Medical Grade Power Supply for DC Motor Drives
Aug 23, 2024
In many medical applications, powerful DC motors are required. For example, they are used for heavy-duty lifting, rotating, or tilting of medical beds and operating tables, maneuvering precision surgical robots, or controlling air pressure in ventilators. DC motors offer very high power-to-weight ratios, reliable performance, and high torque at low speeds, making them ideal for integration inside robotic arms or bed mechanisms. With appropriate gearing, they can also accurately pump fluids in dialysis and transfusion equipment.
However, at standstill, a stationary rotor produces no generator effect, and the motor drive current is limited only by the DC resistance of the windings. Inrush current can reach three times the running current for a brief period until the motor spins up (Figure 1).
One method to reduce peak motor start-up current is to install a PTC (positive temperature coefficient) resistor in series with the motor. PTCs initially limit high inrush current due to higher cold resistance and allow nominal current through once warmed, providing minimal impedance.
For the following example, a 24VDC, 125W motor with a three-times higher inrush current for 200ms is used, aiming to reduce the peak current by 50% (green curve in Fig. 1).
One method to reduce peak motor start-up current is to install a PTC (positive temperature coefficient) resistor in series with the motor. PTCs initially limit high inrush current due to higher cold resistance and allow nominal current through once warmed, providing minimal impedance.
For the following example, a 24VDC, 125W motor with a three-times higher inrush current for 200ms is used, aiming to reduce the peak current by 50% (green curve in Fig. 1).
Limiting Start-Up Current with PTC Resistors
The required PTC specification can be determined as follows:Step 1: find nominal current and inrush current:
Step 2: Calculate the energy rating of the PTC:
Step 3: Calculate the resistance of the PTC needed to reduce the peak inrush current by 50%:
A suitable inrush-limiting device would be a 3 Ohm NTC with at least 100J energy rating and a steady current rating of 6A or higher. Even with peak inrush current limited to 50% of its maximum, the medical power supply must handle this short-duration start-up current.
Selecting the Right Medical Grade Power Supply
Instead of a 125W power supply for steady consumption, a 200W power supply is required to manage the high inrush current. Using a PTC with higher resistance to further reduce inrush current is not recommended, as increased series resistance can affect motor performance and risk stalling under high load. A better approach is to select a medical grade power supply that delivers temporary high inrush current while maintaining short-circuit protection and load fault tolerance.RECOM’s RACM140E-K series meets these requirements. It provides 140W continuous and 210W boost power for up to 10 seconds, sufficient to manage peak start-up and short-term motor stall current. The unit also features full protection against continuous output short circuits and overcurrent conditions. Another concern with DC motors is that under sudden load disconnection or rapid deceleration, the kinetic energy of the motor and geartrain can turn the motor into a generator, causing back-emf to exceed the driving voltage.
Excess energy can be absorbed by a capacitor across the motor (C1), but excessive capacitance increases motor start-up inrush current. A more effective solution is a braking resistor switched across the motor by Q1 only when motor voltage exceeds the driving voltage (Figure 2).
The AC/DC power supply must accommodate motor back-emf, either with a simple motor capacitor or a switched braking resistor configuration.
The RACM140E series features output over-voltage protection set well above nominal voltage to safely handle motor-generated back-emf voltages (Table 1).
The AC/DC power supply must accommodate motor back-emf, either with a simple motor capacitor or a switched braking resistor configuration.
The RACM140E series features output over-voltage protection set well above nominal voltage to safely handle motor-generated back-emf voltages (Table 1).
| Parameter | Model | Output Voltage | OVP Level |
|---|---|---|---|
| Over-Voltage Protection (Hiccup mode) |
RACM140E-12SK | 12V | 30V |
| RACM140E-15SK | 15V | 30V | |
| RACM140E-24SK | 24V | 40V | |
| RACM140E-36SK | 36V | 48V | |
| RACM140E-48SK | 48V | 65V |
Table 1: RACM140E series output Over Voltage Protection (OVP) levels
Medical Certifications and Protection Features
The “M” in RACM140E-K stands for ‘Medical,’ as this medical AC/DC power supply is certified for medical applications. It provides 2MOPP (two means of patient protection) reinforced isolation per EN/IEC 60601-1 and ANSI/AAMI ES 60601-1 medical safety standards, and complies with medical EMC standard EN 60601-1-2. Output leakage current to ground is below 300µA, suitable for Body Floating (BF) patient connections, and touch current is below 100µA during normal operation.
The RACM140E power supply is available with Molex-type connectors, push-in fittings, or spade terminals, and can be panel- or chassis-mounted. With a height of 40mm or less, it fits inside 1HE rack-mount enclosures. In addition to medical certification, it meets household and industrial safety and EMC standards with overvoltage category OVC III up to 2000m or OVC II up to 5000m altitude.
Dimensions:
For simplified system integration, the RACM140E series provides ample margins to EN55032 ‘Class B’ EMC limits with increased surge and burst immunity. The universal input voltage range is 80-264VAC or 120-370VDC, and nominal output voltages of 12, 15, 24, 36, or 48VDC are adjustable by +20% via an onboard trim potentiometer.
With its compact size, cost-effectiveness, high peak load capability, and extensive medical, household, and industrial safety and EMC certifications, the RACM140E series is ideal not only for powering DC motors but also for building and home automation, medical robots, data and telecom, industrial automation, intralogistics, and test & measurement applications.
Dimensions:
- Enclosed: 147 x 81.5 x 40mm
- Open Frame: 147 x 81.5 x 38mm
For simplified system integration, the RACM140E series provides ample margins to EN55032 ‘Class B’ EMC limits with increased surge and burst immunity. The universal input voltage range is 80-264VAC or 120-370VDC, and nominal output voltages of 12, 15, 24, 36, or 48VDC are adjustable by +20% via an onboard trim potentiometer.
With its compact size, cost-effectiveness, high peak load capability, and extensive medical, household, and industrial safety and EMC certifications, the RACM140E series is ideal not only for powering DC motors but also for building and home automation, medical robots, data and telecom, industrial automation, intralogistics, and test & measurement applications.
Applications
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