amps x PF

Where PF = power factor 

Power in KW = AC Voltage x AC Current in Amps x PF/1000

Also  Power in W = Power in VA x PF

         Power in KW = Power in KVA x PF

Suppose we want to size an inverter to carry the following loads:

1. Lighting load, 300W

2. 3 Standing fans of 70W, each

3. 2 LCD TV, 100W

4. 1 Home Theatre Music System, 200W

5. 1 Juice extractor, 150W

Applying Power in KW = Power in KVA x PF

Power in KVA  = Power in KW/PF = Power in KW/0.8    (Nominal PF = 0.8, which is standard for homes)

Total load in Watts = 300 + (3 x 70) + 200 + 200 + 150 = 1060W = 1.06KW

Power in KVA = 1.06/0.8 = 1.325

An inverter of standard rating 1.5KVA is required to carry the loads above.

How to Calculate Inverter Battery Backup Time

The backup time for batteries in an inverter system depends on the number of batteries as well as their capacity in Amp-hours.

Inverter battery backup time is calculated as:

Back up time = Battery Power in Watt hour (Wh)/Connected Load in Watts (W)

Battery Power in Wh = Battery Capacity in AH x Battery Voltage (V) x Number of Batteries

Let us shorten the formula by using the following Symbols:

Let BUT = battery backup time in hours

            C = battery capacity in AH 

    V = battery voltage in volts

    N = Number of batteries in series or parallel as the case may be.

    $P_L$ = connected load in watts (W)

Now

$$BUT = {\frac{C*V*N}{ P_L}}$$

In our example above, suppose we have selected a 24V, 1.5KVA inverter system that is to use two 12V batteries in series connection and suppose further that the capacity of our batteries are 200AH each, then :

C = 200AH

V = 12V

N = 2

$P_L$   = 1,060W

$$BUT = {\frac{200 * 12 * 2}{1060}} = 4.53 hrs$$

How UPS (Uninterruptible Power Supply) Systems Works

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UPS stands for Uninterruptible Power Supply. A UPS system is an autonomous source of alternate power that is used to supply sensitive electronic loads such as computer centers, telephone exchanges and many industrial-process control and monitoring systems. These applications require power that is availability and of good quality.

A UPS solution for sensitive electrical loads is used to provide a power interface between the utility and the sensitive loads, providing voltage that is:

1. Free of all disturbances present in utility power and in compliance with the strict

        tolerances required by loads.

2. Available in the event of a utility outage, within specified tolerances

UPS systems satisfy requirements in 1 & 2 above in terms of power availability and quality by:

1. Supplying loads with voltage complying with strict tolerances, through use of an

        inverter

2. Providing an autonomous alternate source, through use of a battery

3. Stepping in to replace utility power with no transfer time, i.e. without any interruption in the               supply of power to the load, through use of a static switch.

These characteristics make UPS units the ideal power supply for all sensitive applications because they ensure power quality and availability, whatever the state of utility power.

Basic Parts of a UPS System

A UPS comprises the following main components:

1. Rectifier/charger, which produces DC power to charge a battery and supply an inverter

2. Inverter, which produces quality electrical power free of all utility-power disturbances, notably           micro-outages and that is within tolerances compatible with the requirements of sensitive                     electronic devices.

3. Battery, which provides sufficient backup time to ensure the safety of life and property by                   replacing the utility as required

4. Static switch, a semi-conductor based device which transfers the load from the

        inverter to the utility and back, without any interruption in the supply of power

Types of Static UPS Systems

Types of static UPSs are defined by standard IEC 62040. The standard distinguishes three operating modes for UPSs which are:

1. Passive standby (also called off-line)

2. Line interactive

3. Double conversion (also called on-line)

These definitions concern UPS operation with respect to the power source including the distribution system upstream of the UPS. IEC Standard 62040 defines the following terms:

a. Primary power: power normally continuously available which is usually supplied by

        an electrical utility company, but sometimes by the user’s own generation

b. Standby power: power intended to replace the primary power in the event of

        primary-power failure

c. Bypass power: power supplied via the bypass

UPS Operating in Passive Standby Mode

Operating Principle:

The inverter is connected in parallel with the AC input in a standby as shown below:

UPS in Passive Standby Mode. Photo Credit: Schneider Electric

Normal Mode Operation

In normal mode operation, the load is supplied by utility power via a filter which eliminates certain disturbances and provides some degree of voltage regulation (IEC 62040 specifies some form of power conditioning). The inverter operates in passive standby mode.

