Control Power Transformer
Control power transformers [CPT] are transformers designed to provide control voltage to contactors, starter control panels, solenoids, relays etc. CPT may also reduce noise interference by isolating control circuit from power circuit.
![Example of a control power transformer [Siemens]](https://voltage-disturbance.com/wp-content/uploads/2022/07/Example-of-a-control-power-transformer.png)
Control power transformers have the following features:
- CPT is designed with low impedance windings for voltage regulation.
- CPT is designed to provide stable secondary voltage while allowing high inrush current loads (relays, contactors).
- CPT could be multitap with different voltage options.
- CPT is available in different VA ratings.
- CPT is sized based on the continuous VA and inrush VA of control circuit.
- CPT do not have any accuracy requirements.
- CPT utilizes compensated windings to achieve high voltage regulation.
Important ratings relevant to CPT:
- Thermal burden (VA rating)
- Primary and Secondary voltage
- Multi tap or Single tap [Primary and Secondary]
- Temperature rating
- Rated frequency
Read: AC voltage drop and power factor
Potential Transformer
Potential transformers [PT] are used to get scaled down voltage for metering, relaying purposes. PTs are usually wired to power meters, protection relays, signal transducers etc.
![Example of a potential transformer [Square D]](https://voltage-disturbance.com/wp-content/uploads/2022/07/Example-of-a-potential-transformer.png)
Potential transformers have the following features:
- PT is designed to reproduce accurate representation of voltage magnitude and phase angle.
- PT is not intended for use in high inrush applications and are typically used with high impedance loads like recording instruments, protective relays, PLC etc.
- PT has pre-defined primary and secondary rated voltage and is usually not adjustable.
- PT is available in different VA ratings.
- PT is sized based on the VA burden and accuracy requirements.
- PT do not use compensated winding for voltage regulation.
Important ratings relevant to PT:
- Thermal burden (VA rating)
- Primary and Secondary rated voltage
- Accuracy class
- Basic Impulse Level [BIL]
- Rated frequency
Read: PT Accuracy
Can Control power transformer be reverse fed?
No, industrial control power transformers cannot be reverse fed. These transformers are specifically marked with primary and secondary terminals and NEC and UL prohibit connecting source to other than primary terminals. Control transformers also use compensated windings for voltage regulation. A CPT that is reverse fed will result in output voltage lower than desired.