Frequently Asked Questions: Control Transformers

What is a transformer?What is an isolation transformer?

What is a control transformer?

Can a control transformer be reverse connected?

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

Can a transformer be used at higher frequencies?

What is regulation?

Can transformers be used at ambients other than 40°C?

What is the effect of altitude on a transformer?

What is the effect of load on a transformer?What is temperature class?

What is temperature rise?

What is hot spot?

Is one insulation system better than another?

Why is a transformer needed?

Are control transformers current limiting?

Will a control transformer regulate output voltage?

What is a duty cycle?

What is the effect of encapsulation in control transformers?

What effect does a control transformer have on electrical disturbances found on the line?

What is a transformer?

A transformer is a passive electrical device which is designed to change one voltage to another by magnetic induction.

Back to Top

What is an isolation transformer?

An isolation transformer, also referred to as an insulating transformer, is one where the primary and secondary windings are separate, as opposed to an autotransformer where the primary and secondary share a common winding.

Back to Top

What is a control transformer?

A control transformer is an isolation transformer designed to provide a high degree of secondary voltage stability (regulation) during a short period overload condition typically referred to as inrush. Control transformers are also referred to as Industrial Control Transformers, Machine Tool Transformers, or Control Power Transformers (CPT’s).

Back to Top

Can a control transformer be reverse connected?

A control transformer can be reverse connected. However, the output voltage will be less than nameplate due to the compensation factor of the windings.

Back to Top

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

A single phase transformer can be used with a three phase source by connecting the primary leads to any two wires of the three phase system. The transformer will be a single phase.

Back to Top

Can a transformer be used at higher frequencies?

A transformer designed for 50/60Hz operation can be utilized at frequencies up to 400Hz. However, at 400Hz, the inrush capability will be reduced.

Back to Top

What is regulation?

Regulation is the change in output voltage when the load is reduced from rated value (full load) to zero (no load) with input voltage remaining constant.

Back to Top

Can transformers be used at ambients other than 40°C?

Transformers may be used at ambients less than 40°C at full nameplate capacity. For ambients above 40°C, they must be derated as follows:

Maximum Ambient Temperature Maximum Percent of Load
180°C Units                105°/130° Units
40°

50°

60°

70°

     100%                     100%
       90%                       78%
       79%                       50%
       68%                       30%

Back to Top

What is the effect of altitude on a transformer?

A transformer may be used at full nameplate capacity up tp 3300 feet (1000 meters). Above that altitude, the capacity of the transformer should be derated by 0.3% for each 300 feet of elevation above 3300 feet.

Back to Top

What is the effect of load on a transformer?

A control transformer is designed to provide rated output voltage at full VA. As the load decreases, the output voltage will go up. Conversely, increases in load will result in lower output voltages. Typically, the smaller the VA size of the unit, the greater difference there is between no-load and full-load voltage.

Back to Top

What is temperature class?

Temperature class is the rating of the transformer insulation system. It is determined by adding the ambient temperature, temperature rise, and hottest spot temperature. The standard insulation system classifications per UL506 are as follows:

Ambient Temperature Average Winding
Temperature Rise*
Hot Spot
Temperature
Temperature Class
40°C 55° 10° 105°C
40°C 80° 10° 130°C
40°C 100° 15° 155°C
40°C 120° 20° 180°C
*Measured by change-in-resistance method

Back to Top

What is temperature rise?

Temperature rise is the difference between the average temperature of the transformer windings and the ambient temperature.

Back to Top

What is hot spot?

The hot spot is an allowance selected to approximate the difference between the highest temperature inside the transformer coil and the average temperature of the transformer coil.

Back to Top

Is one insulation system better than another?

One insulation system is not necessarily better than another. Each will typically provide a comparable life expectancy. The choice of an insulation system depends upon application, performance, and cost considerations.

Back to Top

Why is a transformer needed?

A control transformer is required to supply voltage to a load which requires significantly more current when initially energized than under normal steady state operating conditions. A control transformer is designed to provide secondary voltage stability under a short period of specific overload referred to as inrush.

Back to Top

Are control transformers current limiting?

A control transformer is not current limiting and will allow as much current to pass through as is demanded by the load. As such, a secondary overcurrent device should be utilized.

Back to Top

Will a control transformer regulate output voltage?

Control transformers are not voltage regulating. Because voltage changes are a function of the transformer’s turns ratio, variations in input voltage will be proportionally reflected to the output.

Back to Top

What is a duty cycle?

Duty cycle is the period and duration when a transformer will be loaded. The transformer is designed to run continuously at full load without exceeding the temperature limits. Transformers may also be operated for short time duty. Depending upon the time and cycle of the maximum load, the transformer VA size may be smaller than for continuous duty.

Back to Top

What is the effect of encapsulation in control transformers?

Encapsulating the coils of a control transformer will help to protect the unit from moisture, dust, dirt and industrial contaminants. Encapsulation helps provide maximum protection in hostile environments while allowing the unit the run cooler than a non-encapsulated unit.

Back to Top

What effect does a control transformer have on electrical disturbances found on the line?
Because a control transformer has primary and secondary windings, it will provide some degree of “clean-up” with regard to electrical noise, spikes, surges, and transients. It will not, however, provide the same degree of power conditioning found in products designed for that purpose.

Back to Top