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Hurst Manufacturing

Synchronous Motors - Hysteresis

For nearly 40 years Hurst has offered a line of hysteresis synchronous instrument motors known for their quality and reliability. The hysteresis motor contains a rotor of hardened magnet alloy steel with a uniform structure. Starting and accelerating torques are developed by induced magnetic poles and currents in the rotor and at synchronous speed the induced poles become fixed. Due to the uniform rotor structure synchronism can occur at any random angular position of the rotor. Thus hysteresis motors are exceptionally smooth, quiet, and vibration free. They are insensitive to inertial loads and can synchronize any load that they can accelerate.

The Hurst line includes an extensive number of gear reductions for output speeds from several hundred RPM to fractions of an RPM and gear trains ratings up to 600 oz-in. Both permanent-split capacitor and shaded pole types are offered. An inverted rotor construction is available as well as gear motors with an integral clutch.

Click here for gear train construction details.

Permanent Magnet Stepping Motor Application Notes

The Hurst permanent magnet synchronous motors are reversible permanent-split capacitor motors identical in construction to the Hurst stepping motors. In operation the permanent magnet rotor poles lock-in with the effectively rotating stator field and the motor runs at synchronous speed. The 60 Hz can-stack motors operate at synchronous speeds of 300 and 600 RPM. High quality gearing is available for the can-stack motors.

The ceramic rotor magnet material provides a relatively high flux resulting in a good torque to size ratio at moderate cost. In addition the permanent magnet construction provides inherent dynamic braking and low rotor speed for quiet operation and rapid acceleration.

The disadvantages of permanent magnet motors are a limited ability to accelerate inertial loads and a high sensitivity to the parameters of voltage and phasing capacitor. The first of these problems may be minimized by gearing or in some cases flexible couplings. The sensitivity to voltage and phasing capacitor directly affects the directional reliability of both starting and running under load. In Hurst motor designs directional reliability is a primary consideration and is assured when motors are operated with the recommended capacitor within a voltage range of +10% of nominal.

Variable Frequency Operation of AC Motor

Sometimes a single motor is specified for both 50 and 60 Hz operation. Since inductive reactance and capacitive reactance vary dissimilarly with frequency, optimum performance must usually be compromised when a single winding/capacitor combination is operated at different frequencies. Some types of motors are more sensitive than others to frequency changes. As a general rule synchronous motors and especially permanent magnet motors are more sensitive than induction motors. The factory should be consulted before 50/60Hz operation of a motor is planned. Operation over a wider frequency range is extremely difficult to accomplish with capacitor phased motors and is not recommended. Reliable operation requires a two phase power supply with applied voltage a function of frequency.

Applying 50Hz to synchronous motors design

Permanent magnet synchronous motors designed for optimum performance at 60Hz will be directionally un-reliable when operated at 50Hz. Rated torque will be 20% to 25% lower at 50Hz. Additionally, the output speed will be 5/6 the speed while applying 50Hz to the motor.

By increasing the capacitor value of the recommended capacitor by 30%, 50Hz may be applied to 60Hz motors without sacrificing reliability. 50Hz torque will be approximately 5% less than rated 60Hz torque.

Why Capacitor Start AC Synchronous Motors?

All Permanent Magnet AC Synchronous motors manufactured at Hurst are of the capacitor start variation. These motors, which are classified in the sub-fractional group, find various applications in situations requiring frequent and prolonged starting periods. As the name suggest these motors run at synchronous speed. The speed of a single phase AC synchronous motor can be determined using the formula Synchronous speed (in RPM) = 120f/p where f is the frequency of the power supply & pis the # of poles For the same starting torque, the capacitor motor when compared to the split-phase motor requires half of the current for starting. The auxiliary winding of the capacitor motor has twice the number of turns of the split-phase AC motor. A split phase AC motor basically has an inductive auxiliary winding. The lesser current in the auxiliary winding of the capacitor motor results in less copper loss and subsequently less heat generated by that motor. Because of the capacitor in the winding, the capacitor motor has the advantage of a greater phase shift between the current of the stator winding and that of the auxiliary winding. Phase shift is typically about 80% compared to 20% for the split-phase motor. The increased phase shift translates into easier starting for the capacitor start motor.

Starting & Stopping Characteristics

Virtually instant starting and stopping characteristics are among the principal advantages of an AC Synchronous motor. Generally, the motor will start within 1 1/2 cycles of the applied frequency and will stop within 5 mechanical degrees. The motor will start and reach its full synchronous speed within 5 to 25 milliseconds. The unusually short stopping distance of an AC Synchronous motor is obtained by simply de-energizing the motor. No mechanical or electrical braking is necessary. The quick stopping is the result of the slow rotor speed and the presence of a no-load reluctance torque produced by the permanent magnet and the tooth construction of the stator and rotor.