Test stands benefit from direct torque control

Rigs constructed to test rotating machines like gears, engines, and complete cars require high accuracy and dynamic load control. Torque control capability in ac motors that drive the test stands is particularly useful for newer, complex electronic functions (such as antilock brake system, electric power steering, or dual-clutch transmissions for automotive applications).


Rigs constructed to test rotating machines like gears, engines, and complete cars require high accuracy and dynamic load control. Torque control capability in ac motors that drive the test stands is particularly useful for newer, complex electronic functions (such as antilock brake system, electric power steering, or dual-clutch transmissions for automotive applications).

Control of speed and torque is paramount when manufacturing test rigs for engines, transmission, or chassis dynamometers with high-performance requirements, including how the drive (inverter) controls the ac motor.

Drives for ac motors can use direct-torque control (DTC) technology, developed by ABB. The principle of DTC can be illustrated most accurately via this mechanical analogy: the continuous calculation of the best angle at which to rotate a shaft, with a given arm length and the available forces. Electrical force vectors are generated with the help of semiconductor switches called insulated-gate bipolar transistors (IGBTs).

DTC can improve testing; here's how. Chassis dynamometers are typically used to test the vehicle performance, exhaust emission, fuel consumption, noise and fine-tuning of exhaust and catalytic converter and motor fuel-injection system. Dynamometers with ac motor technology offer high accuracy, dynamics, and energy savings.

Dynamometers should precisely simulate a real highway. During acceleration and deceleration, dynamometers should dynamically compensate roll inertia to match the mass of the vehicle under test and an actual road. For such online compensation, roll motor load torque must be controlled with precision at each speed.

Motors, Devices & Motion Control

Direct-torque control continuously calculates the correct angle to rotate the shaft with the forces available, as this diagram from ABB shows.

Using DTC, motor status can be evaluated every 25values for leakage and magnetizing inductances, stator resistances, and their saturation behavior. Thus, the motor model guarantees that the dynamics, accuracy, and repeatability are optimized, although the operational point might vary significantly (according to testing needs).

Torque accuracy for DTC is

Torque linearity means that, with a certain torque reference, actual shaft torque must remain the same, regardless of drive speed and torque (motoring or generating torque). For DTC, the non-linearity is

Engine test stand dynamics

To ensure accurate simulation and testing for engine dynamometers, dynamic performance is key to verify the engine performs as designed. Test system dynamic performance can be quantified by looking at the delay from reference change to change in ac motor torque.

Electrical system dynamics include electrical and mechanical characteristics of the ac machine (leakage inductance and inertia); torque-control cycle of the ac drive; and any delay from speed/torque reference via any drive interface to the control-cycle loop itself. DTC controls motor torque every 25

Once torque reference is changed, DTC automatically selects the best voltage vector to achieve desired torque and checks every 25namometers, leakage inductances typically are smaller, requiring shorter current- and torque-rise times.

Transmission test stands

Testing gear-shifting-and-synchronization, calibration of automatic transmissions, clutching, and durability—are typical testing needs. Inherently, these cases require a capability to change load torque very quickly. And, transmission test-stand configuration can include several motors—one simulating the engine, and two or more for load simulation. This requires mutual coordination of drives operation; faster, more accurate coordination more closely simulates real-world conditions like differential-gear operation.

By exchanging speed/torque signals via optical link, DTC-based drives provide reference to follower drives to assess and react to desired load sharing (or as additional inputs to the main drive speed/torque reference). Other complex functions to calculate speed/torque references to individual drives also can be achieved. This is useful in setting up and delivering testing in time-critical operations.

Repeatability is of particular importance when testing engines and complete cars for emissions. Torque repeatability for DTC is less than

Torque control principles

With DTC, IGBT switch changes are based directly on the electromagnetic state of the motor. Optimal switching is determined for every control cycle at 25

In PWM drives, output frequency and voltage are the primary control reference signals for the power switches, rather than the desired motor-shaft torque. Because torque and flux references are compared to actual values in hysteresis controllers every 25quency.

Torque is calculated as a cross product between stator flux and stator current. Stator flux is estimated from the stator voltage vector and stator current. Six voltage vectors and two zero-vectors control stator flux and torque. Stator-flux amplitude is controlled to be constant.

Regeneration—energy generated from the motor, when the momentum of a load continues to turn the motor shaft—also is an inherent part of test stands. A common dc bus construction/installation facilitates connection of several inverters to the same dc bus—another inverter also running the equipment under the test can use power from the regenerating inverter. Fully regenerative inverters also can feed energy into the supplying ac network (grid or self-standing generating equipment), saving significant energy.

When high speeds and high masses are involved, the safety of a complete test cell is an essential design aspect. DTC drives also offer over-speed limits in torque control mode; limitation of torque; prevention of unexpected start; emergency stop; and other built-in safety functions. For example, if the speed feedback signal from the encoder is lost, a DTC drive can recognize this within a few milliseconds, automatically switching to motor-model estimated speed, and alerts the operator via an alarm or fault signal. Most importantly, it prevents motor acceleration to over speed under an alarm event.


Author Information

Vesa Manninen is ABB OEM sales manager for drives; Steve Weingarth is a manager of the U.S. drives applications team; Julian Hobbins is a senior U.K. sales engineer;

Look inside direct-torque control

For graphics illustrating the following points of DTC, along with equations for torque and stator flux, read this online at

Linearity performance: Slow reversing from nominal-generating torque to nominal-motoring torque at 40 Hz mechanical speed;

Torque rise time seen by DTC control for 10% change (for the motor cited, about 0.4 ms);

Standard 1,500-rpm ac motor; torque rise time for nominal load step measured from torque sensor when shaft has been mechanically locked. Damping oscillation in measurement signal is due to mechanical torsion in locked shaft and sensor itself;

DTC core with torque and flux comparators, adaptive motor model, switching logic and dc voltage and phase-current measurements; and

Torque is calculated as a cross product between the stator flux and the stator current:

Ô = Ø s x i s

Stator flux is estimated from the stator voltage vector and the stator current:
Ø s = (U s - R s i s ) dt

i s Stator current vector
Ô Torque
Ø s Stator flux vector
R s Stator resistance
U s Stator voltage vector

Stator flux, stator current and rotor flux in stator all coordinate. Amplitude of the stator flux is kept constant.

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