Battery Backup Mode Operation

In battery backup mode operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a very short less than 10 ms transfer time. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Application

This configuration is a compromise between an acceptable level of protection against disturbances and cost. It can be used only with low power ratings less than 2 kVA.

Limitations

This UPS operates without a real static switch, so a certain time is required to transfer the load to the inverter. This time is acceptable for certain individual applications, but

incompatible with the performance required by more sophisticated, sensitive systems

(large computer centers, telephone exchanges, etc.). Furthermore, the frequency is not regulated and there is no bypass.

UPS Operating in Line-interactive Mode

The inverter is connected in parallel with the AC input in a standby configuration, but also charges the battery. It thus interacts with the AC input source as shown below:

UPS in Line-interactive Mode. Photo Credit: Schneider Electric

Normal Mode Operation

In normal mode operation, the load is supplied with conditioned power via a parallel connection of the AC input and the inverter. The inverter operates to provide output-voltage conditioning and/or charge the battery. The output frequency depends on the AC-input frequency.

Battery Backup Mode Operation

In this mode of operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch which also disconnects the AC input to prevent power from the inverter from flowing upstream. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation

This type of UPS may be equipped with a bypass. In the bypass mode, If one of the UPS functions fails, the load can be transferred to the bypass AC input (supplied with utility or standby power, depending on the installation).

Application and Limitation

This UPS configuration is not well suited to regulation of sensitive loads in the medium to high-power range because frequency regulation is not possible. For this reason, it is rarely used other than for low power ratings.

UPS Operating in Double Conversion (On-line) Mode

Operating Principle:

In this type of UPS, the inverter is connected in series between the AC input and the application as shown below:

UPS in Double-Conversion Mode. Photo Credit: Schneider Electric

Normal Mode Operation

During normal operation, all the power supplied to the load passes through the rectifier/charger and inverter which together perform a double conversion (AC to DC to AC), hence the name.

Battery Backup Mode Operation

In battery backup mode, When the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch. The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation

This type of UPS is generally equipped with a static bypass, sometimes referred to as a static switch. The load can be transferred without interruption to the bypass AC input (supplied with utility or standby power, depending on the installation), in the event of UPS failure, load current transient (inrush or fault currents) or load peaks. The presence of a bypass assumes that the input and output frequencies are identical and if the voltage levels are not the same, a bypass transformer is required.

For certain types of load, the UPS must be synchronized with the bypass power to ensure load-supply continuity. Furthermore, when the UPS is in bypass mode, a disturbance on the AC input source may be transmitted directly to the load because the inverter no longer steps in. Another bypass line, often called the maintenance bypass, is available for maintenance purposes. It is closed by a manual switch.

Common Causes of Battery Failures

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All batteries have a limited life span. However the life span can be considerably shortened by certain factors which tend to cause premature battery failure. The factors discussed below are some of the most common causes of battery failure. Given the roles batteries play and will continue to play in our everyday life, a thorough understanding of these factors will enable engineers and technicians involved in the maintenance of batteries prevent the occurrence of some of these factors in order to prolong battery life.

The factors discussed below are mostly applicable to VRLA (Valve Regulated Lead Acid) batteries which have almost completely replaced conventional lead acid batteries with refillable liquid sulphuric acid electrolyte. VRLA batteries are designed to be maintenance free and the hydrogen that is emitted is recombined internally so that the electrolyte (essentially a paste) does not need replacing over the life of the battery, a valve is installed to release any excess pressure that may build up if the battery were failing. A key drawback of VRLA batteries is the short service life which was procured at the expense of low maintenance.

Elevated Temperatures

Anticipated battery life is specified by the manufacturer for batteries installed in an environment at or near the reference temperature of 25°C (77°F). Above this temperature, battery life is reduced. The chief aging mechanism is accelerated corrosion of the positive plates, grid structure, and strap, which increases exponentially as a function of temperature. Elevated temperatures reduce battery life. An increase of 8.3°C (15°F) can reduce lead-acid battery life by 50% or more. 

