Torque start

[GGOSS]

The manufacturers promotional literature suggests starting currents averaging 30% below that of competing soft starter products.

 

From my own experience (gained through field trialing several soft starter products with and without torque start/control functionality) I have not noted any significant improvements in either starting current or acceleration profile over product offering closed loop controlled current technology. In fact on several occassions the humble open loop controlled voltage current limiting soft starter appeared to outperform the more advanced technology.

 

I would be interested to receive your outline/explanation of how the MSF achieves such low starting currents. If you could also indicate the types of opposition soft starter products the MSF has been compared against (without mentioning names and the types of applications , that too would be greatly appreciated.

 

Thanks in advance & regards,

GGOSS

[marke]

Hello GGOSS

 

I believe that the coments regarding the reduction in start current refer to a comparison of the minimum start current that can be achieved by a closed loop starter as opposed to an open loop starter. The impedance curve of an induction motor is roughly square and if you start with a linear voltage ramp, you will have a higher start current than you actually need. If you use a closed loop soft starter, you can control this current to minimise the start current to what is required by the motor. This can be achieved by a controlled current soft starter, or by a controlled torque soft starter. The controlled torque option will reduce the current when the maximum start current is not needed, so the start current will typically be less than the start current from a controlled current soft starter, however if the start current is lowered, the start time will be extended. I would expect that the area under the curve will be about the same.

 

Best regards,

[GGOSS]

Thanks Marke, although your response doesn't really enlighten me any further.

 

Closed loop controlled current starters allow the motor to provide bulk acceleration torque particularly up towards rated speed. With an open loop current limiting starter the voltage at the motor terminals remains constant (or almost constant) throughout the entire speed range. This acts to reduce the available motor torque by the same percentage throughout the speed range. As a result, the torque curve produced by the motor via an open loop current limiting soft starter (or primary resistance starter or a specially tapped auto-transofrmer starter) more closely resembles the pump curve, than the torque produced by the motor under closed loop controlled current starting ie either constant current or current ramp.

 

Soft starters offering torque control technology are said to sample motor torque and increase/decrease this (though increasing or decreasing voltage) to maintain a certain level of acceleration torque. If this acceleration torque remains constant throughout the speed range, then the pump will accelerate in a more linear fashion than that achievable with a closed loop controlled current starter, but that doesn't mean the current will be lower. In fact the current could be higher in certain applications and in all cases the start time would be longer.

 

What are your thoughts?

[marke]

Hello GGOSS

 

For a moment, forget your experiences, and consider what would be the ideal for starting motors and loads.

The load has two characteristics to consider, the speed torque curve and the inertia. At all speeds, you must provide sufficient torque to overcome the speed torque curve and provide sufficient torque to accelerate the load.

 

The ideal torque curve for a purely inertial load, is a straight line constant torque, and for a variable torque load such as a pump, you want and sloping torque curve with the ideal being a square law cuve.

 

A TVR open loop soft starter provides a rising torque curve and is OK for pumps etc, but no good for inertial loads. If you apply a voltage ramp to an inertial load, you will have a high start current towards the end of the starting period.

The controlled current soft start provides an flatter torque curve up to about 75% full speed, and then the torque curve lifts. With simple closed loop starters, I would recommend a ramping start for pumps and a current limited start for an inertial load.

 

Unfortuately, the voltage control achieved by SCRs on a motor is not truely load independant. This can result in instability under true open loop conditions. In most cases, the effective torque boost as the motor approaches full speed is not a problem and can be a benefit. In the controlled current soft start, the current feedback causes an accelerated increase in voltage as the motor approaches full speed and consequently the torque boost is higher than with a TVR approach.

 

The more modern technique is to apply a mathematical model, a little bit like the vector control model on drives, and to monitor the effective shaft torque produced by the motor. This can them be used to control the magnitude of the voltage applied to the motor in the same way as the current feed back can. The control system can theoretically be designed to provide a constant torque, or a ramping torque, or a predefined torque/speed profile.

This requires a higher processing power than was needed with a controlled current or TVR soft starter, but can provide a higher level of start characterisic control if it is required.

 

Best regards,

[mariomaggi]

Dear all,

I've noted that usually soft starters are seen like a "black box" that must generate "torque".

The torque generator is the motor, and there are big differences between motors, specially in rotor construction. I've found big differences between rotor characteristics also fo the same size of motor of the same manufacturer. Depending from rotor availability, sometimes you could buy a copper bar rotor and sometimes a Silumin bar rotor, reading the same main data on catalogues and on the motor label.

But performances during starting using a soft starter could be very different in such cases.

There is also the problem of sub-harmonics, that could modify performances during start-up, at the same level of starting current with similar motors.

 

Regards

Mario