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


bench setup

Undocumented words and idioms for our ESL friends

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