Parker: Frameless Brushless Torque Motor Kit Version (400STK Series)
With the STK range, Parker offers a solution for direct drive applications. Examples include:
Applications needing total integration of the motor within the servomechanism due to volume and/or weight considerations.
Applications needing a large-diameter hollow shaft demanding a ring-type technology.
Continuous torque from 14 to 2700 Nm with natural convection (according to size), and up to 6100 Nm with fluid cooling.
Six external diameters from 145 to 800 mm.
Internal diameters from 56 to 630 mm.
Various windings available from 30 to 1500 min-1 according to size.
Parker’s STK range of permanent magnet brushless motors has been specially designed for direct drive (without a gearbox), particularly in applications needing very low volume and weight in relation to the torque and power requirements.
Electromagnetic and thermal optimization has permitted continuous torque-to-weight ratios up to 14 Nm/kg to be achieved with natural convection and up to 31 Nm/kg with fluid cooling.
The Parker STK range of frameless motors addresses very demanding industrial applications, not only in terms of performance (dynamic performance, torque capability and precise rotation at low speed) but also in terms of cost. Particular attention has therefore been paid in the design to the combination of technical and economic optimisation.
Industrial users can sometimes be dissatisfied with direct drive servo systems because they are frequently based on torque motors with limited power and speed capability. With the STK range, Parker set out to counter this objection by creating motors that could not only satisfy low-speed applications with their associated speed regulation, but could also address direct drive applications up to 30 kW in natural convection at speeds up to 1500 min-1. The various speed ratings are achieved by having a range of available windings. Some of these are illustrated in the catalogue, but numerous winding variations are possible to allow the drive system to be optimised.
STATOR: this consists of iron laminations incorporating the windings and attached to the external aluminium housing. The windings are encapsulated in resin. The housing may incorporate a cooling circuit when fluid cooling is required.
ROTOR: rare earth magnets protected against corrosion are attached to the periphery of a magnetic iron ring. Stator and rotor mounting: STK armatures and rotors can be optionally shipped mounted on a centering and positioning flange which eliminates the need to mount and centre the rotor inside the stator.
The stator is the source of copper losses as well as hysteresis and eddy current losses. It is necessary to take this into account when incorporating the motor. Here are the main points to be taken in consideration:
Continuous torque ratings are indicated for a copper temperature rise of 120°C for housings in contact with ambient air, or in contact over the full peripheral area with a metal component in contact with ambient air. In addition, the motor housing must be fixed on a metal flange having an area equal to at least twice the housing area.
For example, for a 400 mm diameter motor, the minimum flange area will be equal to
2x (π x 0.4²/4) = 0.25 m²
Avoid an enclosed environment, or if this is unavoidable, consult us for calculation of the motor derating.
Be sure that the materials located in the vicinity of the motor can withstand high temperatures. If this is not the case, consult us for recommendations on derating the motor.
To avoid overheating due to environmental factors, or to achieve a higher continuous torque rating than is attainable with natural convection, it is possible to use fluid cooling.
Fluid cooling specifications are based on two operating points:
The outside of the stator is machined with a helical groove which forms a cooling circuit when the stator is fitted inside an outer housing. Circular grooves at each end of the helix form input and output channels; the input and output ports in the outer housing must be axially aligned with these grooves. Additional circular grooves at each end of the stator are fitted with O-rings to form a seal.
Use either softened water with a glycol additive or a fluid approved for closed-circuit cooling in order to minimise the risk of corrosion and deposits.
Parker STK frameless motors have been designed for minimum torque harmonics when they are driven by a sinusoidal brushless drive. Various positioning sensors are available, and the drive may use 4 types of positioning interface:
In a high-quality resolver (e.g. Parker RES FC6 72 32), the accuracy is limited to around 1 arc-minute. In most of the resolvers currently available, accuracy is of the order of several arc-minutes. It is necessary to check whether the desired positioning accuracy is compatible with the resolver accuracy.
On another hand, resolvers can limit the range of applications when they obstruct a hollow shaft. Finally, resolvers are available with resolutions up to several hundred thousand counts per revolution, but such a high resolution is generally not useable because it is not compatible with the accuracy of the measurement system.
TTL Encoder or TTL Optical Scale
TTL encoders usually have a limited resolution (from 500 to 5000 counts/rev.). However it is possible to find encoders with hollow shafts up to 50 mm internal diameter and with resolutions between 15 000 and 20 000 counts/rev. (multiplied by 4 after decoding).
The best solution in terms of resolution and accuracy is generally offered by optical scales with pitches from 0.5 to 5 μm that can be attached to a hollow hub with the appropriate diameter. They can offer resolutions of 1 million counts/rev or more depending on the diameter.
The associated read heads have a maximum operating frequency which limits the speed for a given resolution.
Absolute encoders offer the characteristics described for TTL or Sin-Cos encoders without the need for a startup sequence for phase commutation (see following paragraph). Also they avoid the need for a homing sequence to locate the axis zero point.
SinCos® Encoder or SinCos® Scales
These encoders and scales are very widely used. The same limitations as previously mentioned apply to these devices. However, the advantage of this technology lies in the possibility of interpolating the sinusoidal output using an appropriate drive interface. Thus an encoder with 1024 sine waves per revolution used with x1000 interpolation will produce one million counts/rev. The Sin-Cos optical scale has the advantage of producing high resolution either directly or by internal drive interpolation, together with an unrestricted hollow shaft, since it may be attached to an appropriate diameter hub.
Permanent magnet synchronous motors require a constant phase angle between the rotating fields of the stator and rotor in order to control the torque. The resolver provides the necessary phasing information and simultaneously gives the axis position (within one polar pitch). Absolute encoders also provide this phasing information, but this is not the case with incremental encoders or scales. Encoder suppliers therefore offer specific ranges for brushless motors including either:
Three-phase commutation rectangular signals (U, V, W, /U, /V, /W) in the case of a TTL encoder; these waveforms must have the same number of cycles per revolution as the number of pole-pairs in the motor.
Sinewave signals (1 cycle per revolution), sine and cosine, giving the absolute position within one revolution in the case of sin/cos encoders. The drive electronic interface multiplies this frequency by the number of motor pole pairs.
In the case of an optical scale mounted on a hub, the information related to the phasing between the stator and rotor fields is not known. Therefore an initialisation sequence is needed during start-up; during that sequence the rotor will perform an indexing motion, or at least a small oscillation.
Model No.: 400STK2M, 400STK4M, 400STK6M, 400STK8M