AC Synchronous Stepper Motors — Speed-Locked to AC Frequency

An AC synchronous stepper motor is a hybrid stepping motor wound to operate directly from an AC power source. Unlike standard stepper motors that require a DC drive or controller, AC synchronous motors run directly on line-frequency AC power — typically 50 Hz or 60 Hz. Output shaft speed is locked to the supply frequency, making these motors inherently consistent without any closed-loop electronics.

At 120 VAC / 60 Hz, a standard AC synchronous motor produces 72 RPM. At 100 VAC / 50 Hz, the same motor produces 60 RPM. Speed adjustment requires either a frequency inverter or a mechanical transmission (gearbox, belt drive, or chain drive) — the motor itself has no variable-speed electronics.

This is the primary application advantage: a stable, repeatable speed from a simple AC supply, with no stepper drive, encoder, or motion controller in the signal chain.

The synchronous speed formula for AC motors is:

RPM = (120 × Frequency) ÷ Number of Poles

AC synchronous stepper motors are built with a fixed pole count matched to produce useful speeds (e.g., 72 RPM or 300/600 RPM variants) at standard line frequencies. Haydon Kerk motors, for example, are available wound for 300 RPM or 600 RPM operation at 60 Hz.

For linear actuator configurations, the linear output speed derives from motor RPM and lead screw pitch. A .001 in/step motor receiving 60 Hz produces 240 steps per second, resulting in 0.240 in/second of linear travel.

AC synchronous stepper motors are selected when the application needs a fixed, repeatable speed from an AC line supply without drive electronics. Common use cases:

• Conveyors and material handling — constant-speed feed systems where line frequency provides adequate timing
• Timing and synchronization equipment — clocks, chart recorders, and instrumentation where shaft position tracks elapsed cycles
• Medical and laboratory instruments — peristaltic pumps, sample carousels, and fluid dispensing systems
• HVAC and building automation — damper actuators and valve positioners where speed precision is secondary to reliability
• Packaging machinery — label applicators, indexing tables, and auger fillers running at fixed throughput rates
• Hazardous environments — oil and gas, chemical processing, and mining where certified motor enclosures eliminate drive electronics from the hazardous zone

Electromate is the exclusive Canadian distributor for Kollmorgen. The Kollmorgen AC synchronous motor lineup is divided into two performance families:

Key Specifications at a Glance

What is the difference between an AC synchronous motor and a standard stepper motor?

A standard stepper motor requires a DC stepper drive to generate the step pulses that move the rotor. An AC synchronous stepper motor is wound to operate directly from an AC line supply — the alternating current itself creates the rotating magnetic field that drives the rotor. No external drive is needed. Speed is fixed to the supply frequency.

Can an AC synchronous motor be used at variable speeds?

Not directly. Output speed is locked to AC input frequency. Variable speed requires either a variable-frequency drive (VFD) on the AC supply or a mechanical transmission (gearbox, belt, or chain). For electronically variable speed, a DC or brushless servo system with a drive is the standard alternative.

What torque levels are available in AC synchronous stepper motors?

Kollmorgen KS/SS and Hazardous Duty series reach up to 1,500 oz-in (1,059 N-cm) in NEMA 42 and 66 frame sizes. Smaller NEMA 23 models produce considerably lower torque — consult datasheets or request sizing support.

Are AC synchronous motors suitable for hazardous locations?

Yes. Kollmorgen Hazardous Duty synchronous motors are UL Listed for Class I, Division 1, Group D and Class I, Division 2, Groups E, F, and G locations, and carry ATEX certification. These eliminate drive electronics from the hazardous zone entirely.

What is an AC synchronous linear actuator?

It is an AC synchronous stepper motor with an integrated lead screw. The rotary output is converted to linear motion at a speed and force determined by the lead screw pitch and the motor's AC frequency. Haydon Kerk produces these in configurations compatible with 60 Hz North American supply.

Do I need a stepper drive or motion controller for an AC synchronous motor?

No. This is the defining characteristic of AC synchronous motors. They connect directly to an AC line supply. No stepper drive, no encoder, and no motion controller are required for fixed-speed operation.

What frame sizes are available?

Kollmorgen AC synchronous motors are available in NEMA 23 (60 mm), NEMA 34 (90 mm), NEMA 42 (110 mm), and NEMA 66 (170 mm) for the hazardous duty line. Haydon Kerk rotary and linear models follow NEMA hybrid stepper frame standards.

We carry AC Synchronous Stepper Motors from both Haydon Kerk and Kollmorgen. 

AC Synchronous Motors from Haydon Kerk, are stepping motors that are run on AC (Alternating Current).  With synchronous motor technology, speed is directly proportional to AC input frequency – for example, at 120 Vac, 60 Hz, an AC synchronous motor will turn at 72 rpm. This speed can be varied by varying the frequency, although most applications simply use gearing or a belt- or chain-drive system to achieve the desired load speed. Other motor technologies (Ie. induction, DC, servo and step motors) either need gear reducers, or electronic drives to match the fixed speed of synchronous motors.

Kollmorgen AC Synchronous motors are available in two families in the ST/SN Series and KS/SS Series. High performance motors in these series are offered in NEMA 23, 34 and 42 frame sizes, with two- and three-phase models available in each size. Resistor-capacitor kits are available for operating two-phase synchronous motors from a single-phase power source. We also offer hazardous duty AC synchronous motors, certified to UL Class I, Division 1, Group D; UL Class 1, Division 2, Groups E, F and G; and ATEX standards.

	ST and SN Series ST and SN Series synchronous motors provide high performance in three frame sizes, NEMA 23, 34 and 42 (60, 90 and 110 mm). Resistor-capacitor kits are also available.
	KS and SS Series These high-torque synchronous motors are available in NEMA 23, 34 and 42 frame sizes (60, 90 and 110 mm), providing up to 1,500 oz-in (1,059 N-cm) of torque.
	Hazardous Duty Synchronous Motors Providing torque up to 1,500 oz-in (1,059 N-cm), these synchronous motors are available in UL Listed versions suitable for use in Class I, Division 1, Group D hazardous locations. Frame sizes are NEMA 42 and 66 (110 mm and 170 mm).

 AC Synchronous Linear Actuator Configurations

AC Synchronous Motors are also available in AC Synchronous Linear Actuator configurations from Haydon Kerk. In the case of a linear actuator the linear speed produced is dependent on the resolution per step of the motor. For example, if 60 hertz is supplied to a .001-in/step motor the resulting speed is .240-in per second (240 steps per second times .001-in/step). Many of the Haydon Kerk™ stepping motors are available as 300 or 600 RPM AC synchronous motors.