ABB has developed and patented a concept for determining the speed of slip-ring motors by measuring the voltage in the rotor.

Drive systems based on slip-ring AC motors and ASTAT speed control from ABB are widely used in industrial cranes. The voltage of the stator winding is usually controlled with thyristors. To ensure stable control of the load, the control system requires information about the motor speed. Up to now, tachometers or encoders have been used to measure the motor speed. However, tachometers with their associated equipment tend to be mechanically sensitive and are often difficult to install, due to space constraints, says ABB. The new control concept uses the frequency of the rotor to determine the speed.

The new approach overcomes the shortcomings of previous methods, ABB claims. A measuring device measures the voltage in the rotor winding over a period of time. A calculation unit uses the voltage input signal to determine the frequency of the rotor voltage. It is then possible to calculate the slip and, from this, the rotor speed.

The concept includes a signal processor to suppress noise in the measured voltage signal prior to the calculations. It also transforms and filters the analogue signal output. In addition, the signal processor includes an A/D converter to give a digital signal output. A mathematical algorithm is used in the calculation unit to determine the voltage frequency, slip and rotor speed. A microprocessor provides the calculation and other functions, which gives a very robust design with no additional movable parts.

Dr Alojz Slutej of ABB Crane Systems in Sweden said: ‘This is a breakthrough and very important for our customers, they will now have better possibilities to handle their cranes.’

The new measurement concept has variants, offering different refinements. In one variant, the rotor with its resistance and contactors is monitored to detect any faults. This variant covers to monitor the individual drives for a crane travel four corner drive. Another variant includes a method to estimate the electromagnetic and shaft torque of a slip-ring motor.

Since the mid-1970s ABB has been supplying advanced AC crane drive systems. With the introduction of the new measurement concept, AC drives with slip-ring motors and the ASTAT control system will provide still more accurate and dependable control of the load, the company says.