Custom Wound Transformers
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Jemelec offers a complete transformer service, from design, through prototyping to manufacture. Whether you are looking for a single 1 kVA or ten at 100 kVA, Jemelec offers a fast and cost-effective service, with urgent jobs completed in as little as 24 hours. Our standard turnaround is typically 5 - 7 days for small orders.
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How to Specify Your Transformer
Total Load Rating of Transformer
Load ratings for transformers are usually given as VA or kVA, these are obtained by multiplying the load current by the supply voltage: this gives the volt-ampere (VA) rating, where 1000 VA = 1 kVA. This is often loosely referred to as the "power" rating of the transformer. In actual practice the power which the transformer can deliver into a given load also depends on a quantity known as the power factor (p.f.), which is defined as the ratio of real power to apparent power, i.e. the absorbed power divided by the volt-ampere (VA) product. For a purely resisitive load the p.f. is unity and the transformer will be able to supply a load power equal to its VA rating. In practice the majority of loads are not purely resistive and have a small reactive (inductive or capacitive) component which should be taken into account. When a p.f. value is not known it is common practice to assume a value of 0.8.
Filament lamps and coiled heater elements may be assumed to have a power factor of approximately 0.95, which is often ignored when specifying a trasnsformer. Discharge lamps (which includes fluorescent lamps) present a lower power factor, which in industrial and commercial situations is normally adjusted by the connection of a p.f. correction capacitor to a value of 0.95. Uncorrected power factors for discharge lamps can be as low as 0.5, so it pays to correct.
Motors also have fairly low power factors. A single phase induction motor will typically have a p.f. in the range 0.6 - 0.85, cheaper motors being worse. Again, p.f. correction is advisable. The power factor is normally marked on the motor rating plate as a cos f (phi) value.
Induction motors are frequently marked with the shaft power rather than the input power which makes the situation more confusing. To obtain the actual input kVA for such a motor it may be possible to measure the input current (preferably with a clamp meter) whilst the motor is under full load. Multiplying the value obtained by the supply voltage will then give the kVA loading of the motor.
E.g. A particular 2 kW load has a p.f. of 0.8, what VA rating should be allowed for the transformer supplying this load ?
A. 2000 / 0.8 = 2 500 VA = 2.5 kVA
This transformer should be specified with a minimum rating of 2.5 kVA.
Phases
Transformers are normally wound as either single or three phase units for connection to a single or three phase electrical supply respectively. A single phase transformer requires a single live (phase) connection and a neutral to the primary for operation. A three phase unit requires three lives (phases) and sometimes a neutral. The phase to phase voltage for a 3 phase plus neutral system is 1.732 x Phase to neutral voltage.
E.g. A 415 V 3 phase supply has a phase to neutral voltage of 415 V / 1.732 = 240 V
Note: 1.732 is the square root of 3, to 4 significant figures.
Winding Type
Transformers may be manufactured as either isolating or auto-wound.
An isolating transformer has primary and secondary windings which are not in any way electrically connected to one another, they are purely coupled by magnetic effects in the iron core. This has the benefit that the secondary windings may be electrically connected to a different ground system to the primary, or not grounded at all, simply left floating with respect to ground.
An auto-transformer has a point which is electrically common to the input and output of the unit. It does not, therefore, provide isolation between input and output, the circuits are electrically connected. The benefits of using an auto-winding are a reduction in the size and cost, as the transformer does not actually have to be large enough to handle the full load power, it merely has to provide the balance of the power. Hence, if the difference between the supply voltage and output voltage is small the transformer can be small.
e.g. A single phase supply voltage of 230 volts is available and it is desired to connect a 240 volt load rated at 24 kW what part of the load is the auto-transformer required to handle?
Ans. Calculate the load current, 24 000 W / 240 V = 100 A
Calculate the difference between supply voltage and load voltage, 240 V - 230 V = 10 V
Calculate the transformer loading, 100 A x 10 V = 1 000 W = 1 kW
The transformer has to handle 1 kVA (assuming a load power factor of unity. See Total Load Rating of Transformer for further information).
When the Total Load Rating of the Transformer is specified in the form (for an auto-transformer) it is essential that the actual load rating is given, NOT the portion of the load power handled by the transformer, i.e. enter only the larger of the two values, 24 kVA in this example.
Supply Frequency
Transformers will only work with an alternating current (a.c.) supply, in which the voltage (and current) reverse direction repeatedly and regularly. The rate at which these periodic reversals occur is known as the frequency and is measured in hertz (Hz). 1 Hz is equal to one cycle per second.
