TechArchive

Original article date: October 1998

Three-phase induction moors can be braked mechanically or electrically. This article from Siemens describes the methods available for electrical braking.

All methods of electrical braking are based on the retarding force exerted on a conductor moving in as magnetic field and carrying induced current. There is no reliance on friction and consequently no mechanical wear and no need for maintenance. Another advantage of electrical braking methods is that they can subsequently be implemented on existing motors.

The disadvantage is that braking is purely dynamic and no braking is present when the motor is at rest – there is no holding brake.

Plug braking also known as reversal braking is the most popular and simplest method of electrical braking and requires little additional circuitry. Interchanging any two phase connections reverses the direction of the rotating field and causes the motor to brake with a mean torque slightly higher than the locked-rotor torque. Ac speed monitor is required to prevent the motor from running up in the reverse direction immediately after stopping.

A variant of plug braking used in hoisting gear drives with slipring induction motors is dynamic lowering braking. During lowering lowering the rotor resistance is artificially increased so as to shift the torque characteristic of the motor to a point below the load characteristic at which gentle lowering of the load is ensured.

Regenerative braking is not very popular because of the high outlay for the capacitor bank. Here the roles of the motor and driven machine are reversed temporarily in the super-synchronous range. If the braking energy is returned to the system the driven machine cannot be braked to less than synchronous speed.

To permit braking to continue at lower speeds the motor can be switched onto a capacitor bank which maintains regenerative operation for a certain time down to values below synchronous speed by supplying reactive current.

With short-circuit braking the stator terminals are disconnected from the supply and are subsequently short-circuited. It is important that the arc of the line switch be extinguished before the motor is short-circuited because otherwise a line short-circuit would occur. Short-circuit braking is very simple but does not permit prediction of the time required for braking to standstill. The method cannot be used where high moments of inertia are involved because the magnetic flux in the motor decays rapidly as does the braking effect.

With DC injection braking the stator is disconnected from the line and is excited with a DC current. The resulting braking torque curve closely resembles the mirror image of the motor torque curve its height depends on the DC current injected.

The magnetic field produced by the DC braking current flowing in the stator winding can be determined from the magnetisation curve of the motor after converting the DC current into the equivalent AC phase current. – this is the current producing the same MMF and hence the same torque as the DC current.

As the motor torque is proportional to the air-gap power which in turn is proportional to the rotor power loss it holds that for a given slip and with the magnetising current being neglected the motor torque is also proportional to the square of the load current.

But calculating the braking torque as a function of speed at any point involves the planning engineer in extensive calculation work. Hence normal practice is to calculate the initial braking torque calculate one point of the braking torque characteristic from catalogue data and the equivalent AC phase current and assume linearity. This is reasonably accurate for cage induction motors.

• Siemens
• 0161 446 5308

October 1998