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Choose AC drives or soft starters for motor controls

In order to understand better the starting requirements using a AC drive and the selection of an AC drive for your motors, a number of motor starting simulation studies (ETAP or other software) can be conducted for the load types and AC drive control mechanism of the motor, in order to determine the AC drive size, control method and settings. These studies can verify that the drive and motor system meet the operational requirements. There are usually a set of speed-torques curves for each application. During the acceleration, frequency and voltage are generally controlled such that full voltage is reached at the design rated RPM of the motor following a ramp. Motor voltage and current are regulated to provide speed and torque control to the load. The voltage to frequency ratio is held roughly proportional. In practice torque developed at very low frequencies falls off and extra voltage above the value of the volts / HZ ratio (or other ctl) is automatically applied to regain the lost torque. The variable frequency AC drive modify the speed but also the input voltage in direct proportion.

The issues that need to be considered include:
• Type of load the motor is driving: Does it require a high starting torque?
• The time required for the motor to reach speed
• The best control method of an AC drive for the load: Is voltage boost required in order to obtain the required starting torque? What is the minimum operating frequency of the AC drives? Will it cause high starting current?

Also, we must not get confused and think very simply that soft starter and variable frequency drive have the same role. AC drives are more expensive and soft starters are not able to regulate frequency. There are many other considerations before deciding if a variable frequency drive or soft -start is needed for your load.

Another formula is the rated torque of the motor which is the design value on the motor nameplate that is developed with rated voltage and rated frequency at the rated slip RPM below the synchronous RPM. In English units, this rated torque can be calculated as:

Rated Torque in lb-ft = Rated HP x 5252 / Rated RPM

With AC drive control, the motor can deliver constant torque from zero speed to rated speed.

I think there can be a lot of reasons for using an AC drive for such a motor, but in that voltage / power range, there are LOTS of other starting methods that are nearly as good, and a lot cheaper.
Just tell us the characteristics of your supply system, and what your concerns are during start, and I am sure quite a number of electrical engineers will be able to advise which starting mode will be cheaper, easier to run, and easier to maintain, than an AC drive. I have done a few myself.
The main application for the AC drive is when the process driven by the motor requires special consideration, mainly in terms of a variation in motor speed, and there can be a lot of reasons for that, too.

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