It is important to know the supply frequency when designing a transformer as it is partly responsible for determing the size of the magnetic core required and the number of turns of wire per volt. The frequency of the electrical supply is normally marked on the electricity supply meter and is generally 50 or 60 Hz, depending on location (50 Hz in the U.K. and 60 Hz in the U.S.). It can however be almost any value, depending on the application, e.g. aircraft typically use 400 Hz to permit smaller generators and transformers and some telcomms apps require 25 Hz. If you are unsure contact your electricity supplier or design engineer or ask our technical staff.
Primary Voltages
The primary (or input) voltage of a transformer must be matched closely to its supply voltage in order for it to operate correctly. If the transformer is to be used with a range of different supply voltages then it is possible to provide it with either a muliple-tapped primary or a number of independent primaries which may be connected in series or parallel to match the supply.
Some common arrangements are 0-115 V, 0-115 V or 0-220-240 V. The former provides two primaries which may be connected either in parallel to operate from a 115 V supply or in series for a 230 V supply. The latter provides for 220 V or 240 V operation.
An alternative arrangement offering greater flexibility is to provide one or more taps for fine adjustment to the other side of the zero tap.
E.g. 10-0-200-220-240 provides no fewer than six possible values for the primary voltage. The values 200, 220 and 240 V are available when the neutral is connected to the 0 V tap. Connecting the neutral to the 10 V tap makes 210, 230 and 250 V connections available.
Auto-wound Voltages
The voltages for an auto-wound transformer are normally specified as a string of numbers starting with a zero and ending with the highest voltage. There is no primary or secondary as such, any two points on the winding can be connected to the supply and likewise a load may be connected across any two points. A minimum of two voltage taps are required for an auto-transformer to perform a useful task. An auto-transformer does not provide electrical isolation between the input and output so must not be used in safety critical applications such as portable tool transformers, arc welders or car battery chargers. Suitable applications are in supply voltage matching where only a small difference exists between input and output voltages. see also Winding Types for further information.
E.g. a 0-220-230 V winding will allow an auto-transformer to be connected to a 220 V supply and deliver 230 V to the load. Alternatively it may be connected to a 230 V supply and output 220 V.
Multiple tappings are possible, e.g. 0-380-400-415-440-480 V.
It is also permissible to have one or more taps before the zero to permit fine adjustment of the output voltage.
E.g. 10-5-0-380-400-415 V, which will allow selection of the following voltages: 380, 385, 390, 400, 405, 410, 415, 420 and 425 V. 5 and 10 V are also available but would not generally be used as outputs (or inputs) in their own right.
Secondary Ratings
Each secondary winding (of which there may be many) on a transformer has a voltage associated with it, along with a maximum current or power which it can deliver to a load. The total VA rating of all the windings will normally equal the VA rating specified as the Total Load Rating of Transformer.
The secondary voltages for each set of secondary windings must be specified, along with the current or VA in each case. The only exception to this rule is when there is only one secondary winding, in which case it will have a rating equal to the total load rating for the transformer.
The voltages should be specified as either a single number in box 1 of each row, where the winding is not multi-tapped, or as a string of numbers of the form 0-12-15-18 V, or 24-0-24 V. For centre-tapped windings the form 110 V C.T. or 110 V C.T.E. for a centre-tapped earth are acceptable.
The loading of each winding on a multiple wound transformer should be specified in box 2 of this row next to its corresponding voltage, and its units, mA, A, VA, kVA selected in box 3 of each row.
In the case of multiple windings it is possible for the sum of the individual loads to exceed the total load rating of the transformer. In this case it must be realised that the actual load applied to the unit should not exceed the total VA rating.
Interwinding Screen
An electrostatic screen comprising a single turn of insulated copper foil inserted between the primary and secondary windings of the transformer and connected to ground reduces mains borne interference from being impressed on the secondary voltages, i.e. results in a cleaner supply.
It also increases safety, by preventing a breakover between primary and secondary windings.
Electrostatic screens screens can only be fitted to isolating transformers.
Duty Cycle
The operating duty cycle of a transformer is normally specified as the percentage of time for which the load is drawing its full rated current . It is usual to specify the duty cycle based on a time period of 30 minutes.
Providing a transformer with a low duty cycle enables a physically smaller transformer to be used in situations where it is known that the load will only be used for a certain time during every 30 minute period. This is normally the case with 110 V tool transformers, portable arc welders and car starter-chargers. For most applications a continuous rating is applied.