Repeated Cycling

Repeated cycling from fully charge to fully discharge and back may cause loss of active materials from the positive plates. This reduces battery capacity and its useful life.

Overcharging

Overcharging by the battery charging system causes excessive gassing and high internal heat. Too much gassing can lead to the removal of active material from the plates. Too much heat can also oxidize the positive plate material and warp the plates.

Undercharging

A faulty charging system will not maintain the battery at full charge. Severe undercharging allows sulfate on the plates to become hard and impossible to remove by normal charging. The undercharged battery may fail to deliver the required power needed for its application.

Over discharge

Over discharge leads to hydration. Hydration occurs in a lead-acid battery that is over discharged and not promptly recharged. Hydration results when the lead and lead compounds of the plates dissolve in the water of a discharged cell and form lead hydrate, which is deposited on the separators. When the cell is recharged, multiple internal short circuits occur between the positive and negative plates. Once hydration is evident, the cell is permanently damaged. Hydration is not visible in VRLA cells because the containers are opaque

Vibration

A battery must be mounted securely. Vibrations can loosen connection, crack the case and damage internal components.

DC Ripple Current

Excessive DC ripple current might contribute to battery aging. VRLA batteries are extremely susceptible to ripple current since it can lead to cell heating and will accelerate the degradation of cells which are at risk from thermal runaway.

Improper Storage

Storing wet cells beyond the manufacturer’s recommended duration promotes sulfation, and decreases cell capacity and life.

Misapplications

Batteries are commonly designed for a specific use. If the battery is not designed for a given application, it might not meet its life or performance expectations.

Understanding Battery Technical Specifications.

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Commonly in a specification sheet for a typical battery, you have all kinds of technical terms that need to be understood so as to be able to use the battery in the right way to get maximum benefit from  the battery in a particular application. Summarized below are some of the key technical terms used in battery specifications:

Nominal Voltage (V)

This is the reference voltage of the battery, also sometimes thought of as the “normal” voltage of the battery.

Cut-off Voltage (V)

This is the minimum allowable voltage of a battery. It is this voltage that generally defines the “empty” state of the battery.

Capacity or Nominal Capacity (AH for a specific C rate)

This is the total Amp-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Capacity is calculated by multiplying the discharge current (in Amps) by the discharge time (in hours) and decreases with increasing C-rate.

State of Charge (% SOC)
SOC is defined as the remaining capacity of a battery and it is affected by its operating conditions such as load current and temperature. It is calculated as:

$$  SOC  = {\frac {Remaining \ Capacity}{Rated \ Capacity}} $$

Depth of Discharge
DOD is used to indicate the percentage of the total battery capacity that has been discharged.

$$ DOD = 1 - SOC $$

Energy or Nominal Energy (Wh for a specific C rate)

This is the “energy capacity” of the battery, the total Watt-hours available when the battery is discharged at a certain discharge current (specified as a C-rate) from 100 percent state-of-charge to the cut-off voltage. Energy is calculated by multiplying the discharge power (in Watts) by the discharge time (in hours). Like capacity, energy decreases with increasing C-rate.
The rated Wh capacity of a battery can be calculated as:

$Rated \ Wh = Rated \ Ah \ Capacity \ * \ Rated \ Battery \ Voltage$

Cycle Life (Number for a specific DOD)

This is the number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the DOD, the lower the cycle life.

Specific Energy (Wh/Kg)

This is the nominal battery energy per unit mass, sometimes referred to as the gravimetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. It is expressed in Watt-hours per kilogram (Wh/kg) as:

$$Specific \ Energy = {\frac {Rated \ Wh \ Capacity}{Battery \ Mass \ in \ Kg}}$$

Specific Power (W/Kg)

This is the maximum available power per unit mass. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery weight required to achieve a given performance target. It is expressed in W/kg as:

$$Specific \ Power = {\frac {Rated \ Peak \ Power}{Battery \ Mass \ in \ Kg}}$$

Peak Power
The peak power of a battery is defined as:
$$P = {\frac{2V_{oc}^2}{9r}}$$
Where:
$V_{oc}$ is the open circuit voltage of battery
$r$ is the internal resistance of the battery

Energy Density (Wh/L)

This is the nominal battery energy per unit volume, sometimes referred to as the volumetric energy density. Specific energy is a characteristic of the battery chemistry and packaging. Along with the energy consumption of the vehicle, it determines the battery size required to achieve a given electric range.

