Relay Control of Induction Motors

[neilparker62]

Would like to check if the following relay control circuit work for driving forward and reverse terminals of a gate motor will work? Relays are connected for 'electrical interlock' meaning it should not be possible for both fwd and rvs terminals to be activated at the same time. Physically that is implemented as a jumper wire between the relay commons. If the fwd n/o is closed, the right hand motor will be connected and vv for the reverse. If both fwd and rvs are somehow activated at the same time, both n/c s will be open and there will be no connection to either terminal.

What is the "conventional wisdom" on dealing with transients from small induction motors such as these ? Are we still using standard R/C snubber circuits or are the bi-directional TVS diodes a better option. Or maybe even both ?

Design notes / recommendations on fuses and TVS diodes below are from Chatgpt.

Design Notes
Motors: 230 V AC, 0.5 HP induction motors.
Motor windings: 10 ohm resistance, 90 mH inductance.
TVS Diodes: SMCJ440CA — bidirectional, 440 V clamping.
Fuses: 500 mA fast-blow, 600 V AC rated — placed in series with each TVS diode.
Purpose: Suppress voltage transients across motor windings during switching by relays.

 

[.Scott]

The first thing I noticed is that if you blow one of those fuses, the system will still operate, but the voltage transients will be entirely unsuppressed. You will have a silent failure. On the other hand, this motor looks like it's 1/4HP or less. So those transients are more likely to show up as EMF interference than actual circuit damage.

The second thing is that supposedly, you are trying to protecting other devices that share the same power source as this motor - or perhaps suppress EMF. And you are depending on the impedance/resistance of the power lines to this motor system to make sure that most of the transient power is absorbed through the diodes and not transmitted. This may or may not be good enough. If it isn't, you may want to loop the live power line just before it reaches the switches to provide a bit more impedance.

The third thing: I know you're using break before make from your discussion. But I can't tell whether those extra lines in your schematic are intended to show that.

 

[Baluncore]

Where is the phase shift capacitor that generates the rotating field?

If you switch from forward to reverse without waiting for the armature to stop, you will generate a high current pulse, and a physical shock to the motor.

 

[neilparker62]

The third thing: I know you're using break before make from your discussion. But I can't tell whether those extra lines in your schematic are intended to show that.

If you mean these:

They are the normally closed switched on the relays. Current paths to the respective motors are as shown below. If both relays are activated at the same time , both normally closed switches will be open. So the red and green current paths are mutually exclusive. In effect the relay connections are implementing an XOR gate.

 

[neilparker62]

Where is the phase shift capacitor that generates the rotating field?

If you switch from forward to reverse without waiting for the armature to stop, you will generate a high current pulse, and a physical shock to the motor.

The relays are driven from program logic. A motor opening a gate will be stopped for a period before the reverse motor/winding is activated to close the gate.

 

[neilparker62]

The first thing I noticed is that if you blow one of those fuses, the system will still operate, but the voltage transients will be entirely unsuppressed. You will have a silent failure. On the other hand, this motor looks like it's 1/4HP or less. So those transients are more likely to show up as EMF interference than actual circuit damage.

The second thing is that supposedly, you are trying to protecting other devices that share the same power source as this motor - or perhaps suppress EMF. And you are depending on the impedance/resistance of the power lines to this motor system to make sure that most of the transient power is absorbed through the diodes and not transmitted. This may or may not be good enough. If it isn't, you may want to loop the live power line just before it reaches the switches to provide a bit more impedance.

Noted - re silent failure. Thanks for pointing it out. We'll have to figure out how to deal with that situation.

Re protection - no other devices are connected. We are just trying to deal with arcing across the relay contacts caused by high voltage transient spikes when the motors are turned off/on.