Compact Soft Starters

[GGOSS]

Many compact soft starters control only 1 or 2 phases to a 3-phase motor hence imbalanced currents can be expected during starting. What are the long term ramifications of this on the motor?

 

Regards,

GGOSS

[marke]

Hello GGOSS

 

Many compact soft starters control only 1 or 2 phases to a 3-phase motor hence imbalanced currents can be expected during starting. What are the long term ramifications of this on the motor?  

As you will have an imbalance in current and more importantly the current vectors, you will have in effect a combination of posative and negative sequence currents flowing in the rotor resulting in an increased heating. If you are using this technology for frequent and/or high inertia starts, then the motor life could be compromised.

If the motor has plenty of capacity relative to the start duty, then there will be little effect.

Best regards,

[GGOSS]

Hello marke,

 

Many thanks for your reply.

 

As you are most probably already aware, most 1 and 2 phase controllers offer timed voltage ramp starting as standard. Therefore if one were to compensate for motor life by increasing motor size, start performance (particularly torque reduction) would be compromised. Do you have any thoughts on this?

 

Regards,

GGOSS

[marke]

Hello GGOSS

 

Yes, you have increased heating in the motor when the field is assymetric. If you are really running the motor to it's limits, then you need to look at increasing the frame size. This will result in increased starting current and a larger starter. You may be better to look at a starter with a symmetric field. (this does not mean equal current with a two phase controller).

It is possible to fiddle the firing angles to balance up the currents on a two phase controller, but this does not mean a symmetric field, especially with a delta connected motor. The field assymmetry will reduce the torque delivered by the motor and will increase the losses in the motor.

 

I would only consider assymetric staters where there is plenty of start capacity, (particularly thermal) and where the starting time is relatively short, i.e. pumps. For high inertia machines, I would not consider an assymmetric TVR starter.

 

Best regards,

[jraef]

GGOSS,

Another consideration of the versions using 1 or 2 phase control is the failure mode. As you know, in a 3 phase-6 SCR starter one SCR shorting has no immediate effect on the motor itself because if all other SCRs are off, there is no path for current flow. In the 1 or 2 phase control versions, the uncontrolled phase(s) provide an immediate completion of 1/2 wave current flow through some of the windings (depending upon the number of uncontrolled phases). This runs the immediate risk of motor damage unless current flow can be detected and an upstream isolation device such as a shunt trip circuit breaker or contactor is used. While we all know that this is a good idea as a method of protecting the motor from multiple SCR failures, the fact is that many if not most users ignore this precaution in the interest of cost savings. Using the 1 or 2 phase soft starters exposes these common users to considerably more risk. I have found that if I get a chance to adequately explain this to potential users, they tend to shy away from using this style of soft starter.

[GGOSS]

Good information jraef,

 

certainly worth consideration for those looking at the more economical 1 and 2 phase soft start alternatives.

 

Regards,

GGOSS

[GGOSS]

Hello Marke & jraef,

 

Have heard a rumour regarding a 2 phase controller that is soon to be released onto the market. The manufacturer of this product claims some 'trick' technology has been included to balance the current in all three phases (during start and during run). How is this possible? Aside from the magnitude of current flowing into each phase, are there any other considerations worth noting?

 

Any advice would be appreciated.

 

Regards,

GGOSS

[marke]

Hello GGOSS

 

Balancing the current is no real problem, however the interesting thing is what is the flux field doing. I wonder if they / you have compared the torque output from a motor at a particular current for the "balanced" two phase controller and a balanced three phase controller.

I expect that the magnitude of the current may appear balanced, but what about the phase angle? If the phase angle is not also balanced, there will negative sequence currents which reduce the torque and increase the heating.

Having not done any testing on the unit, I can not comment beyond this, but to achieve the best results, (maximum torque and lowest losses in the motor) the current vectors need to be balanced.

I am interested to hear more.

Best regards,

[GGOSS]

Thanks Marke,

 

How do we go about testing for these things?

 

Regards,

GGOSS