Static Reversing 3 Phase SSS Schematic

Static Reversing the 3 Phase Induction Motor

This solid-state thyristor (SCR) switch circuit is perhaps the simplest means of reversing a 3 phase motor. “Static” is a catch-all term that essentially means without moving (mechanical) contacts –the traditional means of reversing is via a pair of contactors that swap two of the three AC lines. However, there are disadvantages to contactors as they are expensive, and have a finite life in repetitive reversing applications. Note that this circuit does not provide speed control as the motor runs at its base speed, nor does it provide zero voltage switching.

Static Reversing 3 Phase SSS Schematic

Static Reversing 3 Phase SSS Schematic

Gate Driver Control Schematic

Gate Driver Control Schematic

How it works

The SCRs that are to be conducting are given a repetitive gate pulse train that both turns them on and keeps them conducting. The gate pulses are generated via a 555 oscillator and isolated via a 4secondary pulse transformer. There is one such circuit for each direction. When the 555 is inhibited, the gate pulses cease and the SCR current is AC line commutated.

Why SCRs?

SCRs are a good deal more robust than TRIACs because they are specified at a higher junction temperature, have lower conduction losses, higher voltage rating, higher dV/dT rating, and higher I²T fault current rating. Of course, the disadvantage is the requirement for a pair of devices to conduct both half-cycles.

The semiconductor fuse

The semiconductor fuse is an absolute necessity to interrupt line-to-line fault current quickly enough to save the SCRs from destruction. If both directions get turned on simultaneously, a line-to-line short circuit occurs. The interrupting I²T of the fuse must be substantially less than the I²T of the power semiconductor. They tend to be expensive so I selected one that is on sale by Newark Electronics for $4 due to overstock. Make sure that you purchase about 5pieces –if you have only one available, Murphy’s Law comes into play… If your circuit is wired correctly, it should never need to be replaced.

The snubber

An R-C snubber is connected across each cell (anti-parallel pair). The resistor tends to absorb the energy caused by a line noise burst –such can occur when the power is switched on and can cause the SCRs to false-fire. It also absorbs the energy of the SCR turn-off voltage spike that is a function of rate of change of the SCR recovered charge current and series circuit inductance.

Gate-to-cathode capacitor

The addition of a 0.1uf capacitor between gate and cathode substantially increases the device dV /dT rating, and also reduces the noise sensitivity of the gate circuit.

Gate pulse transformer

Described is a unit that can be easily fabricated with the proper materials. Otherwise, the experimenter may have difficulty finding a pulse transformer with (4) secondaries. This particular one is designed for a peak primary voltage of 24V, but it may be modified to lower voltages via adjusting the number of primary turns. Another approach would be to parallel two pulse transformers with two secondaries each.

Pulse Transformer Photo

pulse transformer photo

Pulse Transformer Specifications

4 Secondary Pulse Transformer


The required logic needs to be a function accomplished in your microcontroller.

  1. It must NEVER set both outputs high at the same time.
  2. It must allow a minimum of (1) line cycle for the SCRs to line commutate before changing directions.
  3. It must otherwise allow a period of time between directions to allow the motor speed to decay. While the speed does not necessarily need to drop to zero, this will reduce motor temperature rise because “plugging” the motor when it running in the wrong direction results in very high motor currents.

Single phase test circuit

Since I do not have three phase power available to operate a motor, testing had to be single phase. Note that this requires only two SCRs connected anti-parallel and half of the control circuit.

Single Phase Test Circuit

Single phase test oscillograph

Single Phase Test Oscillograph


Undocumented words and idioms for our ESL friends

Murphy’s Law –idiom –what ever can go wrong, will go wrong…


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  • frfrfr15873yahoo-com

    in Static Reversing the 3 Phase Induction Motor diagrams i have some problems think we are in forward state then SCR ,s Q1,Q2,Q5 AND Q6 fire ON and in this case lines L1 and L2 short circuited to each other and connected M1 pin of motor i think this is uncorrect please help me to undrestand the problem

  • victor ghenno

    I didn’t say that your circuit does not work. I just suggests some kind of current protector to avoid destruction of components, This can be implemented with conventional electromechanical current relays. Rating of this relay must be selected in order to protect the weakest link in whole system. If you select 20 amps SCR, it can support three times this current for ten cycles. The driven motor consumes this same current to start -at least, nonetheless this can be larger- so the rating of driven motor must be of 10 running amps as a maximum to prevent possible risk of destruction on SCR’S and the current relay must be in that order. These relays are relatively slow,so the motor would start without any problem.

  • Jim Keith

    In a previous life, I designed a 3 phase, phased controlled static reversing induction motor drive control. Speed feedback was via a DC tach. The motor was a modified NEMA D design that avoided the unstable voltage control region prevalent in NEMA B motors. Efficiency was poor when operated at low speed, but it was responsive and reliable –it made the company lots of money at the time (well before the age of the modern PWM inverter drive control). All legs were phase controlled (required (5) anti-parallel SCR pairs). Line voltage was 460VAC and the SCRs were rated at 1200V. So per my substantial experience, the static reversing drive described here WILL WORK!

  • reon

    to many things that can go wrong but still a interesting article

    • Erich

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  • victor ghenno

    Please be aware of loaded motors. To implement this securely you must use some kind of current protector in order to avoid dangerous reverse voltages on thyristors and loaded motor. Sometimes is better to protect the motor instead of thyristors.
    Great article.

    • Francys

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    • Jim Keith

      Good point! The magnetic field in the rotor does not collapse immediately when the stator current ceases –as a result, the voltage across the thyristors equals line voltage + CEMF generated (as it gets out of phase with the AC line). For this reason, the voltage rating of the thyristors must exceed the peak-to-peak line voltage.

      Minimum voltage rating for a 10% high 230VAC line is 715V. Use an 800 to 1000V thyristor rather than the suggested 600V device.

      Over-current protection is an issue not covered –this must be sized according to the nameplate FLA and could be interfaced to the microcontroller.