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Four-in-one: simplifying the reversing of three-phase motors

14 June 2011

In some motorised applications there is a need to reverse the direction of the motor to achieve a specific aim – sun-tracking solar panel systems being a good example. In this article, Doug Sherman describes an elegant method of reversing the direction of three-phase motors using a solid state relay package designed specifically for the purpose

Three-phase motors are used in many industrial applications to control loads such as pumps, compressors, valves, conveyors, and numerous other motor-driven devices. They are relatively simple in design, efficient, and have a high starting torque compared with single-phase motors. They are smaller and less expensive than single-phase motors with comparable ratings, and also tend to last longer than the latter at the same power rating.

Many loads driven by three-phase motors simply require the motor to turn on and off in order for them to perform their required function (see Figure 1). A typical load might be an industrial fan, which only needs the motor to rotate in one direction to achieve the desired air circulation. A compressor is another example. These applications typically use a simple three-phase solid state relay, contactor, or soft starter to energise the motor.

However, some applications are not so straightforward and require a bit more in the way of control than simply turning a motor on and off. Sun-tracking solar panel systems, for example, utilise motors to move the panels throughout the day so that they can follow the path of the sun across the sky. However, at the end of the day they must return to their original position in order to greet the sun on the following morning. This requires a controller that is not only able to energise the motor, but also reverse its direction when needed.

Reversing a three-phase motor
Figure 1 shows a simple wiring diagram for controlling a three-phase motor with a contactor. When the contactor is energised, it switches the three phases of the ac mains supply to the motor and it begins to rotate accordingly. It will continue to rotate at a constant speed and direction for as long as the contacts remain closed. However, if the connection of any two phases of the ac mains to the contactor are changed (connect L1 to terminal #2 and L2 to terminal #1, for example) the direction of the motor will reverse when it is re-energised.

Of course, physically changing the connection on the contactor every time you want to change the direction of the motor is not the most practical of methods, so a device is needed that can do this automatically when a ‘change of direction’ command is issued by the controller. Traditionally this was accomplished by using discrete components, multiple mechanical relays or, more conveniently, a three-phase motor-reversing contactor. But mechanical systems have their drawbacks as any user of electromechanical devices will attest. The most significant of these drawbacks is life expectancy, especially in applications where the motor is ‘bumped’ or ‘inched’ when required to reach a specific position.

One way of avoiding the problems associated with mechanical devices is to use multiple single-phase solid state relays. Referring to  Figure 2, phase L1 is connected directly to the motor. Solid state relay (SSR) #1 and SSR #3 connect either L2 or L3 to the second leg of the motor, while SSR #2 and SSR #4 connects either L2 and L3 to the third leg of the motor. When SSRs #1 and #2 are energised, the motor will rotate in one direction. To reverse the direction, SSRs #1 and #2 are de-energised, and SSRs #3 and #4 are energised, effectively swapping the connection of L2 and L3 to the motor.

There are, however, a few important points to note when using multiple SSRs in a motor-reversing application:
* The system controlling the SSRs must have an interlock circuit that prevents the ‘forward’ and ‘reverse’ relays from turning on simultaneously! Failure to comply with this requirement may result in a phase-to-phase short-circuit via the relays – with inevitable consequences!
* Relays with internal over-voltage protection must not be used in motor-reversing applications. An internal transient voltage surge may switch on the output of the SSR, effectively creating a phase-to-phase short-circuit. A Metal Oxide Varistor (MOV) may be placed across the output of each SSR to provide protection from such transients.
* A fifth SSR can be used to switch the third phase of the motor if this is required by the application. It is not necessary for this relay to be part of the interlock circuit, but it must be energised at the same time as the ‘forward’ or ‘reverse’ relays to prevent the motor from being damaged.

Motor-reversing solid state relays
Yet another, and often more preferred method, is to use a motor-reversing SSR. The Crydom 53 series motor-reversing SSR, for example, offers two significant advantages over the methods described above. Namely, all four SSRs are contained in one industry-standard three-phase SSR package, and the interlock circuit is already built into the relay.

As shown in Figure 3, two of the three phases are wired through the SSR and the third phase is connected directly to the motor. When a logic signal is applied to the ‘forward’ terminal, the SSR switches L1 and L2 directly to the motor. When the signal is removed from the ‘forward’ terminal and applied to the ‘reverse’ terminal, the SSR switches the connection of L1 and L2, effectively reversing the direction of the motor’s rotation. If a logic signal is simultaneously placed on the ‘forward’ and ‘reverse’ terminals, the relay will shut off. Table 1 shows the load-current path through the SSR outputs for a given input.

MOVs can be added externally to the SSR’s output circuit(s) to provide additional protection from over-voltage conditions. As shown in Figure 2, the SSR has four separate output circuits to provide the motor-reversing function (two for forward, and two for reverse), so four MOVs would be required. Moreover, as with any electrical circuit, proper fusing and a suitable disconnect from the ac mains supply will be required.

All SSRs dissipate thermal energy in their conduction of load current, and therefore often require the use of external heat sinks to maintain allowed operating temperatures. Information about heat sink selection is readily available from most SSR manufacturers.
Motor-reversing SSRs are suitable for many three-phase applications rated to 10hp (600V), 7.5hp (480V) and 3hp (240V). Renewable energy is a growth area for such devices, as are electronic valve controllers. However, there are many more potential applications, including materials handling machines, hoists, access control systems and machine tools.

Doug Sherman is Crydom’s field sales application engineering manager, EMEA region

NB - Please refer to the digital issue (accessible from the home page) in order to view the diagrams

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