Why Use AC 10 Thyristor Control?
The AC10 Thyristor Control system has been trusted for over forty (40) years, this system together with the slip-ring motor, is the preferred system for hot metal cranes and cranes operating in difficult environments. Motor size and drive setup are also considerations in the selection of the preferred drive setup. It’s a question of weighing up the risk of and probability of drive failure with the cost of production loss resulting from failure.
When failure does occur from conditions (heat, voltage, stability etc.), what action can be taken to get back into production? The first comparison is the frequency controller and cage motor combination. This unit stands out as robust, with no-slip rings and is half the price of larger motors. A natural preference for any drive system however, the only control for this system is the variable frequency system. The frequency control system is a higher technology than AC10 control, and, correctly set up, the control it provides is matchless. However, the rules applicable to frequency control must be followed. Most basic of rules pertain to temperature.
All controller makes have a temperature limit of 40C fully loaded. With certain de-rating, they can go to 50C. Both temperatures are dangerous for the long-term life span of the control card. An ambient temperature around 22-23C is safe. The fully loaded controller itself dissipates 3% of input power as heat. If the controller is 200kw, it dissipates 6kw to the environment it is in. If the crane handles molten metal, the normal ambient (especially in summer, in hot climates), will easily be 40C. The metal is around 1600C, tremendous heat is radiated from it. In this well understood situation, frequency controllers must be cooled down with air conditioning of adequate capacity.
Then the reliability of the air condition unit comes into question. There are excellent units, but they must be checked and maintained. Our experience with them in certain countries has been disappointing and knowing this situation, some metal producers decline air conditioner units, saying that sooner or later they will fail. Hot metal hoists go up 480t capacity.
Large hoists will have multiple motors, and if frequency-controlled, the controllers will be large. Usually, a failed controller must be changed. The hot metal load can be hanging and roasting the crane. At least one (very expensive) controller must be held in stock, programmed and ready to use. The objective here is to simply compare Frequency Control with AC10 Thyristor Control so the customer can make an informed decision.
AC10 Electronic Assemblies
All closed-loop control systems have a power converter to control magnitude and polarity of power to the motor, a unit to control the power assembly, and a reference generator to convert digital inputs from the crane operator. With AC10, they are separate units: A1 for the thyristor power assembly, A2 for the modular control unit, and A3 for the reference generator. The A1 power assembly is based on Semikron SKKT Semipak devices (PIV rating 1800V) on a common heatsink, up to 180kw motor rating.
Five sets of back-to-back disc devices in cassettes are employed for higher ratings – up to 800kw total motor capacity. The assembly is complete with cooling fan, temperature device and snubbers across devices.
The A2 control assembly – known as the AC10 Eurorack – has separate plug-in cards for distinct control functions. They are separately listed and described. This is the original analogue controller – AC10 A. A digital
option – AC10 D, with built-in reference card, is also available.
The A3 reference unit has terminals for logic inputs from the master controller for the motion. Fixed speeds (up to five) are set for the controller notch positions, with 0 = full speed hoist of fwd., 5 = neutral, and 10 = full speed lower or rev. This is also a useful replacement for old-fashioned AC10 reference cards – with individual pots – dating well into last century. It is low cost, and easy to install. Once set, the speeds do not drift.
AC10 CONTROL PANEL – HOIST
The assembled control panel has a current-limiting circuit breaker for isolation and fault protection. The stator has a single power-on contractor, and three CT’s feeding an M330 Crane Protection Monitor. Three separate
CT’s feed an ammeter in each stator phase.
The meter cluster also shows -90-0+90V for tacho feedback, and 0-10V for speed reference. The E-Control system has a supply V/23V synchronizing transformer feeding the A2 controller. For correct phase switching the control vectors must be in line with the fundamental.
Two rotor contactors switch the external rotor resistor, one intermediate for optimum torque and one final to liftlower the motor rated speed, in controller notch 4 – H or L. The system includes LC1 – D12 contractors for master relay MR, and brake contractor BC, both with GV2 CB protection. Control logic and small-time functions (brake control and Rc1) are solid-state in a LOGO smart relay, with screen. Overall, the system is straightforward, robust, and modular.
AC10 TRAVEL DRIVE
The electronic units (A1-A2-A3) are the same as on a hoist. Thyristors are sized to the load, in this case a twomotor drive. Standard PIV ratings are 1600-1800V. Travel motors have a fixed slip resistor per phase (kx0.25). There are no rotor contactors. Standard thermal protection monitors are used for each motor, and a single ammeter is provided per motor, plus then tacho and reference meters.
