Resolver Output and Voltage
A resolver consists of a primary winding and a pair of secondary windings, each with a sinusoidally varying winding density. The primary winding rotates with the load. The secondary windings, known as the sine and cosine windings, are fixed in position with a 90° offset from one another; hence the names.
A resolver outputs an analog signal caused by current passing to the primary winding which generates a magnetic field. When the winding turns with the load, it excites separate output voltages in the sine winding and the cosine winding. The ratio of these two voltages can be processed to calculate the angular position of the load.
The key electrical characteristics for a resolver are:
- Input excitation voltage
- Output voltage
- Transformation ratio
Resolver Input Excitation Voltage
Input excitation voltage and frequency refer to the input AC voltage (Vrms) and frequency supplied to the leads of the primary resolver winding by the system. These voltages typically range from 1 to 26 VAC, at a frequency of 400 to 10,000 Hz. A given winding can operate over a range of voltages in frequencies without significant change to performance. Large departures from the recommended voltages and frequencies can introduce errors and low resolver output voltage that can potentially damage the device. Properly specifying the resolver for the application is important.
Maximum Input Current
As the name suggests, a resolver’s maximum input current is the peak current in amps that can be used to energize the primary winding at the system input voltage and frequency. Each resolver has a maximum current rating. Per Ohm’s law, current is given by the ratio of input voltage to winding resistance. Typical input current ranges from 10 mA to 100 mA.
The current for a resolver is supplied by a resolver interface card installed the drive. The card has a maximum voltage rating; it is essential to ensure that voltage is large enough to drive the resolver. Typical interface cards provide 4 V to 12 V; this will handle roughly 90% of the resolvers available.
Resolver Transformation Ratio
The transformation ratio of a resolver is the ratio of its output to the input voltage when the output voltage is at maximum magnetic coupling. Recall that a resolver is a specialized transformer. During operation, it takes the input voltage and steps it down to an output voltage. The transformation ratio is a useful metric for determining whether a resolver will operate appropriately in a system.
How To Troubleshoot Resolver Output
If the feedback system isn’t working, check the output voltage. The resolver interface card determines how much voltage can be supplied to the resolver for operation, but the interface card also requires a minimum output voltage from the resolver in order to read it as a signal. Here, the transformation ratio may provide an important clue to the problem. Consider an interface card that provides up to 7 V of input to the resolver but requires a minimum of 2 V RMS of output. If the transformation ratio of the resolver is 0.3, that means it steps down a 7 V input to a 2.1-volt output. That is barely above the threshold required by the resolver interface card, which means that the signal may be lost in the noise. It would be better to specify a resolver with a higher transformation ratio in this scenario.
Winding design determines the transformation ratio of a resolver; roughly speaking, transformation ratio is the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. As a result, a manufacturer can customize resolver windings to produce transformation ratio required for the system without affecting performance.
Although a different resolver interface card that provides higher drive voltage or accepts a lower output voltage from the resolver might also solve the issue above, it would typically carry a higher cost.
Transformation ratio is not the only factor to consider when trying to maintain appropriate resolver output voltage. Cabling losses can also be a problem. If the system has a 300-foot cable between resolver and drive, voltage drop across the cable may lower the drive voltage carried from drive to resolver. The lower drive voltage will also reduce the output signal voltage.
Resolvers do not require any onboard electronics; as a result, they can tolerate very harsh conditions such as temperature extremes, contamination, high radiation, and shock and vibration. However, resolvers should be specified with care. To optimize your results, work closely with your vendor to ensure that you choose the appropriate device for your application.
Additional Resolver Resources
Learn more about resolver technology here
Learn how to specify resolver speed and accuracy here
Download our white paper on the key differences between encoders and resolvers here
Download our white paper on how to specify the best resolver for your application here
Dynapar's Most Popular Resolvers
Dynapar and Harowe offer frameless resolver and housed resolver options. Frameless resolvers with rotor and stator assemblies that can be housed in servo motors, direct drive motors, rotary platforms and more to provide motion feedback. Housed resolvers provide reliable feedback in a sealed package with various IP rated models. See our most popular resolver models: