Tram Controls - The "Handle" - For Power Application

For Power Application

The "handle", which is normally the larger of the two levers on the controller is the device whereby the motorman controls the amount of power to be applied to the motors. It is normally located in the centre of the top of the controller. It can only be operated when the key (see above) is in the forward or reverse position. When the key is in the off position, or is removed, the handle is locked. Interlocking devices also normally ensure that the key can only be removed when the handle is in the off position.

In most traditional trams increased power is applied by turning the handle clockwise through a series of steps (or "notches") until full power is applied. Power is cut off by turning the handle anti-clockwise.

DC traction motors have the advantage of being able to deliver their full torque virtually immediately, but this comes with the risk of either the wheels slipping or rough, jerky starts, which is both uncomfortable to passengers and can damage the vehicle. The power must be gradually applied to the motors. On older trams, this was achieved by the use of a number of resistors, which are interposed between the controller and the motors and which are normally located under the car, or on its roof. The more resistors that are activated, the less power is available for traction purposes. As fewer resistors are activated, more power is available for the motors.

The activation of all or some of the resistors is achieved by opening and closing various electrical circuits between the controller, resistors and motors. By operating the handle on the controller, the motorman is actually opening and closing those circuits. On older vehicles the driver opens and closes circuits directly (sometimes called "direct control") and on newer vehicles the circuits are opened and closed by relays (sometimes called "remote control").

In both direct and remote control systems the handle is connected to a vertical shaft inside the controller and circuits are connected to the shaft by means of copper contactors or "fingers". Direct control controllers have larger contactors and fingers, owing to the larger electrical currents flowing through these components, while remote control controllers, relying on relays (and thus smaller currents), can utilise smaller contacts and fingers. The handle is rotated around the axis of this shaft to redirect the power to different circuits. Notches in the shaft allow the motorman to know when he or she has cleanly opened the correct circuit. A notch or "position" corresponds to a circuit. Different controllers have different numbers of circuits attached to different numbers of resistors and therefore different numbers of notches.

In addition, the motors can be operated in series circuits for slow speed or in parallel circuits for higher speeds.

While acceleration requires the gradual application of power to the motors, the opposite is the case when slowing the tram down. Moving the handle slowly back through the notches (called "back-notching") can result in damage to the controller circuits caused by electrical arcing. Instead the handle is quickly rotated anti-clockwise back to the off position and the brakes used to slow the tram.

Running a tram in anything but full series or full parallel wastes electricity and runs the risk of overheating the resistors and thereby causing a fire. The normal operating positions on the controller are therefore either full series or full parallel, with speed being controlled by judicious application of power, coasting (with no power applied) and braking.

From a standing start the motorman moves the handle from the off position, releases the brake, and then turns it through the series notches, thereby reducing the amount of power to the resistors and increasing the amount of power to the motors. When the "full series" notch is reached all power is routed to the motors running in series (effectively half speed). The motorman may safely continue to operate the tram in full series. If greater speed is required the motorman continues to turn the handle past a "transition point" between series and parallel notches and the motorman progresses gradually through the parallel notches. Power is again partially diverted to the resistors, until "full parallel" is reached, when all power is again routed to the motors, this time running in parallel.