AC10-A CIRCUIT
The A2 controller in this schematic is for the original AC10 analogue (AC10-A) configuration. The A3 reference generator is external to the A2 controller. The new AC10-AR2 reference module has opto-isolated inputs and an electronic potentiometer with display. It locks the reference set for each speed – zero drift.
AC10-D CIRCUIT
The control and safety functions between A and D versions are the same. The A2 controller is a sophisticated upgrade, with speed-setting, screen for reference-speed, and fault lamps. Control logic and time functions are
in a LOGO smart relay.
A2 Control Assembly – A&D
A2 Controller A is the original A is the original analogue format. All components in it are discrete, and speeds and direction are set by steps of 0-10V from an external reference unit. It is analogue because there are no digital settings.
Individual AC10 cards, most with large LED status indication, are plugged into a Eurorack, control rack, with motherboard at the back. The arrangement is modular. It has the following cards:
- A124 I/O connect card.
- A128 Speed Loop Supervisor card. Specify poles.
- A171 Monitor card.
- A130 Hoist amplifier.
- A131 Travel amplifier. Specify poles.
- A139 Rotor contactor card. Specify poles.
- A121 Phase shi@ card.
- A123 10V supply card.
- A119 Gate driver cards (Qty 5), to thyristors T1-T5.
- A125 Gate connect card.
AC10-A is specified by long standing clients for upgrades and commonality between old and new machines to the plant.
A2 controller D typically goes to new plants (this century). It is digital because speeds and direcRon are set by
number. It is digital because speeds and direcRon are set by numbers. It has the following different cards:
- A151 Reference card
- A128A Monitor card A
- A128B Monitor card B
The remaining active cards are common to A & D controllers. The A128A card does the speed loop function. It will be described in this document. The AC10-D circuit diagram shows the inclusion of a LOGO smart relay. Control logic and time functions are controlled by it. Time concerns application of a service break 0.5-1 sec and is critical.
A3 Reference Module for AC10- A Control
The AC10-AR2 reference module converts the stepped logic inputs for direction (H-L) and speeds (ST2-4) per controller notch, to set analogue values in the range 0-10V. This then is the direction-speed input reference to the AC10-A controller. The two program buttons are for Hoist, (of FWD) range 5-0V, and lower (or REV), range 5-10V. With AC10, the slowest speed is 10% so the effective settings are 4.5-0V for Hoist, and 5.5-10V. With AC10, the slowest speed is 10%, so the effective settings are 4.5-0V. This then is the direction-speed input reference to the AC10-A controller. The two program buttons are for hoist, (or FWD) range 5-0V, and lower (or Rev), range 5-10V. With AC10, the slowest speed is 10%, so the effective settings are 4.5-0V for hoist, and 5.5-10V for lower. With the controller in neutral the display is 5.0. The unit does not allow 5V with FWD-H input, or 5V with Rev-L input. Also, it only allows increased speeds for higher notch positions.
To program speeds for each notch:
- With the controller in any notch, the A3 to A2 controller unit knows the notch position and direction.
With the controller in any notch:
- Depress the ↑ and ↓ buttons at the same time to set Program Mode.
- The Program Lamp illuminates, if the controller is positioned to notch 1 Hoist (input Fwd./H), click the ↓ button to say 4.0. This is 20% hoist speed. Switch to another notch, and the notch 1 hoist speed is locked.
- If now in notch 2 Hoist (inputs Fwd./ H + ST2), press the ↑ ↓ buttons to program, then click the ↓ button to program, then click the ↓ button to a faster reference speed. Set to 3.0, the hoist speed will be 40%. Switch out, and this speed is locked.
- In notch 3 Hoist (inputs Fwd. / H + ST2 + ST3), typical reference values will be 2.5 (50% speed), or 2.0 (60% speed). With AC10, this is the high intermediate hoist (and lower) speed.
- If now in notch 3 Hoist (inputs Fwd. / H + ST2 + ST3), typical reference values will be 2.5 (50% speed), or 2.0 (60% speed). With AC10, this is the high intermediate hoist (and lower) speed.
- In notch 4 Hoist (reference 0.0) or notch 4 lower (reference 10.0) the motor will accelerate to rated hoist-lower speeds. Intermediate lower reference speeds (Rev/L+–) will typically be 6.0 (20% speed), 7.0 (40% speed) and 7.5 or 8.0 (50 and 60% speeds).
AC10 CONTROL TECHNOLOGY
AC10 is a fixed-frequency control. The motor is a slip-ring unit, and it drives a DC tachogenerator to provide exact speed and direction to the A2 controller.
CB is a conventional current-limiting circuit breaker – typical NSX series from Schneider Electric. MC is a conventional contactor – AC1 rating is fine, but to be conservative, we use AC3 ratings, also for rotor contactors
Rc1 & Rc2.
