20-750-UFB-1 — 755 Series Universal Feedback Option Card

The 20-750-UFB-1 is a universal feedback option card for the 755 series AC drive. It installs into one of the drive's option card slots and adds a dedicated feedback interface that supports multiple encoder and resolver types — incremental encoders (TTL/HTL), absolute encoders (SSI), and resolvers — all from the same card. Once installed, it unlocks closed-loop flux vector control on the drive, enabling tight speed and position regulation that sensorless vector mode cannot achieve.

The word "universal" is accurate: most feedback devices used on industrial servo and induction motors connect to this card without additional hardware. That matters for system integrators working across multiple motor brands and encoder suppliers — rather than stocking multiple feedback cards for different encoder protocols, the 20-750-UFB-1 covers the field with a single part number. The card reads the feedback, scales it to the drive's internal units, and the drive's flux vector control algorithm uses it to close the speed and torque loops.

Specifications

Parameter	Value
Part Number	20-750-UFB-1
Compatible Drive	755 Series AC Drive
Feedback Types Supported	Incremental encoder (TTL differential, HTL single-ended), SSI absolute encoder, Resolver
Max Encoder Frequency	500 kHz (TTL differential)
Encoder Supply Voltage	5V DC or 12V DC (selectable via jumper)
SSI Clock Frequency	Up to 1 MHz
SSI Word Length	10–32 bits (configurable)
Resolver Excitation	Provided by card (2–10V RMS, 2.5–10 kHz)
Simulation Output	Encoder simulation output (A, B, Z)
Connector	15-pin D-sub (HD15)
Installation	755 drive option card slot
Operating Temperature	−10°C to 60°C
Standards	UL, CE

What Closed-Loop Feedback Changes on a 755 Drive

Without encoder feedback, the 755 drive runs in sensorless vector mode — it estimates rotor speed from the motor's current and voltage signatures. SVC is good enough for most process loads, but estimation accuracy degrades at very low speeds, under rapidly changing loads, and when extremely tight speed regulation (better than ±0.1%) is required.

With the 20-750-UFB-1 installed and an encoder mounted on the motor, the drive knows the actual rotor speed to encoder resolution on every control cycle. Speed regulation tightens dramatically — typical closed-loop speed regulation is ±0.01% or better, versus ±0.5–1% for SVC. At low speeds (below 5 Hz), where SVC struggles to maintain stable torque, closed-loop vector delivers full rated torque down to near-zero speed. This opens the drive to applications that SVC cannot handle: winders maintaining constant web tension, extruders holding precise screw speed under varying material viscosity, cranes and hoists needing full torque at standstill.

Installation and Wiring

The card slides into a designated option card slot on the 755 drive and locks with a retention clip — no tools required. The feedback device connects via the HD15 (15-pin D-sub) connector on the card face. Pre-made feedback cable assemblies are available for common motor and encoder combinations; for custom assemblies, follow the pinout in the card's installation instructions precisely.

Use shielded cable for all feedback signals. Connect the shield at the drive end only to prevent ground loops. At high encoder frequencies (above 100 kHz), poor shielding causes noise-induced count errors that appear as speed hunting or position errors in the drive.
Keep encoder cable separate from motor power cables and other power wiring. Running feedback and motor power in the same conduit is the most common cause of encoder noise problems on new installations.
For TTL encoders, verify the 5V encoder supply current draw is within the card's supply current limit. Encoders drawing more than the card's supply rating need an external 5V supply — the card's supply is for the encoder only, not for additional loads on the cable.
After installation, set the encoder type, line count (for incremental), and word length (for SSI) in the drive parameters before running autotune. Incorrect feedback configuration causes the drive to misinterpret speed data and potentially run unstably.

FAQ

Q: Does installing this card automatically enable closed-loop vector control?

The card provides the hardware interface, but closed-loop vector mode must be selected in the drive's control mode parameter and the feedback parameters must be configured correctly. Installing the card alone doesn't change the control mode — the parameter change and an autotune are also required.

Q: Can the simulation output be used to feed encoder data to another device?

Yes. The encoder simulation output (A, B, Z) generates a TTL-level incremental encoder signal based on the drive's measured speed. This can feed an external PLC counter module, a secondary drive for electronic gearing, or a motion controller that needs position feedback from the driven axis.

Q: Which option card slot does this card occupy on the 755 drive?

The 755 series has multiple option card slots with specific assignments for different card types. The 20-750-UFB-1 installs in the designated feedback card slot — consult the 755 drive's option card installation guide for the correct slot assignment for the specific drive frame size in use.

Q: Can both an encoder and a resolver be connected simultaneously?

No. The card has one feedback input port configured for one feedback type at a time. Only one feedback device can be active per card. If two feedback devices are needed (unusual in most applications), a second feedback card would be required in a separate option slot.

Q: Is an autotune required after installing this card?

Yes. Switching from sensorless vector to closed-loop vector mode requires a rotating autotune to accurately characterize the motor with feedback. Static autotune is not sufficient for closed-loop flux vector. The motor must be able to spin freely (decoupled from load if necessary) during the rotating autotune procedure.

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