PKP Series stepper motors

A stepper driver energises the stator phases in sequence. Each pulse from your PLC or motion controller advances the magnetic field by one step, and the toothed rotor snaps to the next stable position. The number of pulses sets how far the shaft turns, and the pulse frequency sets how fast it turns. There is no encoder and no tuning loop, which is why a stepper responds immediately to commands and is easy to synchronise across several axes.

The accuracy comes from the mechanics. With 50 or 100 rotor teeth, every stopping point is defined by the same tooth geometry, so errors do not build up over many moves. The limitation is equally mechanical: if the load torque exceeds what the motor can produce, the rotor falls out of step and the controller does not know. That is why correct sizing, with a torque margin, is the heart of every open-loop stepper design.

• 1.8° standard type: 200 full steps per revolution; finer resolution through microstepping in the CVD driver
• 0.9° high-resolution type: twice the rotor teeth, better stopping accuracy under friction load and a way to move out of a resonance region
• Holding torque at standstill with no brake needed for horizontal axes (use the brake option for vertical loads and power failure)

NEMA sizes describe the mounting flange, not the torque. A NEMA 17 motor has a 42 mm square flange and a NEMA 23 motor a 56.4 mm flange; torque then depends on stack length and winding. Within the PKP range, a 42 mm motor covers roughly 0.1 to 0.99 N·m and a 56.4 mm motor roughly 0.74 to 3.1 N·m, so the two sizes overlap only slightly at the top of NEMA 17 and bottom of NEMA 23.

Because PKP motors deliver about the torque of the next larger frame of a conventional stepper, many Indian OEMs who were using a long NEMA 23 motor from another brand find that a PKP NEMA 17 or short NEMA 23 does the job, saving space and weight on moving axes.

• 20–35 mm (NEMA 8–14): valves, small dispensers, camera pan/tilt, lab instruments
• 42 mm (NEMA 17): light belt axes, lead-screw stages, feeders, label and print mechanisms
• 56.4 / 60 mm (NEMA 23 / 24): ball-screw tables, conveyor indexing, textile and packaging axes
• 85 mm (NEMA 34): heavier indexing tables and gantry axes, or use a geared 42/60 mm type instead

A gearhead multiplies output torque, divides the step angle and, just as importantly, reduces the load inertia reflected to the motor by the square of the ratio. For large rotating tables or heavy pulleys, a geared stepper is often the cleanest way to keep the inertia ratio in a safe range.

Oriental Motor offers four gear families on PKP 2-phase motors. The choice depends on how much backlash your process can tolerate and how much torque and shaft load you need.

• SH (spur, offset shaft): economical, 28/42/60 mm; now with backlash guaranteed as a specification (for example 60 arcmin or less on some 42 mm ratios) and up to 60% higher permissible speed than earlier models
• CS (spur, centre shaft): larger output shaft and higher permissible torque and load without moving to a bigger gear frame
• Neugart PLE planetary: hardened, honed gears for high torque density and low backlash (about 7–19 arcmin depending on size and ratio); motor and gearhead are supplied separately
• Harmonic: non-backlash, lost motion of 1.5 arcmin or less, ratios of 30, 50 and 100 (0.018° per step at 100:1) for precision stages and large-inertia loads

Unipolar PKP motors have 6 lead wires and run on a simpler unipolar driver; bipolar motors have 4 leads and need a bipolar driver such as the CVD. For the same frame, the bipolar version generally gives more torque: for example, the 42 mm unipolar PKP reaches about 0.75 N·m, while the bipolar version reaches about 0.99 N·m.

Oriental Motor explains the trade-off in terms of how much of the coil is used. Unipolar and bipolar half-coil connections use fewer turns, so low-speed torque is lower but torque holds up to higher speeds. Bipolar series uses the full coil and gives the best low-speed torque, but torque falls faster at speed because of higher inductance. Bipolar parallel gives good torque across the range but needs about 40% more current. For new designs with CVD drivers, bipolar is the usual choice; unipolar motors are typically ordered to replace existing machines built around unipolar drives.

Oriental Motor's sizing method works in the same order for every motor type: define the mechanism, list the required specifications, calculate inertia, torque and speed, pick a motor, and then confirm it against the speed-torque curve and inertia limits. Skipping the inertia step is the most common reason for steppers that stall or vibrate on site.

As a rule of thumb from Oriental Motor, an open-loop stepper can handle roughly 10 times its rotor inertia, compared with about 30 times for a closed-loop stepper such as the AZ Series and about 100 times for a servo. If your load inertia is higher, add a gearhead or move to a closed-loop motor.

• 1. Mechanism: direct drive, ball screw, belt and pulley or rack and pinion — note load mass, diameters, lead, friction and efficiency
• 2. Requirements: stroke, positioning time, stopping accuracy, holding needs, environment and supply voltage
• 3. Load inertia: calculate it and compare it with the motor rotor inertia
• 4. Torque: required torque = (load torque + acceleration torque) × safety factor
• 5. Speed: convert travel and positioning time (including acceleration time) into motor r/min or pulse speed
• 6. Confirm: check the operating point sits well inside the pull-out torque curve of the motor and driver combination, then choose encoder, brake or gear options

Choose a PKP motor when the load is predictable, moves are short and frequent, and the lowest cost per axis matters. Steppers give high torque at low and medium speed, excellent response to pulses and firm holding at standstill, all without tuning. Add a PKP encoder type if you want to confirm position or detect a stall through the controller.

Choose the AZ Series closed-loop stepper when a missed step would scrap product, when loads vary, or when you want absolute position without homing after a power cut. Choose a servo for long, fast, continuous moves, high peak torque at speed, or very high inertia ratios. Our blog post on the difference between servo and stepper motors walks through the trade-offs with a comparison table.