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Technical Support


Frequently Asked Questions

What is a Transformer?

A transformer is an electrical device that transfers energy between two or more circuits through electromagnetic induction.

What is an Isolation Transformer (Double Winding)?

An isolation Transformer is with primary (Input) and secondary (output) winding separated/isolated. It does not have a direct electrical path from the power input to the power output.

What is an Auto Transformer (Single Winding)?

An auto Transformer consist of 1 winding which the primary (Input) and secondary (Output) share a common winding. Its designed to transformer voltage when isolation from the line is not necessary.

What is inrush current?

The inrush current is a current several times of the electrical device’s full load current drawn when the device is first turned on. During this period, the transient current can flow for several cycles before reaching its steady state current. Inrush current is rated at number of times in transformer. Transformers usually have an inrush rating of 8 to 15 times. For very small transformers, the inrush can go up to 25 to 40 times.

What is meant by “Class” in insulation?

Insulation “Class” systems are classified by the temperature rating. The rating is the maximum allowable winding (hot spot) temperature of a transformer operating at an ambient temperature of 40°C.

What is temperature rise in a transformer?

Temperature rise in a transformer is the average temperature of the windings and insulation above the existing ambient temperature.

Can 60 Hz transformers be operated at 50 Hz?

No. 60Hz Transformer should not be using on 50Hz operation. Due to higher losses and resultant heat rise. However, any 50Hz transformer able to operate at 60Hz operation.

Why are transformers rated in kVA not kW?

Generally, there are 2 kind of losses in a transformer, iron losses and copper losses. Since iron losses depend on voltage and copper losses on current, the total losses depend on voltage and current, and no power factor is invoiced. Transformer are rated in kVA as KW wound include power factor.

Can Transformer be connected in Reverse?

Yes. However, the output voltage will be lower than its rating, due to the voltage compensation factor of the windings.

What is a Full Capacity Tap?

A full capacity tap is one through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise.

Can Transformer regulate the output voltage?

Transformer will not regulate the voltage. The output (or secondary voltage) of a transformer is dependent on the magnitude of the primary voltage and the transformer turns ratio. Therefore, fluctuations in the secondary voltage are direct results of any fluctuations in the primary voltage.

What are Impedance Voltage and Load Loss tests?

The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature.

Can transformers be used in parallel?

Single phase transformers can be used in parallel only when their voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers which will cause excess heating and result in a shorter life of the transformer. In addition impedance values of each transformer must be within 7.5% of each other.

Can a single phase transformer be used on a three phase source?

Yes. Any single phase transformer can be used on a three phase source by connecting the primary leads to any two wires of a three phase system, regardless of whether the source is three phase 3-wire or three phase 4-wire. The transformer output will be single phase.


If any of the symptoms listed in below are identified, the transformer should be removed from service at once. Immediate attention may prevent dangerous conditions or expensive repairs. In many instances, the problem can be found quickly and the unit returned to service.

All operation to the Transformer must be carried out with the Transformer de-energized and terminal earthed. If the issue cannot be corrected, do not re-energize the transformer. Contact MES immediately.

Further inspection may require disassembly of the core and coils. This work must be performed by a factory representative or authorized repair facility.



• Continuous overload
• Incorrect external connection
• Poor ventilation
• High surrounding air temperature
• High Harmonic or unbalanced loads
• Clogged air ducts or inadequate ventilation
• High input voltage

Smoke is visible or smelled


• Insulation failure
• Overheating

Insulation failure


• Continuous overloads
• Dirt accumulations on coils
• Mechanical damage in handling
• Lighting or switching surges

Burned insulation


• Lighting surge
• Switching / line disturbance
• Broken busing, taps or arresters
• Excess dirt or dust on coils

Secondary voltage reduced or zero voltage


• Overload
• Tap connection not on identical tap positions
• Neutral ungrounded

Tripping of Fuses or Breakers during energization


• Overcurrent device unsized
• Inrush current
• Low input frequency
• Low power factor

Breakers or fuses trip


• Short circuit
• Overload

Excessive cable heating


• Improperly bolted connection
• Incorrect cable size for load
• Incorrect cable routing

High exciting current


• Low frequency
• High input voltage
• Shorted turns

Noise & Vibration


• Low frequency
• High input voltage
• Core clamps loosened

Installation Manual

MES Transformer offer Installation, Operation and Maintenance manual to help ensure the proper safety procedures and inspections are taken into consideration before installing any MES product.

Download Manual