PT/VT Interface:

     ATE=31 PT-Interface    ATE-31  

                         ATE-31 PT Interface                                            ATE-31 PT Interface

When electrical generators are operated into the national electrical grid, one of the more daunting tasks is synchronizing the unit's alternating current output voltage to the grid-frequency before closing the power-line circuit breakers. If the unit is not synchronized precisely, the electro-mechanical shock that occurs when the circuit breakers are closed can trip the breakers, and even damage control system equipment. This is particularly critical in hydroelectric generating systems where the huge momentum of the water column is added to the rotational inertia of the turbine/generator set. 

Early speed sensing equipment used mechanical gear-trains and tachometers to measure rotating speeds of hydroelectric turbines. These mechanical speeds sensors were expensive and difficult to maintain. Wear and tear over time degraded performance of these mechanical systems to the point that they become unserviceable. To solve this problem, the Potential Transformer Interface (PT Interface) signal conditioner was designed to use the electrical output of the generator itself as the tachometer signal, designed specifically for this purpose by engineers at Woodward Governor Company's hydroelectric controls division over 35 years ago. 

When Woodward's hydro division was dismantled, several of the engineers who had used this product in their governor designs at Woodward moved on to other hydroelectric control companies, yet still wanted access to a product with these same capabilities; but improved using new packaging options and electrical components that have become available in recent years. Contract manufacturing of these customer-specified modules is how ATECo got started building the ATE-30 in 2001. To date, over 300 of these modules have been supplied in various configurations and packages, without any significant problems or product recalls over the life of the product.

One customer redesignated the ATE-3X PT Interface as their own "VT Interface" and acquired a UL certification for it. A subsequent decision was made to manufacture these modules in-house, which released Actuation Test Equipment from the prior "captive-supplier" relationship, hence the ATE-31 VT/PT Interface was made available to the market as a product of Actuation Test Equipment Company.

Inquiries from hydroelectric powerplants for spares of these modules has resulted in direct market sales at reduced prices to these end users.

Weidmuller package
              The ATE-30 is in a Weidmuller enclosure
ATE-31 PTInterface Side
                ATE-31 is in a Phoenix Contact enclosure
                 ATE-31 Cut Sheet Document                                        ATE-31 Cut Sheet Document

The packaging configuration for the ATE-30 and ATE-31 Potential Transformer Interface (PTInterface) modules is shown in the two pictures above. The circuit board layout and design are essentially the same; the electronic circuitry and electrical performance of these two modules are identical. The original ATE-31 was designed into the Weidmuller DIN rail mount (23 mm wide) packaging. This enclosure did not meet the CIE flame retardancy standards, so the design was improved by switching to the Phoenix Contact (23 mm wide) DIN rail mount enclosure.

Both of these signal conditioners utilize the Analog Device AD210 isolation amplifier as the primary element, providing up to 3500 VDC isolation between the input and the output.

For this design and CIE certification, the isolation was tested to 2000 VDC isolation.  Both designs have the same electronic circuit in them.

The input accepts a voltage from an instrumentation potential transformer and converts it to a 24 Volt pulse train for use by PLC-based frequency counting equipment.  The input amplitude can vary from 0.2 VRMS to 180 VRMS.  The input frequency can vary from 6 Hz to 120 Hz.  

A 180 Hz 3-pole Butterworth low-pass filter in the ATE-3X PTInterface prevents high frequency noise, such as that induced by operation of nearby switchgear from getting to the tachometer input.  This filter blocks high frequency transients that may be superimposed upon the input signal voltage from passing through to the frequency counting equipment. 

The amplifier sensitivity of the Model ATE-31  allows it, in conjunction with a frequency counter, to reliably measure the frequency output of a generator operating at low speed prior to the application of field excitation.  

Typical generator output at 10% speed is greater than 0.2VAC.

The ATE-31 takes advantage of the residual magnetism in the rotor and armature of the generator when the unit is "off-line." 

When generator excitation is turned off and the unit is governed in an isolated, "speed-control" mode. 

Accurate and instantaneous information about turbine rotor angular speed is essential for robust dynamic control system performance.

This residual alternating voltage is in exact synchronization with speed, and is ideal for a tachometer or speed controller.

The AD-210 amplifier circuit produces a 24 v pulse train from a sinusoidal input; intended to measure the generator's residual voltage output while spinning at less than 100% speed, with generator excitation off.  

When the excitation is on, the 110 VAC output of the PT circuit is sensed using a 15k Ohm 5W resistor and a pair of high-speed switching diodes. 

The input stage of the Model ATE-31 is fully isolated from the output stage, so the signal input can be connected either from line to neutral or from line to line of the instrumentation potential transformers.

The connector plugs are available in two styles, a screw-terminal and a spring-cage type.

Two schools of thoght exist on which is the superior method, the simple solution for ATECo is to offer both, and let each customer decide which to use. The PT Interface


Phoenix ConnectorsThe Model ATE-31 PT Interface has two connector styles. The upper is a spring-cage connector that is used by pressing on the orange clip lever and inserting the wire into the opening. The advantage with these is that the wire is always held tightly, and cannot work lose because the spring hold tension on them.

A traditional screw-terminal  is also available.
The enclosure is available with an integral grounding wire to connect directly to system ground through the rear of the case..


The Model ATE-31 PT Interface module is useful in applications where the rotating speed of a generator must be accurately measured without the addition of a conventional slotted disk or gear and magnetic pickups.

 It is designed with sufficient sensitivity to operate from the voltage produced at low speeds from the residual magnetism in the rotor of the generator. 

Typically, the residual output of most generators is sufficient for the Model ATE-31 to allow measurement of the generator speed down to and below 10% of the rated speed of the generator, specifically designed for use as an input to the unit braking system.

The ATE-32 is not a creep detector.


The Model ATE-3X utilizes two status LEDs.

The green LED indicates that DC power is applied.

The yellow LED is illuminated when the input signal has positive polarity from terminal 11 to terminal 12.

The yellow LED is extinguished when the input signal is zero or has negative polarity.

At low input frequencies, the yellow LED will flash visibly to indicate the presence of the input signal.

At higher input frequencies, the yellow LED will appear to be illuminated at a reduced intensity due to visual averaging of the flashes.


No calibration is required for the Model ATE-31.



                Supply:                                  18 to 30 VDC

                Consumption:                      2 W typical


Amplitude Range:              0.2 Vrms to

180 Vrms

                Frequency Range:              6 Hz to 120 Hz


                Pulse train 24 Volts peak-to-peak (typical)


                TS32 or TS35 DIN Rail

ATE-30 Pin Connections:

21, 22, 23 & 51, 52, 53 Ground Bus

43    (+) Input

41    (-) Input

12           CT Ground

11         Signal Output

13         DC Power (+)

42         No Connection

ATE-31 Pin Connections:

1, 2, 3, 4 & 13, 14, 15 & 16 Ground Bus

5 & 6    (+) Input

7 & 8    (-) Input

9           CT Ground

10         Signal Output

11         DC Power (+)

12         No Connection


               Operating:           0 to 55 oC (5 to 131 oF)

                Storage:                -25 to 70 oC (-13 to 158 oF)