Power Density (W/L)

The maximum available power per unit volume. Specific power is a characteristic of the battery chemistry and packaging. It determines the battery size required to achieve a given performance target.

Maximum Continuous Discharge Current

This is the maximum current at which the battery can be discharged continuously. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity. 

Maximum 30-sec Discharge Pulse Current

This is the maximum current at which the battery can be discharged for pulses of up to 30 seconds. This limit is usually defined by the battery manufacturer in order to prevent excessive discharge rates that would damage the battery or reduce its capacity.

Charge Voltage (V)

This is the voltage that the battery is charged to when charged to full capacity. Charging schemes generally consist of a constant current charging until the battery voltage reaches the charge voltage, then constant voltage charging, allowing the charge current to taper until it is very small.

Float Voltage (V)

This is the voltage at which the battery is maintained after being charge to 100 percent SOC to maintain that capacity by compensating for self-discharge of the battery.

(Recommended) Charge Current

The ideal current at which the battery is initially charged (to roughly 70 percent SOC) under constant charging scheme before transitioning into constant voltage charging.

Internal Resistance (Maximum)

This is the resistance within the battery, generally different for charging and discharging.

Basic Battery Terminology

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There are certain  basic battery terminology that tends to be misunderstood in practice. These terms commonly refers to the condition of the battery as well as capacity of the battery:

State of Charge (SOC) %

SOC defines the present battery capacity as a percentage of maximum capacity. SOC is generally calculated using current integration to determine the change in battery capacity over time.

Depth of Discharge (DOD) %

This expresses the percentage of battery capacity that has been discharged expressed as a percentage of maximum capacity. A discharge to at least 80 % DOD is referred to as a deep discharge.

Terminal Voltage (V)

This is the voltage between the battery terminals with load applied. Terminal voltage varies with SOC and discharge/charge current.

Open Circuit Voltage (V)

This is the voltage between the battery terminals with no load applied. The open-circuit voltage depends on the battery state of charge (SOC), increasing with state of charge.

Internal Resistance

This is the resistance within the battery, generally different for charging and discharging, also dependent on the battery state of charge. As internal resistance increases, the battery efficiency decreases and thermal stability is reduced as more of the charging energy is converted into heat.

Consider the battery circuit above, where E is the open circuit voltage, I is the load current flowing in amps, V is the terminal voltage across a load resistance $R_L$, r is the battery internal resistance

Power  input to battery is given by: $$P_i = IE$$ $$E = I (R+r)$$ $$P_i = I *I(R+r) = I^2(R+r)$$ power output: $$P_o = IV = I*IR = I^2R$$ Battery Efficiency  $$Eff = {\frac{P_o}{P_i}} = {\frac{I^2R}{ I^2(R+r)}} = {\frac{R}{(R+r)}}$$ As can be seen, the more the internal resistance of the battery, the less efficient the battery becomes due to increasing conversion of useful battery energy to heat.

C and E – Rates

In describing batteries, discharge current is often expressed as a C-rate in order to normalize against battery capacity, which is often very different between batteries. A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity.

C – Rate can be expressed as:

$$I = M * C_n $$     

Where:

I  = Discharge current in Amps

C = Numerical value of rated capacity of the battery in ampere – hours (AH)

n = Time in hours for which rated capacity of battery is declared

M = Multiple or fraction of C

Consider a battery rated at 200AH with a discharge current of 10Amps, the C rate is calculated as: 

M = I/Cn  = 10/200 = 1/20 = 0.05C or C/20 rate

E’’ Rate

An E-rate describes the discharge power. A 1E rate is the discharge power to discharge the entire battery in 1 hour. Just like the C rate, the E rate can be expressed as:

$$P = M * E_n $$    

Where:

I  = Discharge current in Amps

E = Numerical value of rated power of the battery in watt – hours (Wh)

n = Time in hours at which the battery was rated

M = Multiple or fraction of C

Consider a battery with rated at 1200mWh with rated power of 600mW, the E rate is calculated as :

M = P/En  = 600mW/1200mW  = 0.5E or E/2 rate