Motors & Drives Information

Questions concerning various ac motors often arise. Discussions with and answers from various users indicate that the operational behavior of ac motors are still a mystery. This article will clarify some of the concerns raised by a typical user question.

Typical Question

I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?

Can I achieve the benefits of a wound rotor motor (high starting torque with lower starting current) along with the added benefits of reduced maintenance by using a squirrel cage motor and an electronic soft starter?

Discussion Group Answers

• A wound rotor motor with an appropriate secondary resistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not. The negatives are that both the motor and the starter will require a lot more maintenance than a standard cage motor and the purchased price is higher.
• The soft starter and induction motor will approximately accomplish the similar output to the wound rotor motor. However, the soft starter may be more expensive and more demanding on the service. Also, MTBF may be lower for the soft starter.
• I’m not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the 1000 HP+ range, either. Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?
• The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor NEMA Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor.
• I don’t have knowledge for comprehensive evaluation of your options compared to your application. Considering only the motors themselves (not the other parts of the starting/control): In my experience we have much higher failure rate on wound rotor motors than on our squirrel cage motors (although wound rotor motors are probably used in the more demanding applications).
• If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations have been operating for more than twenty years. For this application, it is likely that a wound rotor motor would be required based on the high starting torque requirement.

Clarifications

Wound rotor motors are also squirrel cage motors. A standard squirrel cage motor is normally referred to as a standard induction motor. Wound rotor motors and standard induction motors operating on different principles. The wound rotor motor is a variable % slip motor while the induction motor operates as a fixed % slip motor.

Normally, the supply voltage fed to each motor is fixed frequency (i.e. 60 Hz from the line). Although reduced voltage is sometimes applied, normally, the supply voltage is fixed. With the induction motor, 60 Hz worth of slip occurs forcing the motor to go beyond its pull out torque point (200% to 250% FL). This results in high current (600% to 800% FL) while producing less torque (70% to 120% FL) than the motor is capable of producing.

With the wound rotor motor, increased rotor resistance (higher slip) is initially used and reduced as the motor comes up to operating speed. This method allows the motor to operate without going beyond its pull out torque point. This results in greater starting torque at lower starting currents than achievable using standard induction motors.

When the wound rotor motor is at operating speed, the slip characteristics approach those of the standard induction motor. A wound rotor motor is similar in characteristics of a NEMA D design induction motor.

The use of a electronic soft starter with a standard induction motor does not yield the same performance as a wound rotor motor with a variable rheostat control. Soft start is effectively a reduced voltage start. Since the frequency of the applied voltage is 60Hz, the effect of the reduced voltage is a substantial decrease in starting torque while holding the starting current to some maximum limit.