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1.3
Principles of the SEW brake
1.3.1Principles of project planning
The SEW brake is a DC-operated electromagnetic disc brake with a DC coil which is opened electri-cally and braked using spring force. The system satisfies fundamental safety requirements: thebrake is applied if the power fails.
The principal parts of the brake system are the brake coil itself (accelerator coil + coil section =holding coil), consisting of the brake coil body (9) with an encapsulated winding and a tap (8), themoving pressure plate (6), the brake springs (7), the brake disc (1) and the brake bearing endshield (2).
The significant feature of SEW brakes is their very short length: the brake bearing end shield is apart of both the motor and the brake. The integrated construction of the SEW brake motor permitsparticularly compact and sturdy solutions.
167283910411500871AXX
1Brake disc
5Working air gap 9 Brake coil body
2Brake bearing end shield6Pressure plate10Motor shaft
3Carrier
7Brake spring11 Electromagnetic force
4Spring force8Brake coil
Fig. 1: Block diagram of the brakeDrive Engineering - Practical Implementation Vol. 41
1.3.2Basic function
In contrast to other DC-operated disc brakes, SEW brakes operate with a two coil system. Thepressure plate is forced against the brake disc by the brake springs when the electromagnet is de-energized. The motor is braked. The number and type of the brake springs determine the brakingtorque.
When the brake coil is connected to the appropriate DC voltage, the spring force (4) is overcomeby magnetic force (11), thereby bringing the pressure plate into contact with the brake coil body.The brake disc moves clear and the rotor can turn.Particularly short response timesWhen switching on:A special brake control system ensures that only the accelerator coil is switched on first followedby the holding coil (entire coil). The powerful impulse magnetization (high acceleration current) ofthe accelerator coil produces an especially short response time, particularly in large brakes, with-out however reaching the saturation limit (→ Fig. 2). The brake disc moves clear very swiftly andthe motor starts up with hardly any braking losses.
1)2)MTS3VACBS01873AEN
IBIHt120msAccelerationHolding00868AEN
BS Accelerator coil1) Brake
I Acceleration currentTSCoil section
2) Brake control system
IBBS + TS = Holding coil
H
Holding current
Fig. 2: Functional principles of the two coil brakeDrive Engineering - Practical Implementation Vol. 47
18
The particularly rapid response times of SEW brakes add up to a shorter motor startup time, mini-mum startup heating and therefore less energy consumption and negligible brake wear duringstartup (→ Fig. 3). These factors pay dividends to the user in the form of an extremely high startingfrequency and long brake service life.
1)2)ISISttt1t1MBMBttnntt00869AXX
ISCoil current1)Switch-on procedure for 2)Switch-on procedure for operation MBraking torqueoperation with a rectifier with SEW rectifier with switching nBSpeed
without switching electron-electronics, e.g. BGE (standard from t1
Brake response time
ics
size 112 upwards)
Fig. 3: Shortening the motor startup time with the SEW brake systemThe system switches over to the holding coil electronically as soon as the SEW brake has released.The braking magnet is now only sufficiently magnetized (with a small holding current) to ensurethat the pressure plate is held in the startup position with adequate security and with minimumbrake heating (→ Fig. 2).
Drive Engineering - Practical Implementation Vol. 41
When switching off:This means de-excitation occurs very rapidly when the coil is switched off, so the brake is appliedwith an extremely rapid reaction time, particularly with large brakes. This offers benefits to the userin the form of an especially short braking distance with a high repeat accuracy and high level ofsecurity, e.g. for applications involving drive units for vertical motion.
1)2)ISISttt2t2MBMBttnntt00872AXX
ICoil current1)Brake reaction to cut-off in the 2)Brake reaction to cut-off in the MSBraking torqueAC circuit
DC and AC circuits
nBSpeed
t2
Normal brake reaction time
Fig. 4: Shortening the braking distance by brake cut-off in the DC and AC circuitsThe response time for application of the brake is also dependent on how rapidly the energy storedin the brake coil can be dissipated when the electrical power is switched off. A free-wheeling diodeis used for dissipating the energy for a “cut-off in the AC circuit”. The current decays according toan e-function.
The current decays much more rapidly via the varistor when the DC circuit of the coil is interruptedat the same time, giving “cut-off in the DC and AC circuits”. The response time is significantlyshorter (→ Fig. 4, Fig. 5). Conventionally, “cut-off in the DC and AC circuits” is implemented usingan additional contact on the brake contactor (suitable for DC switching).
Under certain circumstances, it is beneficial to use the electronic relays SR and UR (→ Sec.2.2.2.2) for interrupting the DC circuit.
Drive Engineering - Practical Implementation Vol. 4→ Sec.109
110
MTS3VACBS001240ADE
Fig. 5: Principles of cut-off in the DC and AC circuitsParticularly quiet
Particularly quiet brake motors are demanded in many applications in the power range up toapprox. 5.5 kW in order to reduce noise pollution. SEW satisfies these conditions as standard in allbrake motors up to size 132S by means of appropriate design features, without affecting the partic-ular dynamic properties of the brake system.Particularly safe
The excitation power needed for the holding function may be too small in the event of a power fail-ure or especially severe voltage dips. The brake is applied for reasons of safety. A monitoring sys-tem ensures that the accelerator coil is reactivated when the voltage returns, thereby releasing thebrake.
Drive Engineering - Practical Implementation Vol. 411
98
11.7Working air gap with SEW brakes
Motor sizeBrake typeWorking air gap (mm)New settingReadjust at63B0371/80BMG05, BC0580BMG1, BC05min. 0.25max. 0.690/100BMG2, BC2100BMG4, BC2112/132SBMG8132M/160MBM15160L/180BM30min. 0.3max. 1.2200/225BM31180BM321)200/225BM621)min. 0.4max. 1.21) Double disc brake
Drive Engineering - Practical Implementation Vol. 4
