8 replies to this topic

[MB123]

Hi Mark and All,

 

We have a mixing machine with two motors coupled to the one shaft. Motors are identical. I would like to use one EMX3 soft starter, as I have done with other vendors, but I cant find anything addressing multiple parallel motors in Aucom's documentation

 

If I do this with their starter will they say that it was against their advice/warranty?

 

Thanks Michael

[mariomaggi]

MB123, 

you can use without problems your configuration. 

Best regards, 

Mario 

[marke]

I agree with Mario, there will not be a problem other than the motor protection will not discriminate between motors and will look at the combined current.

 

With two motors on the same shaft, the loading must be identical as the slip will be identical, so it will be effectively one motor with two stator windings. If you wanted to protect each winding, you could add individual overload relays to each winding.

 

Best regards,

Mark.

[MB123]

Thanks all for your quick and valuable responses

 

So the conclusion is that I should not be spooked by the fact that Aucom do not have application guidance for running two motors in parallel?

 

And, Mark, with respect to your comments regarding the slip being the same - are you saying that technically it would not be required to protect the two motors individually? This would make sense as any overload caused either by the process or a mechanical i.e. gearbox issue would present an overload on both motors equally.

 

We will probably still provide individual overloads though

 

Thanks again guys!

[marke]

Hello  MB123

 

Given that the two motors are identical, and the speed is identical, and the supply voltage is identical, the  torque produced by each motor will be close to identical, so there is no way for one motor to overload relative to the other. The only "individual" issue is a stator insulation breakdown.

 

Best regards,

Mark

[Shozza]

Hello All,

 

Wouldn't AS/NZS 3000:2007 clause 4.13.2 dictate that "each" motor must be protected with a means of protection against overload?

I interpret this as an overload would be required for each motor to achieve compliance with the wiring rules.

I'd be interested to hear how others interpret this?

 

Best regards,

Shaun

[marke]

Absolutely, in order to be fully compliant, I would agree, but functionally, identical motors with a hardcoupled shaft, must be running at the same slip and voltage, so would load share well and you would not compromise the reliability by treating them as a single motor with two stator windings.

 

Best regards,

Mark

[MB123]

Hi All

 

Thanks for your response

 

Hey Shozza, thanks for the wiring rules reference. We would always have a thermal overload relay for each motor to ensure they are protected individually.

 

I have another question though on this topic. Another engineer has made the decision in this case to use two EMX3 starters in lieu of one larger one. Hence the two motors, on the same shaft, will each have their own starters.

 

Assuming the two starters are identical, and their settings are identical, can anyone see an issue with this? I.e. when starting I would not expect the voltage/torque produced by both motors to be exactly the same if they are connected to two different starters. How much difference could this be? Over time this may break the machine I would imagine.

 

Or even when it comes to commanding a start, would two EMX3s, each being commanded at the same moment, both start the motor at the same time or would there be a difference?

[marke]

Absolutely no problem with using two separate soft starters on the motors. I have done this many times.

 

If you set both starters up equally, then the start current on each will be similar, but if there is plenty of thermal capacity in the rotors of the motors, I would suggest looking at starting one motor only with a higher start current and then start the second when the machine is up to speed.

 

If starts are relatively frequent, you could add some logic and alternate the starts between the two motors.

 

The torque reduces by the square of the current reduction. If you start both motors at as 400% start current with a LRC of 900% and an LRT of 180%, then the start current is more than halved and the start torque is less than a quarter. 4/9 x 4/9 = 16/81 = 19.75% of the total start torque.

 

If we start one motor only at the same torque, the start current would be SQRT(19.75/90) x 900 = 596%

So this is 596% of one motor as opposed to 400% of two motors which is a much lower current.

 

Best regards,

Mark