nected. The stator is essentially the same as is used in any standard squirrel cage motor. By shorting out external impedance via contactors connected in the rotor circuit, the speed/torque curves of the wound rotor induction motor can be altered, Fig. 2.
The slip ring motor is used primarily to start a high inertia load, or a load that requires a very high starting torque across the motor’s full speed range. By correctly selecting the resistors used in the secondary resistance or slip ring starter, the motor is able to produce very high torque at a relatively low current from zero speed to full speed. The resistor banks are typically brought in to and out of the secondary circuit by a series of heavy-duty contactors.
The main driver for converting a wound rotor based control system to a variable frequency drive is to reduce energy and maintenance costs. To control torque production in the wound rotor motor, resistor banks are switched in and out by means of high current electrical contactors, Fig 3. These resistors
Table 1 - EXAMPLE SPECIFICATION FOR A WOUND ROTOR MOTOR
Rated hp:
200
Rated rpm:
1200
Primary amps:
215
Primary volts, AC:
460
Secondary amps:
254
Secondary volts, AC:
365
Enclosure:
Ambient (°C):
TEFC
40°
Duty:
Service factor:
Continuous
1.0
Table 2 - EXAMPLE SPECIFICATION
New motor:
Rated hp:
Rated rpm:
Enclosure:
Voltage
Ambient (°C):
Duty:
Service factor:
300 1200
TEFC 460 40°
Continuous 1.0
are sized on secondary (rotor) amperes and the application’s duty cycle. In general, if the application operates below 50% speed for a high percentage of its operational cycle, it may be a good candidate for modernizing to a VFD as a way to save energy. If the motor is operated at reduced speeds, more energy is being absorbed in the resistor banks for maintaining speed control. Depending on the application’s horsepower and duty cycle, the energy savings can be in the hundreds of k WH per application.
While the resistor banks typically are not prone to failure, the contactors that switch resistor banks in and out of service for speed control of the wound rotor motor are typically replaced or serviced on a yearly basis. Typical replacement costs for each set of contactors can be up to $5,000 each; depending on the size of the motor and number of control steps, there can be many contactors in the control system.
To size the motor for the application, we first need to determine the capability of the existing wound rotor ( WR) motor by using the following equation.
Thus, we can determine the (100%) full load
torque of the existing WR motor. In this case, for ex-
ample, for a 200-hp, 1,200-rpm wound rotor motor:
Per a chart listed in NEMA MG1-1993, Section 12. 41, a 200-hp, 1,200-rpm motor polyphase wound rotor motor with a continuous rating should have a breakdown torque of 225% full load torque. This should give us a minimum to start with because typically we do not have enough documentation with some older motors to make a judgment call. Based upon the application (hoist, chipper, or pump) this can range from 225% to 275% and it is based on the motor’s application.
Examining some older charts, Fig. 4, a “typical” vintage wound rotor motor can supply up to a maximum of approximately 275% per unit torque.
References:
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