Torque motors do the trick

Torque motors are frameless kit motors consisting of a permanent-magnet rotor and laminated stator.

A bridge keeps the rotor and stator aligned while the torque motor is assembled.

Torque motors come in a variety of sizes, with diameters ranging from less than 100 mm to greater than 2 meters. These ETEL motors, models TmA 0450, 0291 and 0175, have model numbers that correspond to their diameters. Motor diameter is analogous to frame sizes for conventional brushless-dc servomotors. For each diameter, there are several axial lengths, so engineers have a range of sizes to satisfy torque requirements for given applications.

A torque motor is part of this rotary table

Torque motors have been around since at least 1970, and like most servomotors of that era, they used motors with brushes. As servomotors adopted brushless technology, however, torque motors followed suit and brushless torque motors now dominate the market. They have several advantages over other motors. For example, they have:

• Small electrical time constants and, therefore, high dynamic responses.
• Large mechanical air gaps (0.5 to 1.5 mm), so mounting and alignment is easy.
• And they use permanent magnets for high efficiency.

But the most obvious feature of torque motors is that they have relatively large diameter-to-length ratios, and short axial dimensions. Additionally, torque motors can have large outer and inner diameters, resulting in a motor that is a little more than a thin ring. Thus, mass can be quite low as a function of diameter.

The large diameter helps the motor develop high torque levels. It gives the motor a large lever arm to generate high levels of torque. The large diameter also provides ample room along the circumference for powerful rare-earth magnets. As an extreme example, a torque motor for a telescope drive has a diameter of 2.5 m and a length of only 50 mm and it still produces a continuous torque greater than 10,000 N-m.

Torque motors are also "frameless" motors. This means they don't have housings, bearings, or feedback devices. In this sense the motor is a kit and meant to be part of the machine structure. Torque motors can come with a reusable assembly aid called a "bridge," which is set at the factory to ensure the rotor and stator are aligned for assembly. The bridge also keeps the magnetic field within the motor, thereby eliminating the need for special nonferrous assembly areas, and preventing damage to the rotor from metal scraps and loose screws.

Torque motors are designed as direct drives. They eliminate the need for gearboxes, worm-gear drives, and other mechanical-transmission elements and directly couple the payload to the drive. This makes possible drives with high dynamic responses and no hysteresis. Angular stiffness can also be extremely high, on the order of 100 N-m/arc-sec for motors with peak torque of 2,500 N-m.

The large inner diameter of a torque motor can be a plus for machine tools. Essentially it is a large hollow shaft, giving designers more flexibility in locating the motor. In most cases, motors can be optimally located with respect to support bearings, feedback devices, and payloads. This means adding the motor does not add excessive moving mass or inertia.

Torque motors are available in a wide range of sizes, with diameters from smaller than 100 mm to larger than 2 m. But 1.2 m is typically the largest for machine-tool applications. Motor diameter is analogous to frame size for conventional brushless-dc servomotors. And for a given diameter, several axial lengths are available. This lets designers choose from a wide range of motor sizes to satisfy torque requirements.

Torque motors have a relatively large number of magnetic pole-pairs. Consequently, there are many permanent magnets on the rotor. This means torque motors can be built as thin rings. It also means they can have smooth velocity regulation with low ripple. However, eddy-current losses in brushless motors increase with the number of pole pairs, which constrains the maximum practical number of pole pairs. As a result, torque motors are primarily designed for low-speed applications, generally below 1,000 rpm, which is more than enough for many applications.

Torque motors produce high torque at stall and can have high dynamic stiffness. However, the motor alone does not determine dynamic stiffness or precision. To exploit the full benefits of direct-drive motors, a machine must have the necessary standards of precision and stiffness and use a high-performance control system.

Feedback and heat

High-precision, high-resolution feedback is essential for optimal performance of direct drives. Because loads are directly coupled to the drives, higher accuracy is possible. But positioning resolution is also in direct relation to the feedback's resolution, so it takes an optical encoder with a high line count (typically 9,000/ rev or more) and a high-resolution interpolation factor. System resolution should be typically below 1 arc-sec.

An important difference between direct-drive systems and those driven by conventional dc servomotors and gearboxes is that torque motors are found inside the axis and are part of the machine. This makes heat control more important. Conventional motors typically mount in less critical locations (e.g., at the end of a worm gear or sprocket), so heat removal is less of an issue and motors can run at higher temperatures. But most torque motors include provisions for liquid cooling. Liquid cooling effectively increases the continuous torque rating of the motor. Air cooling, while an option, is much less effective than liquid-based cooling.

Sizing and comparing torque motors

Sizing and selecting torque motors is similar to sizing and selecting conventional brushless-dc servomotors. But because of thermal considerations, designers must pay attention to certain factors associated with heat generation, most importantly, the thermal power dissipated by the motor.

The first step in selecting a motor is to determine the torque and speed required. Calculating rms torque lets designers estimate the power dissipation and resulting heat in the motor. Heat produced by the motor must be removed by a cooling system to avoid overheating machine elements.

When comparing torque motors, engineers rely on the motor constant, (K_m N-m/W). It is used to rate the relative efficiencies of motors from various manufacturers and shows the relationship between torque produced and resulting power losses. The motor with the highest K_m value is the most efficient torque generator.

K_m is a function of a motor's design and construction. It includes factors such as packing efficiency of windings, type and design of laminations, and electromagnetic circuit design. Therefore, it is a better indicator of motor performance than the torque constant, K_t (N-m/A), which relates torque output to supplied current. K_t is useful for matching motors to servoamplifiers, but it doesn't give information about the efficiency.

A torque motors is only one element in a complete system. It still needs a mechanical structure with high rigidity, bearings, and a feedback device. It is the overall integration of these elements that determines system performance.