Is Instantaneous Bi-directional Operation Possible With a Three-Phase Induction Motor?
Instantaneous bi-directional operation... I'm trying to give an answer for this connection diagram.
Vex, you seem to be working on an inquiry. Instantaneous bi-directional operation... perhaps it's a reversible motor?
No, it's a three-phase induction motor. I introduced to the customer before. It's for a conveyor, but due to a change in the specifications, it's supposed to reverse instantly instead of stopping when returning. So, I will answer about the wiring in that case.
I wonder what your answer will be?
The answer is to replace any 2 wires R, S, or T of the three-phase power supply. I learned this when I was a rookie.
That's when it stops and then reverses. Three-phase induction motors are not recommended for instantaneous switching of the rotation direction.
Eh? Why not?
There are two main reasons. ① Damage to the gearheads ② Risk of a power supply short circuit Let's look at ① first. At the moment of reversal, the motor will try to reverse, but the load will keep trying to turn in the same direction. Since torque is applied in opposite directions, the gearheads may not have sufficient strength and may be damaged.
I didn't know about that... I almost answered that it was fine.
In fact, three-phase has a higher starting torque than single-phase. At the moment of reversal, the rotation speed is 0, so the generated torque at that time is the starting torque. This has a huge impact.
AC Motor Torque Characteristics Diagram
If you look at the characteristics diagram, it's true that the three-phase motor is higher. So what is ②, the risk of a power supply short circuit? When you switch the rotation direction in a three-phase motor, you have to change the wiring, right?
That's right. But that's the point. We use 2 electromagnetic switches to instantly switch the power supply, but if they are turned ON at the same time, the power supply will short circuit. This could cause the breaker to trip and shut down the equipment.
But isn't that the same with a single-phase reversible motor?
There's no such risk with single-phase motors because there is only one contact.
Connection Diagram for Single-Phase Motor
Connection Diagram for Three-Phase Motor
I see.
However, you can also use something called a reversible electromagnetic switch. It uses two electromagnetic switches to switch the power on and off, but they are interlocked so that the two contacts cannot be turned ON at the same time. So you don't have to worry about short circuits anymore. If you use a round shaft motor, there's no risk of damage to the gearheads, so you can introduce to the customer.
Um... Ms. Ori. In this case, the answer to the customer would be "no," right?
That’s correct. However, there are few ways to use it which I can suggest. The first thing you need to do is to ask them to establish a stop time, even just a short one.
So when they want to reverse direction, you have to stop temporarily, and then switch the connection.
It might also be a good idea to use an inverter to perform a deceleration stop and then switch the rotation direction. The KIIS Series can achieve excellent characteristics when combined with inverters, and we have combination data available, so I think your customer can make their decision with confidence.
Example of Combination Data for Inverters and KIIS Series
If it's combined with an inverter, there's no need to change the motor.
If the customer really needs instantaneous reversing, they can change to a single-phase 200 VAC reversible motor. You can use a single-phase motor by using two of the three-phase motor. In that case, the rated time of the motor will be different, so be careful.
I understand. I'll be careful if the customer says they want to change to a single-phase reversible motor.
Oh! I almost forgot! If you have a large number of motors, you'll need to use them in such a way that they won't skewed toward one of the R, S, or T points of the power supply.
Thanks for all the advice. Now I'm all prepared to give the customer an answer!
That's great.
Well then, I'll switch over instantly and go answer the customer.
Good idea. In front of customers, be the reliable Vex who can answer anything.