Thyristor switches T1-T5 are SKKT Semipak (back-to-back) devices, with an 1800V PIV rating. Separate disc devices are used for higher power ratings. Switches T1T2-T3 are ON for positive torque. Switches T4-T1-T5
are ON for negative torque. There is a blocking period between them, (like a reversing contactor).
Output voltage V2 is determined by the conduction angle (), of the devices. With full conduction ( = 0), V2 = V1 less a small volt drop across the devices. There is a quadratic relationship between voltage and torque, based
on: Torque T = (V2 / V1)2
The basic motor-electronic setup is:
This generalized equation does not quantify the torque. It applies to all motors operating on fixed frequency. It states that the torque developed by the motor varies as the square of the voltage applied to it. It also applies to cage motors on fixed frequency control – the electronic soft starter controls angle during starting.
The slip-ring motor is indicated as wound delta-star, but it can be wound star-delta. Apart from its physical construction, the electrical properties are resistance r-, and reactance x-. Resistance r is constant (with a small heat deviation), and reactance x is derived by: Reactance x = 2 f L with L = inductance
Inductance L is based mainly on the number of turns, which are fixed for each motor, and 2, and f are constants, so on fixed frequency, stator reactance x is a constant. Note- reactance x is a direct function of frequency f. x = f(f). This has strong relevance to variable frequency control. Magnetizing current Im through the stator coils is a direct function of voltage V2, based on:
Im = V2
√ (r12 + x12)
Stator flux Fs is directly proportional to magnetizing amps Im, in turn to voltage V2. Rotor flux Fr is directly proportional to rotor current Ir, and this in turn is a function of speed (motor slip s), rotor external resistance REX, and the driven load.
Motor torque T-MOTOR is the product Fs × Fr. Like a transformer, voltage V2 (then Ir & Fr) is induced by Fs. They change in proportion to V2 (then Im & Fs), and the torque product of the two changes as a quadratic function – such as:
Fs × Fr = T-MOTOR. Hence- Torque T = V2 2 2 2 4 V1
The A130 amplifier drives the polarity and magnitude of output V2 to match speedback fb to reference ref. It starts with full conduction ( = 0) to lift or lower to ref, and then adjusts to match motor torque with load torque to hold at the speed. This is a closed-loop control function.
The sum R1-R3 = rotor k × 0.5. With = 0 and starRng at standsRll, rotor amps 1.0 = In × 2 and motor torque = Tn × 2. (per-unit values and n = nominal or rated motor values). The curve is: Q1 speed-torque 0.5 Rotor k = V2 √3 × I2 0 0 0.5 1.0 1.5 2.0 Torque V2 and I2 are rated motor (nameplate) values.
To finish off the power side: The motor will accelerate as long as T-MOTOR T-LOAD. In the rotor, resistance r2 is constant, but reactance x2 varies with speed and quadrant of operation. Rotor frequency f-r is 50 Hz at standstill
(slip s = 1), 0 Hz at synchronous speed (slip s = 0), and 100 Hz at start of braking from Notch 4 lower (slip s = 2).
The full Q1-Q4 hoist performance curve is:
Rotor voltage Vr is a direct function of slip s, so it reduces close to zero as the motor accelerates in Quadrant 1 (hoist), and jumps to a factor of 2, on start of reverse braking in Quadrant 4 (lower). At this point, reactance x2 doubles with the 100 Hz frequency. Rotor card A139 closes rotor contactors Rc1 and Rc2 at set points in Q1 hoist and Q3 drive-down lower. They are open in Q1 drive-down from hoist, and Q4 retard from lower.
The curves bend due to the impact of reactance x2 in quadrants 1 and 4. The option of four slow speeds is available – no extra. Simply the controller must be five notch. Due to motor regulation and the 5% R3 slip resistor, the motor runs slightly above synchronous speed in L4 Q4. Here it acts as a generator, returning system energy, minus system losses, back to the mains. Thyristors T4, T2 and T5 are ON. The level of returned power is based on mass m × gravity g × speed × efficiency .
AC10 for Travel Drives.
The control circuit is the same for travels and hoists. The power circuit is simplified by the absence of rotor contactors. Each motor (there can be two, four or more, fed from the A1 assembly) has a permanent slip resistor
with a grading of rotor k × 0.25. Like hoist motions, the resistor has a NEMA 90 continuous thermal rating.
Travel drives accelerate and retard the heavy machine plus load inertia. Once the motion is up to speed, there is zero speed change with time (d/dt = 0). The load drops right down to the level to overcome rolling resistance. Resistor R enables correct acceleration torque, and it does not affect rated running speed.
The speeds indicated are typical, and the controller can be 5-notch, with an additional intermediate speed.
